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Pentathlon Science Resource Guide 2015-2016 1

AN INTRODUCTION TO ECOLOGY

2015–2016

PENTATHLON

SCIENCE RESOURCE

GUIDE


INTRODUCTION

Natural ecosystems are driven by the interactions among the many organisms that live within

them as well as by the ways that the local climate, soils, and chemistry define the bounds of what

types of organisms can survive there. These interconnected relationships are the basis for the

field of ecology, which is our topic of study in this resource guide. Ecology is a broad and

continuously developing field. Scientists within this discipline study very diverse topics.

Population ecologists might be interested in the competitive relationships among multiple male

grouse for a mate; community ecologists may study the impacts of the reintroduction of a

predator, such as a wolf or grizzly bear, on other animals in the ecosystem; and landscape

ecologists might be concerned with the impacts that a widespread invasive species is having on

multiple habitat types for many wildlife species across vast areas.

A major emphasis of the discipline of ecology is that the many elements of an ecosystem are

naturally interconnected. Not only do organisms interact in ways that we can observe, such as

when trees grow rapidly and shade out competitors, as animals fight for territory, or as

herbivores graze on herbaceous species, but organisms are also connected by the ways that

energy, nutrients, and water cycle through different ecosystem components and regulate the

structure and function of the ecosystem. These relationships can also change over space and time

as energy sources are changed. Understanding of how complex ecological relationships vary

through space and time is an important and necessary step toward conserving biodiversity in our

natural world.

Finally, it is important to recognize that while humans have had an enormous impact on our

planet, we are simply part of the interconnected world in which we live. Humans have altered

atmospheric chemistry, changed species distributions and interactions, and influenced the

productivity of ecosystems. These changes often come full circle to impact the health, food

security, and well-being of the earth’s human inhabitants as well as that of the other 8.7 million

species with which we share our planet. In the course of this resource guide, we will examine

relationships between organisms and their environment, learn about the interconnectedness of all

life on the planet, and close with a look at how humans have changed these relationships.

NOTE TO STUDENTS: You will notice as you read through the Resource Guide that some key terms and

phrases are boldfaced. These terms are included in the glossary at the end of the Resource Guide.


FIGURE 1.1

Levels of ecological organization. Ecology is the scientific study of the relationships between organisms and their environment.

German biologist Ernst Haeckel in 1860.Haeckel was the first to use the

term “oekologie” (ecology).

SECTION I:

INDIVIDUAL AND POPULATION ECOLOGY

An Introduction to Ecology What is Ecology?

Ecology is the scientific study of the relationships between organisms and their environment.

Ecologists, the scientists who study ecology, are often interested in the interactions that

determine the distribution (geographical location) and abundance (quantity) of a type of

organism. Some of these interactions are between the same or different kinds of organisms

(biotic interactions), and others are between an organism and its physical environment (abiotic

interactions). The more that these interactions are understood, the more we realize that in every

ecological system, organisms (including humans!) and their environment are interconnected, and

that altering one part of an ecosystem usually changes others as well. Ecologists are interested in

answering many types of research questions about these relationships: What accounts for

patterns of distribution and abundance? How do these patterns change over long timescales?

How have humans altered these patterns? And, will these patterns be impacted by a changing

climate?


Ecological communities vary widely in scale and number of species, ranging from microscopic

communities of marine plankton to vast temperate forest communities and from uniform agricultural

communities to ecologically diverse regions, such as tropical rainforests.

A History of Ecology

Ecology as a study began relatively recently. The term “oekologie” (ecology) was first used by

the German biologist Ernst Haeckel (1834–1919) in 1866. The term is derived from the Greek

words oikos (meaning “house” or “dwelling”) and logos (“study of” or “science”). Haeckel

recognized that there was a strong relationship between ecology and the theory of natural

selection. The theory was promoted by Charles Darwin’s The Origin of Species in 1859.

Natural selection is the process by which individuals with better adapted inherited

characteristics tend to survive and reproduce more successfully than other individuals with less

well adapted characteristics. This framework provides a mechanism for the processes that control

the distribution and abundance of all organisms.

It is important to distinguish ecology from both environmental studies and environmentalism.

The field of environmental studies examines human impacts on physical, biological, and

chemical processes. Environmental studies tend to be very broad in scope, integrating ecology,

geology, economics, social studies, and philosophy. It has led to a social movement,

“environmentalism,” which seeks to take steps to minimize these human impacts on planet Earth.


FIGURE 1.2

We can think about ecological interactions on multiple scales, from the individual organism to the entire planet.

Ecology, in contrast, is a broad science encompassing research questions on many spatial and

temporal scales. Sub-disciplines have arisen in more recent years, including Historical Ecology,

Wildlife Ecology, Plant Ecology, Human Ecology, Urban Ecology, Landscape Ecology, and

Disturbance Ecology, as well as many others.

The Ecological Hierarchy

We can think about ecological interactions on multiple scales, from the individual organism to

the entire planet. At each scale, a different set of patterns emerges, and different processes

determine the interactions that we see. Individuals often compete with one another for limiting

resources such as light, water, food, or nutrients. A population is a group of individuals of the

same species that live in a particular area and interact with each other. Populations of plants and

animals interact with one another in many ways, including competing for limiting resources.

Some populations are a food source for another population, while other populations may be

mutually beneficial, each providing a valuable resource or service to the other. The study of

populations of organisms, population ecology, often seeks answers to questions about how and

why the locations and numbers of populations change over space or time.


FIGURE 1.3

Broad-scale patterns of climate and geology allow us to categorize similar landscapes and ecosystems into biomes (large-scale regions dominated by similar types of ecosystems). Collectively, all of the world’s ecosystems together make up the biosphere.

Two or more interacting populations of

plants and animals of different species

in the same area are referred to as

communities. Ecological communities

vary widely in scale and number of

species, ranging from microscopic

communities of marine plankton to vast

temperate forest communities and from

uniform agricultural communities to

ecologically diverse regions, such as

tropical rainforests. The study of the

interactions within and among these

communities is called community

ecology. Community ecologists might

ask research questions such as: “Why

do some places have more species than

others?”; “Why do high and low levels

of disturbance result in reduced species

diversity, but an intermediate level of

disturbance results in higher

diversity?”; and “How is community

structure altered when a top predator is

removed from the area?”

An ecosystem is a community of

organisms and the abiotic environment

(the non-living physical and chemical components) in which they live. Some examples of

ecosystems include tropical rainforests, intertidal regions, high-altitude deserts, and freshwater

marshes. We will discuss many of these ecosystems in depth a little later in this section.

Scientists who study ecosystem ecology are interested in the way climate alters the distribution

of biotic communities, the rates at which nutrients and water enter and cycle through a

community, and the way that soils and precipitation patterns alter the relationships between plant

communities.

Landscapes are patchworks of multiple communities and ecosystems, and are typically studied

at larger spatial scales, such as what one would view while standing on a mountaintop or from an

airplane. At the landscape scale, scientists are interested in researching topics including how

variability in soils and topography influences patterns of community composition and species

diversity. A major component of landscape ecology is the study of the effect of ecological

patterns (e.g., patches of habitat types, rivers, mountains, agricultural areas) on processes (e.g.,

wind and water movement, disturbance events, nutrient cycling).1

Broad-scale patterns of climate and geology allow us to classify similar landscapes and

ecosystems into biomes (large-scale regions dominated by similar types of ecosystems), such as

tropical grasslands, deserts, temperate forests, arctic tundra, etc. Collectively, all of the world’s

ecosystems together make up the biosphere, the highest level of biological organization. The


FIGURE 1.4

The Scientific Method. Within each level of the ecological hierarchy, scientists use the scientific method to answer ecological research questions.

biosphere consists of all living organisms on the planet and their physical environment. At the

biome and biosphere level, scientists are interested in studying topics such as connectivity and

energy flow between biomes, global changes in climate, and large-scale extremes and norms of

carbon, energy, nutrients, and temperature.

The Scientific Method

Within each level of the ecological hierarchy, scientists use the scientific method to answer

ecological research questions. All scientific studies begin with observations of the natural world.

These observations lead to questions about the processes driving the phenomena observed. A

scientist then makes a hypothesis, or educated guess, based on prior experience and knowledge

about what is driving the observed process. A hypothesis should be very specific and should

include a statement of cause and effect and must also be formed in such a way that it is

quantifiable by the data to be collected. Testable predictions about the outcome of hypothesis

testing or experiments are then made. Then, experiments or observational studies are designed

and conducted, and data are collected and analyzed. The outcomes of experiments are interpreted


to see whether they support or

reject the original hypothesis. If

the outcome of the experiment

supports the original hypothesis,

then conclusions are drawn about

the studied system, additional

questions or hypotheses may be

made to expand the scope of the

question, and additional data

collected. If the data collected are

not in support of the hypothesis,

then the original conceptual

relationships about how the

system works must be

reevaluated, and a new

hypothesis might be formed.

When a greater understanding is

gained as the result of the testing

of multiple related hypotheses, a

theory is formed to collectively

explain the results of a large

number of experimental

observations. The large body of

scientific literature and knowledge has come from centuries of scientists following the scientific

method to build upon our understanding of the natural world. Because ecological data are

constantly being collected at field sites and in laboratories across the world, our understanding of

ecology is always changing.

Geographic Ecology: The Abiotic Environment The physical environment sets the ultimate limits of an organism’s potential distribution. Long-

term trends in temperature, wind patterns, and precipitation, patterns of salinity or acidity, and

soil properties are among the important factors that limit where an organism can survive, grow,

and reproduce. The environment determines whether a particular area will be a tropical

rainforest, a montane lake, or a temperate grassland, setting the stage for all of the organisms that

will occupy the area. Within the bounds set by abiotic factors, competitive relationships between

organisms further limit the distribution of a species.

Temperature and Climate

We are all aware of the weather around us. The current temperature, humidity levels,

precipitation, and wind determine the clothing we choose, our method of transportation, and our

activity choices for the day. Over long periods of time, the long-term average weather pattern, or

climate, places limitations on the biological life that can successfully endure, grow, and

reproduce in an area. Variation in climate on large scales is due to the amount of solar radiation

that reaches the Earth’s surface at a given geographical location. We typically see maritime

climates (high humidity with little daily or seasonal fluctuations in temperature) in coastal

http://upload.wikimedia.org/wikipedia/commons/d/df/ClimateMap_World.png


FIGURE 1.6

A moving air mass picks up moisture as it travels over a body of water. As the air mass moves up the mountain range, it cools and condenses, releasing much of its moisture as rain or snow. The drier air mass then warms as it travels down the inland side of the mountain range.

The rain shadow effect is responsible for the temperate, rainy climates of the Pacific Northwest on the west side of the Cascade Mountains (left) and the arid deserts of the interior

Great Basin on the east side (right).

regions with an oceanic influence and continental climates (large seasonal and daily variation in

temperature) in the interiors of large land masses.

Topography often creates regional patterns in

temperature and precipitation, creating vastly

different ecosystem types within a small

geographic area. For example, we commonly

see moist, mild climates and tall, green

vegetation on the windward side of mountain

ranges near the ocean, and a harsher, arid

climate with drought-tolerant desert vegetation

on the inland (or leeward) side of mountains. A

moving air mass picks up moisture as it travels

over a body of water. As the air mass moves up

the mountain range, it cools and condenses,

releasing much of its moisture as rain or snow.

The drier air mass then warms as it travels down

the inland side of the mountain range. This

orographic or rain shadow effect is

responsible for the temperate, rainy climates of

the Pacific Northwest on the west side of the

Cascade Mountains, and the arid deserts of the interior Great Basin on the east side. Spatial

patterns in temperature and precipitation drive differences in vegetation (and their associated

animal) communities from the high-latitude tundra to temperate deciduous forests in the mid

latitudes to tropical rainforests at the equator.

Climate is variable in time as well as in space. Daily and seasonal patterns are driven by the

Earth’s rotation on its axis and its revolution around the sun. Longer term variation in climate is

impacted by large-scale cyclic patterns in ocean and atmospheric currents (e.g., El Niño/La Niña

events).


FIGURE 1.7

The best-known fluctuation in high- and low-pressure cells in the Earth’s atmosphere is the El Niño Southern Oscillation (ENSO), in which unusually warm sea surface temperatures occur at the equator.

As Earth orbits the sun, its location,

relative to the sun, changes. This causes

different intensities of sunlight to reach the

Earth’s surface. The patterns of differential

solar radiation create seasonal climate

differences. Seasonal differences in

temperature are more noticeable in the

polar and temperate regions than in the

tropics.

Over time scales of years to decades,

climatic variation is caused by fluctuations

in high- and low-pressure cells in the

Earth’s atmosphere. The best-known of

these fluctuations is the El Niño Southern

Oscillation (ENSO), in which unusually

warm sea surface temperatures occur at the

equator. In North America, El Niño tends

to cause warmer and drier winters in the

Northwest and Midwest, and cooler and

wetter than average winters from Mexico

up to California. La Niña events produce

the opposite patterns, and usually follow El

Niño events. ENSO events are not well

understood, and are difficult to predict, but

generally occur at irregular intervals about

every three to seven years.

Over the past 500 million years, variation in Earth’s orbit has caused many episodes of cooling

and warming. Scientists speculate that these cooling and warming patterns are a result of

changing concentrations of greenhouse gases (atmospheric gases that absorb and reradiate the

earth’s radiation) in the earth’s atmosphere as well as due to variability in Earth’s orbit around

the sun. Clearly, these significant changes in climate over time have had a large impact on the

distributions of plants and animals. For example, although Antarctica is now covered with ice,

and plant and animal life is limited, fossil evidence shows it was once home to a diverse array of

plant and animal life.

Soils

Soil is another important part of the physical environment that helps to determine the distribution

of organisms. Soil is the medium in which plants grow. It provides physical support for plant

growth, assists air, nutrient, and water movement to plant roots, moderates temperature, and

protects from toxins. Soil properties influence the type of vegetation present, and thus, the

number and types of biological organisms that the ecosystem can support.

When we think about the plants that surround us, we often imagine the plant material consisting

of leaves, branches, stems, and flowers. Much of the plant, however, exists below ground. The


FIGURE 1.8

Decomposition, the breakdown of complex organic compounds (i.e., those which make up plants and animals) into simpler ones, adds nutrient materials to the soil.

soil surrounding the roots anchors the plant and

provides support. Pores in the soil ventilate the soil:

roots take up oxygen (O2) and release carbon dioxide

(CO2), a process called respiration. Pores in the soil

also absorb water and hold it where it can later be

used by the plant roots. Plants need water

continuously to transport nutrients, maintain plant

tissues, regulate temperature, and photosynthesize.

Because precipitation is not continuous, it is

important that soils have enough water-holding

capacity to allow plants to survive during dry

periods. Soils also supply plants with the dissolved

mineral nutrients that are necessary for plant survival

and growth. Plants take up these nutrients through

their roots and incorporate them into the plant

tissues. Animals (including humans) then get their

nutrients by eating plant materials.

Soil is formed from the weathering and breaking

down of parent material, the underlying rock or

mineral substrate (usually bedrock or material that

has been transported by wind or water). Mechanical

weathering occurs as parent material starts to

physically break down. Processes such as

freeze/thaw and wet/dry cycles cause parent material to expand and contract, slowly breaking the

rock into smaller pieces over time and increasing the surface area. At the same time, the

influence of water, oxygen, and acids further break down rock materials in a process called

chemical weathering. Decomposition, the breakdown of complex organic compounds (i.e.,

those which make up plants and animals) into simpler ones, adds nutrient materials to the soil.

The processes of soil formation are very slow. It can take many thousands of years to develop a

complex organic soil that is capable of supporting diverse plant and animal communities.

Physical and chemical properties of soils vary. Physical properties such as color, texture,

moisture-holding capacity, and depth allow us to classify and map soils and predict their capacity

to support life. The color of a soil is easy to distinguish and tells us a lot about the composition.

Soils rich in organic matter are very dark. Reddish and yellow-brown soils indicate iron oxides,

manganese oxides give a purple hue, and quartz, gypsum, and calcium and magnesium

carbonates make the soil appear white. Soil particles can be classified into gravel, sand, silt, and

clay.

Soil texture refers to the amounts of different-sized soil particles found in soil. Soils are

considered to be clay, sand, silt, or a mixture of two or more of these types, depending on their

composition. Soil texture plays a major role in the soil’s water-holding capacity and is rated from

fine to coarse. Fine soils with high clay content hold water for much longer than sandy, well-

drained soils. Fine soils have much smaller pore spaces, which impacts the movement of air and

water and decreases the ability of plant roots to penetrate. Soil depth can vary widely across a


FIGURE 1.9

Fine soils with high clay content hold water for much longer than sandy, well-drained soils. Fine soils have

much smaller pore spaces, which impacts the movement of air and water and decreases the ability

of plant roots to penetrate.

FIGURE 1.10

Over time soil becomes diversified with depth from the soil surface, becoming layered with the downward movement of organic material through the soil. This vertical layering is the soil profile, and each layer is referred to as a horizon.

landscape, depending on parent material,

slope, and vegetation type. Mountaintops

tend to have shallow, rocky soils, and level

ground at the base of slopes generally has

much more soil accumulation.

While a soil in a given area originates from

the same parent material, over time it begins

to become varied with depth from the soil

surface. It becomes layered with the

downward movement of organic material

through the soil. We call this vertical

layering the soil profile, and each layer

within the profile is referred to as a horizon.

The surface layer, or O horizon, consists of

organic material that accumulates from

decomposing plant materials such as needles,

twigs, and leaves. Just below this is the A

horizon, more commonly referred to as the

topsoil. While this layer is made up primarily

of weathered parent material, it tends to be

rich in organic materials due to the trickling

of materials downward from the O horizon.

The B horizon, sometimes called the

subsoil, has limited organic matter and often

has accumulated mineral particles due to leaching from

the topsoil. This horizon is usually quite dense, making

it difficult for many plants to extend roots down into

this layer. Finally, the C horizon lies beneath the

subsoil, and is comprised of unconsolidated materials.

There is low biological activity in this layer, and it

retains many characteristics of the parent material.

Below the C horizon is the bedrock or parent material

(sometimes called the R horizon). Very different soils

arise from regional differences in parent material,

climate, and vegetation communities.

Water and Light

Water is a critical component for all organisms, as its

presence in biotic tissues is needed to facilitate all

physiological functioning. The amount of water

present in an ecosystem is an important determinant of

what types of plants and animals can live there. As we

saw earlier, ocean and atmospheric currents largely

drive large-scale precipitation patterns, with regional

influences by topography. In addition, the moisture-


FIGURE 1.11

The conversion of sunlight into carbon compounds by photosynthesis drives the production of energy for use by biological organisms.

holding capacity of an area’s soils impacts

how much of this precipitation is

effectively stored in the ecosystem for use

by biological organisms. Water availability

is also influenced by surface runoff

(overland flow of excess water), downward

percolation through pores in the soil,

evapotranspiration (the sum of the water

lost from evaporation plus transpiration—

the movement of water through a plant),

and groundwater flows. These processes

are the major pathways by which water

flows through an ecosystem. Extreme

conditions of very low or very high water

availability create unique challenges for

organisms that inhabit these areas, and

many have evolved unique strategies to

persist in these harsh environments.

The conversion of sunlight into carbon

compounds by photosynthesis drives the

production of energy for use by biological

organisms. Photosynthesis has two major

steps: the first is harvesting energy from

sunlight and the second is fixing carbon to

generate carbohydrates. The rate of

photosynthesis determines the supply of

energy available to organisms. This

impacts individual growth rates,

reproductive success, and, ultimately, the geographic range of a species. The availability of light,

then, is an important driver of ecosystem productivity.

In aquatic ecosystems, the greatest light availability is at the water surface. There is a rapid

decline of the absorption of light as the depth of the water increases. In terrestrial environments,

light is absorbed by the plants themselves, influencing the amount of light that can penetrate to

the Earth’s surface. In a dense forest, much of the light entering the ecosystem is absorbed by the

highest layers of the forest canopy, which is why it is often very dark in these types of forests,

even in the middle of the day. The amount of sunlight that does penetrate the canopy depends on

the type, quantity, and orientation of branches and leaves.

Seasonal changes in many ecosystems influence the amount of sunlight that is available at the

Earth’s surface. Temperate forests tend to have many deciduous trees, which lose their leaves in

the fall, allowing much more light to penetrate the canopy than in the summer months. In high-

latitude ecosystems, winter months bring much shorter days than summer months, reducing the

amount of time each day that sunlight is available for use by biological organisms. In tropical

ecosystems near the equator, day length, and thus light availability, is fairly constant year round.


Some plants that live in high-salinity environments have roots that do not allow salt uptake, others have the ability to secrete excess salt through special glands on their leaves, and some isolate salt in internal organs.

Other Abiotic Factors: Wind, Salt, pH, Nutrients

Many other abiotic factors can limit or facilitate the successful survival, growth, or ability of an

organism to reproduce in an environment. Wind can cause significant erosion, either by

transporting existing particles, or by wearing down surfaces. Plants in areas prone to high wind

conditions have evolved strategies, such as flexible stems that bend without breaking, succulent

tissues that retain moisture, and narrow leaves (e.g., grasses, needles) and are often small

statured, to avoid the drying effects of high wind.

Salinity (a measure of the dissolved salt content in water) alters properties of water and can limit

an organism’s ability to absorb water. We usually think of the ocean when we talk about salt

water, as the Earth’s oceans contain 97 percent of its water. The salinity of the ocean varies. The

highest salinity in ocean water is found near the equator. Soils adjacent to oceans, such as those

in salt marshes, are similarly very saline. This creates very unique ecosystems that are capable

of persisting in these high-salinity environments. Some plants that live in high-salinity

environments have roots which exclude salt uptake, others have the ability to secrete excess salt

through special glands on their leaves, and some isolate salt in internal organs. Many animals

that live in estuary and tidal marsh habitats will move with the changing tide to maintain their

own salinity requirements. Some fish can drink salt water and excrete the salt through their gills.

Sea birds often excrete excess salt through specialized salt glands in their nasal cavities. Marine

mammals, while they live in high-salinity conditions, get most of the fresh water that they need

to survive from the food that they eat.


FIGURE 1.12

Freshwater and marine ecosystems are linked as components of the hydrologic cycle, the process

by which water travels from the air to the earth and back to the atmosphere.

Nutrients are chemical elements that are required by all organisms for metabolism and growth.

Nutrients must occur in the environment for plants and animals to survive, but nutrient

concentrations vary considerably in different ecosystem types. Increases in some mineral

elements such as aluminum, hydrogen, and manganese can cause increased soil acidity, resulting

in an environment that is toxic to many organisms and sometimes restricts plant growth. Various

nutrient requirements and tolerances to each element determine the biotic life that can live and

thrive in these unique environments. On the other hand, certain necessary nutrients may become

limited, or in short supply, restricting growth of individual plants or preventing some species

from establishing in these limiting types of environments.

Geographic Ecology: Biomes

AQUATIC ENVIRONMENTS

Water covers about 75 percent of the Earth’s surface, which means that aquatic environments are

more widespread than terrestrial environments. The salinity of water greatly influences the

adaptations of organisms that live in these environments. Because of this, aquatic environments

are subdivided into two major groups: saltwater (or marine) and freshwater, each with unique

organisms and ecosystem processes. Freshwater and marine ecosystems are linked as

components of the hydrologic cycle, the process by which water travels from the air to the earth

and back to the atmosphere.

http://water.usgs.gov/edu/watercycle.html


FIGURE 1.13

Rocky intertidal zones lie at the boundary of the ocean and the beach and are influenced by the changing tide and the action of pounding waves.

Oceans cover about 71 percent of Earth’s surface and contain diverse marine ecosystems. Marine

ecosystems are often categorized by their location to shorelines and depth in the water column.

Differences in temperature, surface material, light availability, interactions with other organisms,

and water column pressure determine the distribution of marine organisms.

The area where the river meets the ocean is called an estuary and is characterized by variation in

salinity that fluctuates with the tides. Estuaries are very productive environments and are

important habitats for juvenile fish, shellfish, crabs, and sea grasses. Marshes are formed when

upland sediment is carried downriver and accumulates near ocean shorelines. They are

dominated by grasses, rushes, and forbs that are rooted underwater. Like estuaries, they are very

productive as nutrients are deposited in these areas, and they provide an important habitat for

many types of fish, crabs, birds, and mammals. Mangroves are coastal ecosystems inhabited by

salt-tolerant trees and shrubs. The plant roots trap mud and sediment, which build up and alter

the shoreline. They also are very productive and provide a habitat for many unique species across

the globe, such as monitor lizards, monkeys, manatees, and many fish and bird species.

Rocky intertidal zones (the rocky zone occupying the area between high and low tide) lie at the

boundary of the ocean and the beach and are influenced by the changing tide and the action of

pounding waves. Diverse plant and animal communities are tightly anchored to the rocky surface


FIGURE 1.13

The open ocean, or pelagic zone, is stratified vertically. The benthic zone refers to the ecological zone at the bottom of a body of water. In the ocean, the benthic zone starts in the rocky intertidal area and continues along the ocean floor out to sea.

of these areas to keep from being washed away. Sessile (fixed) organisms (such as mussels) in

these communities must be able to cope with salinity, drying winds, wave action, and

temperature fluctuations. Mobile organisms (such as crabs and sea stars) often move to tide pools

during periods of low tide.

Sandy beaches may appear to have very little life associated with them. The changing tides and

constant wave action limit establishment by plants. However, beneath the sand surface small

organisms such as clams, crustaceans, and polychaete worms survive, protected from the wave

action and temperature and moisture fluctuations. These organisms feed on plankton and detritus

during high tide periods.

Coral reefs are usually found in warm, shallow coastal regions, generally at depths of less than

fifty meters. Corals are an aggregation of live organisms individually referred to as “coral

polyps,” which are animals closely related to jellyfish. Corals build their hard skeleton structure

by extracting calcium carbonate from seawater. These skeletons encase each individual polyp,

and the polyps live on top of the calcium carbonate skeletons of previous polyps, which

accumulate over time, building large structures called reefs. Corals live in close association with

algae. The algae live inside each polyp and provide energy through photosynthesis, while the

corals provide protection from grazing to the algae. The reefs provide structure and protection

for other organisms as well, making coral reefs much more diverse and productive than the open

ocean that surrounds them.

The open ocean, or pelagic zone, is

layered vertically. Light availability

decreases rapidly with depth in the water

column, which leads to rapidly changing

habitats. Throughout the pelagic zone,

the dominant organisms are tiny

phytoplankton (microscopic plants) and

zooplankton (microscopic animals and

juvenile stages of larger animals), with

the highest concentrations of

photosynthetic organisms in the layers

nearest to the water surface. These very

tiny organisms absorb nutrients directly

from the seawater and in turn are a food

source for much larger oceanic

organisms. Deeper in the water column,

light becomes limited, and biota become

few and far between. Small and

microscopic marine crustaceans feed on

the decaying material that falls from

layers above it. Fish, larger crustaceans,

octopuses, and squid are common deep

sea predators.


Freshwater and marine ecosystems are linked as components of the hydrologic cycle, the process by which

water travels from the air to the earth and back to the atmosphere.

The benthic zone refers to the ecological zone at the bottom of a body of water. In the ocean, the

benthic zone starts in the rocky intertidal area and continues along the ocean floor out to sea. The

ocean bottom is sparsely populated by very unique organisms that often are not well studied due

to the difficulties of examining the ocean floor. The base of the benthic food chain is made up of

detritus from dead phytoplankton, marine mammals, birds, fish, and invertebrates. Polychaete

worms and crustaceans are diverse and abundant in these areas. Sea cucumbers and sea stars

graze on the organic matter on the ocean floor or filter food out of the water. Benthic predators

often use bioluminescence (the biochemical emission of visible light by living organisms) to

lure prey.

Freshwater ecosystems make up a small portion of the earth’s surface, but create important

linkages between terrestrial and marine environments. Rivers and streams transport nutrients and

material from terrestrial uplands downhill to the ocean. The smallest and highest elevation

streams are called first-order streams. When two first-order streams merge, they create a

second-order stream. The Amazon River in South America, the world’s largest river by

discharge, is a twelfth-order stream. Each stream has stretches of riffles, fast-moving portions

flowing over coarse surface, and pools, deep, slow-moving stretches with fine sediment. Fish

and other swimming organisms inhabit fast-moving portions of the main channels, and

invertebrates tend to live in or on the river substrate, feeding on dead organic matter.

The environments change from the high-elevation headwater streams to the lowland large rivers.

Headwater streams (orders 1–3) are often cold and well-shaded, and productivity is limited.

Dominant organisms tend to be aquatic insects that shred leaves and other organic matter,


FIGURE 1.14

The smallest and highest elevation streams are called first-order streams. When two first-order streams merge, they create a second-order stream. FIGURE 1.15

Riparian ecosystem is a transition between the aquatic ecosystem and the adjacent terrestrial ecosystem. Riparian ecosystems occur at the boundary of land and water along rivers or other water channels.

breaking it down into small particles that

accumulate on the stream bottom. Medium-

sized creeks and smaller rivers (orders 4–6)

have more surface water exposed to the sun, so

water temperature is higher. As the slope

becomes less steep in the lower elevations, the

current slows down. Vegetation can more

easily establish and persist in these streams,

and there is increased production of algae.

Aquatic larvae feed on organic matter that has

been transported downstream as well as on

algae and rooted plants. Predators shift to

warmer-water species, and bottom-feeding fish

such as catfish and suckers are common. In

streams and rivers of orders 6 and above, the

channel becomes wide and slow-moving, with

increased flow volume. There is increased

sediment accumulation on the substrate, and

bottom-dwelling collectors of detritus are the

primary consumers. Most rivers eventually

meet the ocean, linking inputs from upland

terrestrial ecosystems to the open sea.

Riparian ecosystem is a transition between the

aquatic ecosystem and the adjacent terrestrial

ecosystem. Riparian ecosystems occur at the

boundary of land and water along rivers or

other water channels. Vegetation communities

are generally very different in these ecosystems

than in the adjacent upland areas due to greater

water availability. Riparian areas are very

important ecosystems for water and nutrient

cycling, energy flow, and wildlife habitat.

Freshwater wetlands are places where the land

is covered by shallow water, sometimes

seasonally, as in floodplains, swamps, ponds,

and the edges of rivers and lakes. Wetland soils

are typically saturated with water, and the

vegetation that grows in them is characteristic

of aquatic plants, dominated by flora such as

submerged water plants, floating vegetation,

and cattails, often surrounded by trees and


FIGURE 1.16

This map shows the location (lighter areas) of the major deserts of the world.

shrubs. Many fish, amphibians, reptiles, mammals, and invertebrates are dependent on the

world’s freshwater wetland ecosystems.

TERRISTRIAL ENVIRONMENTS

Terrestrial biomes are classified in terms of temperature and precipitation gradients as well as by

the major plant life form that they contain (trees, grasses, and/or shrubs). They vary

considerably from warm, moist tropical forests to cold, dry polar regions. Plant growth forms are

indicative of the types of environments in which they live, from evergreen, broadleaved trees in

the tropics that are able to photosynthesize year round to cacti and shrubs with succulent (thick,

heavy foliage that can store water) leaves or stems that can store water for long periods of time in

desert regions to deciduous (annual shedding of leaves) forests in temperate regions that have

seasonal patterns of wet and dry. We will briefly discuss several of the major types of terrestrial

biomes in terms of their unique climate, seasonality, plant communities, and physical

characteristics.

Tropical Rainforests Tropical rainforests are found near the equator in areas where rainfall exceeds 2000 mm per year.

There is little seasonality or variability in the constant warm temperatures and persistent

moisture, causing continuous growth by plant species. These are some of the most productive

ecosystems in the world, and they contain a disproportionate amount of the planet’s biological

diversity. Both evergreen and deciduous trees are found in tropical rainforests, and the

availability of light determines the structure of the vegetation. Plant species must either be very


FIGURE 1.17

Temperature and precipitation drive differences in vegetation (and their associated animal) communities.

competitive to use the sunlight in the upper

canopy, or they must adapt to the lower

light environment beneath dense layers of

vegetation.

Tropical Seasonal Forests and Savannas At latitudes nearer the Tropics of Capricorn

(23.5°S) and Cancer (23.5°N), rainfall

becomes much more seasonal, with distinct

wet and dry periods. Vegetation responses

to this seasonality include lower tree

densities, shorter-statured vegetation, loss

of leaves on deciduous trees during the dry

season, and a greater abundance of

understory plants (shrubs, grasses) relative

to rainforests. Frequent fires during the dry

periods and seasonal flooding both

contribute to the development of

woodlands or savannas (vegetation

communities in tropical and subtropical

ecosystems with trees and shrubs

intermixed with a dense grass understory).

Hot Deserts Deserts are sparsely populated with plants and animals, reflecting the harsh conditions of high

temperatures and long periods without precipitation. Many desert plants are succulent, allowing

them to store water in their tissues for long periods of time so that the plant can survive the

period of low water availability. Some desert annuals carry out their entire life cycle in just a

couple of weeks. After a period of precipitation, they germinate, flower, produce seeds, and then

die. The major deserts of the world include the Sahara, the Arabian deserts, the Atacama Desert,

the Chihuahuan Desert, the Sonoran Desert, and the Mojave Desert.

Temperate Grasslands Large expanses of grassland exist in both the Northern and Southern hemispheres at temperate

latitudes. These areas have great seasonal variability in temperature, with long periods of

freezing temperatures during a cold, dry winter, and warm, moist summers. Fire and grazing by

large herbivores are important natural disturbance processes in these ecosystems, limiting the

establishment of woody tree and shrub species and favoring an ecosystem dominated by grasses

and small herbs.

Temperate Shrublands and Woodlands Shrublands and woodlands develop in temperate regions with a winter rainy season. The

vegetation in these ecosystems tends to be small statured, with thick, stiff evergreen leaves. They

are generally well adapted to long dry periods and will slowly grow and photosynthesize in a

moisture-limited environment. Some coastal temperate shrublands include the fynbos of

Australia and the chaparral of North America. Inland shrublands and woodlands areas are

associated with the seasonally cold climates and often fall in the rain shadow of mountain ranges.


Temperate deciduous forests occur where cold periods are prolonged enough to make ongoing photosynthesis inefficient, but where the growing season is long enough and the soils nutrient rich

enough to make regrowth possible in the spring.

The Great Basin of North America sits between the Cascade/Sierra Nevada crest to the west and

the Rocky Mountains to the east and is characterized by shrubs and infrequent, small trees.

Temperate Deciduous Forests Temperate deciduous forests occur where cold periods are prolonged enough to make ongoing photosynthesis inefficient, but where the growing season is long enough and the soils nutrient

rich enough to make regrowth possible in the spring. These forests occur in eastern North

America and on both the eastern and western edges of Eurasia. There are multiple vertical layers

to these forests, with a subcanopy of trees as well as shrubs and forbs below the upper canopy.

Temperate Evergreen Forests There is quite a bit of variability in the environmental conditions that support evergreen forest

growth in the temperate regions, from warm coastal areas to cool inland ecosystems. At the high

precipitation end of the spectrum, these forests are sometimes referred to as temperate

rainforests, and at the low precipitation end, drier forest types support frequent fire return

intervals of ten to twenty years, which promote the persistence of the species present. Soils in

temperate evergreen forests tend to be nutrient poor, in part because of the acidity in the leaves

that becomes incorporated into the soil profile. The diversity of these forests is usually lower

than that of either deciduous or tropical forest types. Dominant tree species in North America are

needle-leaved conifers such as pines, firs, hemlocks, and junipers.


Boreal forests are primarily composed of coniferous species that resist damage from hard winter freezes.

The tundra in Greenland. The tundra is found in the Arctic as well as at the edges of the Antarctic

Peninsula.

Boreal Forests Continuous temperatures below freezing for up to six months are common in continental forests

above 50° latitude, presenting significant challenges for vegetation persistence. The air

temperatures can reach –50°C, and the soils may freeze for long periods of time, obstructing

water drainage. The forests in these regions are primarily composed of coniferous species that

resist damage from hard winter freezes. Decomposition of plant material is very slow due to the

cold temperatures, and the rate of plant growth exceeds that of decomposition, leading to a large

buildup of organic material on the forest floor. During seasonal summer droughts, fires can burn

slowly in this organic layer for many months or even sometimes years.

Tundra Above about 65° latitude, trees no longer dominate vegetation structure. In the tundra, there is a

permanently frozen subsurface soil layer called permafrost. Above the permafrost, the soil

thaws each summer and is frozen each winter. Permafrost keeps this soil layer chilled, even

during the growing season, limiting plant growth, microbial activity, and nutrient accumulation.

Low statured sedges, grasses, forbs, and shrubs persist, and mosses and lichens make up an

important component of the ecosystem. Vegetation development is simplified, normally with few

species and short-statured growth forms. The tundra is found in the Arctic as well as at the edges

of the Antarctic Peninsula.


The Hawaiian hoary bat (Lasiurus cinereus) is the only species of terrestrial mammal native to the Hawaiian Islands. No other mammals have been able to cross the Pacific Ocean to disperse to the

Hawaiian Islands on their own.

THE ORGANISM AND ITS ENVIRONMENT

As we have seen, environment largely determines which species can live and successfully

reproduce in the unique abiotic conditions of an area. A range is the geographical area where a

particular species can be found. A species’ physical and behavioral characteristics reflect its

adaptation to a particular environment. Different environments provide a differing set of

challenges, and the set of characteristics that make a species well adapted to one environment

often limit its success in another. The diversity of life represents many evolutionary strategies

that allow different organisms to successfully survive, grow, and reproduce in their respective

environments.

Limits to Dispersal: Physical and Chemical Limits

All species have a niche, a set of optimal environmental conditions that are most conducive to

their successful photosynthesis, growth, survival, and reproduction and that define the way that

the species fits into an ecological community or ecosystem. This ecological niche is a result of

natural selection, as a species’ composition and behavior become ever more adapted to its

environment. As conditions move away from this set of optimal conditions, rates of these

processes decrease. Stress, then, is an environmental condition (which may be temporary) that

limits biological processes, lowering an organism’s rate of growth, survival, or reproduction. At

the limits of a population’s range, the physical environment limits an organism’s ability to obtain

the resources needed to grow and reproduce, thereby limiting the population within a fixed

geographical area (fundamental niche). Extreme conditions such as temperature or water stress


(e.g., flooding, drought) or the presence of toxic chemicals can surpass an organism’s tolerance

levels, even within its normal geographic range.

Limits to Dispersal: Biotic Limits

Biotic interactions with other organisms can further limit the geographic distribution of a species

(realized niche). Competition between two or more species, herbivory, or predation may limit a

species’ distribution, even under optimal abiotic conditions. Similarly, two species may require

that they have overlapping distributions. For example, some pollinators visit specific flowers in

search of pollen and nectar and in the process pollinate the flowers. Both the plants containing

the flowers and the pollinators are limited by the range of the other organism. Organisms can

also be excluded from an area due to pathogens or parasites.

Adaptation and Natural Selection

Differential success is the idea that those organisms best adapted to a given environment will be

most likely to survive to reproductive age and have offspring of their own. Organisms that are

successful in their environments will be more likely to be successful in reproduction, and

therefore the better-adapted organisms will reproduce at a greater rate than the less well-adapted

organisms. The differential success of individuals within a population of organisms occurs when

individuals with particular heritable traits consistently have more offspring than individuals with

other heritable traits. For example, more favorable traits such as camouflage, rapid movement, or

large size might allow an organism to escape predation, thus living longer and having more

opportunities to reproduce. This process of natural selection is the key mechanism driving the

evolution (directional change) in biological populations over time. Evolution occurs as a species

gradually becomes increasingly different from its ancestors and better adapted to its (often

changing) environment.

Evolution is often described in terms of genetic change. The genes in all living organisms are

composed of deoxyribonucleic acid (DNA), and they specify how to build the proteins that are

the building blocks of biotic life. Any given gene can have two or more different forms (called

alleles) that result in the production of different versions of the protein. During sexual

reproduction, a single copy of each gene is inherited from an organism’s mother and another

from its father. Multiple combinations of alleles together designate an organism’s genetic

makeup, or genotype. The physical manifestation of that genetic makeup is called the

phenotype.

Over time, the frequencies of alleles in a population can shift toward phenotypes that make a

population better adapted to its environment. Natural selection serves to sort individuals in a

population, favoring those that have more favorable heritable traits over others. In The Origin of

Species, Charles Darwin outlined that natural selection is a product of two major conditions: 1)

variation in some heritable characteristic occurs among individuals within a population, and 2)

this variation results in differential survival and reproductive rates among individuals.

Population Dynamics

A population is a group of individuals of the same species that live in the same geographical area

at the same time and interact with one another. The relationships between individuals in space

and time determine, in part, the distribution of the species. These population dynamics


determine the population size (the number of individuals in the population) and/or abundance

(the number of individuals in a given area).

TABLE 1: Factors that Limit Populations

Factors that cause a population to

increase

Factors that cause a population to

decrease

Abiotic

Favorable light

Favorable temperature

Favorable chemical environment

Too much or too little light

Too cold or too warm

Unfavorable chemical environment

Biotic

Sufficient food

Low number or low effectiveness of

predators

Few or weak diseases and parasites

Ability to compete for resources

Insufficient food

High number or high effectiveness of

predators

Many or strong diseases and parasites

Inability to successfully compete for

resources

Species Interactions

Biotic interactions can occur between members of the same species (intraspecific interactions)

or between two different types of organisms (interspecific interactions). In the framework of

population dynamics, we will discuss these processes as they relate to interactions between two

or more members of the same species, but it is important to note that the terminology and

processes are the same in the framework of community ecology, when multiple species interact

(and which will be discussed in the next section).

When resources (for example, nutrients, light, water, and food) are insufficient to satisfy all of

the organisms that depend on them, they are distributed at a disadvantage to at least some of the

individuals in the ecosystem. In some cases, a resource limitation results in a reduction of that

resource to all organisms equally, often resulting in a decline of the species and sometimes local

extinction. More commonly, resource limitation results in some dominant individuals continuing

to utilize the resources that are needed to the disadvantage of other individuals, negatively

impacting growth and development, and even leading to mortality. This intraspecific competition

in animals can result in limiting reproduction, altering behavior patterns, and increasing the

likelihood of disease and parasites. In plants, competition often results in reduced growth and

seed production.


Section I Summary Ecology is the scientific study of the relationships between organisms and their environment.

Natural selection is the process by which individuals with better adapted heritable

characteristics tend to survive and reproduce more successfully than other individuals.

Ecological interactions take place on multiple scales, from the individual organism to the

entire planet. A population is a group of individuals of the same species that live in a

particular area and interacts with each other. Populations of plants and animals interact with

one another in many ways, including competing for limiting resources. Two or more

interacting populations of plants and animals of different species in the same area are referred

to as communities. An ecosystem is a community of organisms and the physical environment

in which they live. Landscapes are patchworks of multiple communities and ecosystems and

are typically studied at larger spatial scales.

Broad-scale patterns of climate and geology allow us to categorize similar landscapes and

ecosystems into biomes—large-scale regions dominated by similar types of ecosystems.

The scientific method is used by scientists to answer ecological research questions. All

scientific studies begin with observations that lead to questions about the processes driving

the phenomena observed. A hypothesis is then made about what is driving the observation.

Data are then collected and analyzed, and the hypothesis is revisited. When a greater

understanding is gained from an ecosystem as the result of the testing of multiple related

hypotheses, a theory is formed to collectively explain the results of a large number of

experimental observations.

The physical environment sets the ultimate bounds of an organism’s distribution. Long-term

trends in temperature, wind patterns, and precipitation, patterns of salinity or acidity, and soil

properties are among the important determinants that limit where an organism can survive,

grow, and reproduce. The environment determines whether a particular area will be a tropical

rainforest, a montane lake, or a temperate grassland.

Climate is the long-term average weather pattern for an area that places constraints on the

type of biological life that can survive in the area. Within a climatic regime, topography often

creates regional patterns in temperature and precipitation, creating vastly different ecosystem

types within a small geographic area.

Soil is an important component of the physical environment that helps to determine the

distribution of biological organisms. Soil provides physical support for plant growth,

facilitates air, nutrient, and water movement to plant roots, moderates temperature, and

protects plants from toxins. Soil properties help to determine the type of vegetation present

and the number and types of biological organisms that the ecosystem can support.


Water is a critical component for all organisms, as its presence in biotic tissues is needed to

facilitate all physiological functioning. The amount of water present in an ecosystem is an

important determinant of what types of plants and animals can live there.

The conversion of sunlight into chemical energy by photosynthesis drives the production of

energy for use by biological organisms. Photosynthesis has two major steps: harvesting

energy from sunlight and fixing carbon to generate carbohydrates.

Water covers about 75 percent of the Earth’s surface, making it the planet’s dominant

environment. The salinity of water greatly influences the adaptations of organisms that live in

these environments. Because of this, aquatic environments are subdivided into two major

groups: saltwater (or marine) and freshwater, each with unique organisms and ecosystem

processes. Freshwater and marine ecosystems are linked as components of the hydrologic

cycle, the process by which water travels from the air to the Earth and back to the

atmosphere.

Terrestrial biomes are classified in terms of temperature and precipitation gradients as well as

by the major plant life form that they contain—trees, grasses, and/or shrubs.

A range is the geographical area in which a particular species can be found. Within that

range, individuals of that species are dispersed depending on resource availability and

physical and biological constraints to individual establishment or reproduction.

There are many ways that organisms of the same species interact with each other in the same

geographical area, constraining the number of organisms in a population. These population

dynamics determine the population size (the number of individuals in the population) and or

abundance (the number of individuals in a given area). Within populations, the dispersion of

individuals depends on the availability of resources, dispersal, and biotic interactions.


SECTION II:

COMMUNITY ECOLOGY

In any ecosystem or habitat, a community is a unique collection of plants, animals, bacteria, and

fungi that interact with one another and with their environment in the same place and at the same

time. The community’s physical environment is usually loosely defined as a limited geographical

region. For example, a community might be all of the organisms that live within a particular

lake, sand dune, forest, or desert. These organisms may compete with one another for limiting

resources such as light, nutrients, food, and water, may rely on each other for food, or may

interact in a way that is mutually beneficial. Collectively, these interspecific interactions form

the field of ecology known as community ecology.

Community ecologists seek to understand how the number of species, their spatial arrangements,

and the interactions among them form the structure of the ecosystem. A simple measure of this

structure is the number of different species occurring in a defined geographical area, referred to

as species richness. While richness gives us an idea of the complexity of a community, there are

not equal numbers of each species represented. The evenness is the percentage that the

individuals of each species contribute to the total number of organisms of all species present, and

it gives us an idea of the “rareness” or “commonness” of any given species of interest. Species

diversity combines the species richness and evenness measures to give a measure of the

variability and variety of living organisms in an ecosystem.

TABLE 2: Species Richness and Evenness Two communities with the same number of species and total organisms can be divided in numerous ways. Consider the following

example of two simplistic ecological communities. Each community has only two species present, so the species richness is the

same. However, community 1 is much more homogeneous. In contrast, community 2 has a much greater evenness among the

numbers of species present.

TABLE 2:Species Richness and Evenness

Community 1 Community 2

Species 1 97 individuals 25 individuals

Species 2 1 individuals 25 individuals

Species 3 1 individual 25 individuals

Species 4 1 individual 25 individuals


FIGURE 2.1

The term biodiversity can refer to variability at multiple scales, from genes to species to ecosystems.

BIODIVERSITY

Types of Biodiversity

Species diversity is one type of biodiversity, the variation of life. While we generally think of

biodiversity and species diversity as being synonymous, it is important to recognize that the term

biodiversity can refer to variability at multiple scales, from genes to species to ecosystems.

Genetic diversity is the variation in alleles present in a population that results in individual

differences in appearance, function, and behavior. These genetic differences among individuals

within a species increase the chance that the species will continue, as individuals differ in their

ability to grow, reproduce, and continue through multiple changing environmental conditions.

Species diversity tells us the variety of species that are found in an ecological community.

Ecosystem diversity refers to the different ecological communities that are found within a fixed

area. For the purposes of this discussion, we will be referring to biodiversity as the diversity of

species found within an ecological community.

Global Patterns of Biodiversity

Globally, scientists have estimated that there may be somewhere between 5 and 30 million

different species, although these numbers are just guesses, as only about 1.7 million species have

been identified and described.

However, these species are not

evenly distributed across the

planet. There is a huge amount

of variety of life from the very

equatorial tropics to the

species-poor polar regions.

In North America, there is a

trend of greater variety of tree

species in the east, and less

variety in the west, with a

hotspot of tree diversity in the

U.S. southern states. In

contrast, over the same area,

there is a trend of greater bird

diversity in the west, and fewer

species in the eastern part of the

continent. Small island

ecosystems have lower

biodiversity than large islands

or mainland ecosystems, with

very distant islands like the

Hawaiian archipelago being

relatively species poor.

Additionally, there are places of

high biodiversity that have been


greatly threatened by human activity, called biodiversity hotspots. There are currently thirty-

five areas that have been designated as biodiversity hotspots on the planet, and they are often a

focus of biological conservation efforts.

The sub-discipline of community ecology seeks to answer questions about why these

distributions of diversity exist such as the following: Why are there more species in the tropics

than in the temperate regions? Why do we usually find a few dominant species and many less

common species in any one ecosystem? Why do woody species typically replace grasses and

herbs over time? As we start to understand more about an ecosystem, it becomes possible to start

to answer some of these questions.

Causes of Biodiversity

What drives these patterns of biodiversity? Clearly this is a difficult question with multiple

theories that explain these patterns. Some hypotheses for the observed patterns of diversity are

as follows:

1. The evolutionary speed hypothesis says that there are more species in some areas (like the

tropics) because speciation (the formation of new and distinct species by evolution) happens

faster in these areas or has been happening longer. This can occur if increased temperature

increases the rate of speciation, allowing species to diversify more rapidly than in more

temperate regions. Places with a long evolutionary history (like many places in the tropics)

will be more diverse than places with a shorter evolutionary history (for example, the

Hawaiian Islands, which are geologically very young, very distant from the mainland, and

have low species richness).

Biodiversity Hotspots of India

There are two biodiversity hotspots in India: the Eastern Himalaya hotspot and the Western Ghat

hotspot. The Eastern Himalaya hotspot contains the northwestern and northeastern states of India

as well as northern Pakistan, Nepal, and Bhutan, and includes Mt. Everest, the world’s tallest

mountain. Because these mountains rise very abruptly, they contain many unique ecosystems in

a relatively small area, from grasslands to subtropical forests to alpine meadows. There are an

estimated 10,000 plant species, almost a thousand bird species, and about three hundred mammal

species. Logging for agriculture, livestock grazing, and settlement has fragmented habitats in the

area, degraded many ecosystems, and caused large-scale erosion on steep slopes.

The Western Ghat hotspot spreads across six Indian states, encompassing a mountain region that

parallels the country’s western coast. The Western Ghat hotspot contains an estimated five

thousand plant species, over five hundred bird and one hundred mammal species, 179 amphibian

species, and 288 freshwater fish species. Clearcutting for tea, coffee, and teak plantations has

significantly fragmented habitats and threatened the biodiversity in this region.

http://www.thinktheearth.net/earthrium/18bdhotspots/images/map_s.jpg

http://www.kerenvis.nic.in/WriteReadData/userfiles/image/hotspot%20Map.jpg


2. The geographic area hypothesis says that large areas have more diversity than smaller land

areas due to increased room for speciation and buffering against extinction. Larger areas have

more diversity of habitats that support a greater variety of species. Since the tropics contain a

much larger area than temperate zones, they have more species per unit area.

3. The interspecific interactions hypothesis says that the high species diversity in the tropics

is associated with greater competition and higher predation rates than are found in other

regions. In higher latitude regions, where physical conditions are generally more severe than

in tropical regions, abiotic factors (for example, extreme ranges in temperature associated

with seasons) mainly control natural selection. The species most adapted to the severe

seasonal extremes continues. In milder tropical regions, abiotic conditions are generally more

favorable for life; thus, predation and competition are bigger drivers of natural selection in

those areas. In areas with intense competition, natural selection should lead to highly

specialized species niches (the specific habitat requirements and functional role of an

organism in its community).

4. Climate is determined by the solar radiation, water, and temperature of a region, resulting in

varying energy availability in different regions of the globe. The ambient energy hypothesis

says that where there is more energy, there will be greater biodiversity. Global patterns of

biodiversity generally support this hypothesis, as we see higher biodiversity in areas with

climates that are more favorable for plant growth.

5. The productivity hypothesis says that greater production (the rate of generation of biomass

in an ecosystem) should result in greater biological diversity. This hypothesis, however,

seems to be disputed by most available data. Some of the world’s most diverse plant

communities, the fynbos in South Africa and the heath scrublands of Australia, for example,

are fairly low productivity sites. These communities, however, are near sites that occur on

better soils and have more productive vegetation, but are characterized by lower species

diversity.

6. Finally, the intermediate disturbance hypothesis describes smaller scale patterns of

biological diversity. This hypothesis says that in the absence of disturbance (the disruption of

an ecosystem, community, or population that changes the surface and resource availability,

and/or the physical environment), the few most competitive species should eliminate less

dominant competing species. The disturbance should result in a loss in species diversity.

However, when there are small periodic disturbances, such as herbivory, disease, flooding, or

fires, these disturbances open up areas of habitat. The absence of dominant species leads to

greater overall biological diversity.

These factors interact in complex ways to contribute to the patterns of biodiversity that we see

across the globe, and there are many exceptions to these patterns. A community ecologist’s task

is to describe and explain these interactions for a particular ecosystem being studied.


TABLE 3: Causal Factors Driving Patterns of Biodiversity

(adapted from Currie 1991, Krebs 2008, and Willig et. al 2003)

Factor Reason

Evolutionary Speed More time and faster evolution leads to the evolution of new

species

Geographical Area Large, complex habitats provide more evolutionary niches

Interspecific Interactions Competition leads to resource partitioning (dividing a niche), and

predation allows similar species to coexist

Ambient energy Fewer species can tolerate climate extremes

Productivity Richness is limited by the division of energy among species

Disturbance Moderate disturbance slows competitive exclusion

Interspecific Interactions

We will look at the interactions between two or more individuals of different species of in greater

detail here. The different types of interactions are divided into what we will call negative species

interactions and positive species interactions. It is important to note that this is not synonymous

with something ecologically “good” or “bad.” Negative species interactions refer to all

interactions where one of the interacting species benefits from the relationship, and the other

species suffers, or where both species suffer from the association. In contrast, positive species

interactions occur when one or both (or all) species benefit from the association, but no species is

negatively impacted. Additionally, two or more species can coexist in an area but not interact by

utilizing different resources.

NEGATIVE SPECIES INTERACTIONS

Predation We can all picture a mountain lion preying on a deer, or a hawk on a field mouse, but perhaps

not everyone has considered that a Venus flytrap consuming an insect or a ladybug consuming

an aphid are also examples of predation. Predation occurs when one species (a predator) kills

and eats another organism, its prey. In this relationship, one species benefits, and the other is

harmed. Predation can have a strong influence on the population sizes of both predator and prey

species. Most commonly, increasing the number of prey species will result in an increase in the

numbers of the predator as well because there is an increase in its food source. On the other

hand, if predator numbers increase, the result is the decline in prey species as they are consumed.

And, when those prey species decline, eventually the predator abundance will decline due to

limited food supplies. Because it takes time for the population of one species to change in


The process of herbivory differs from predation on animals in that herbivory

usually does not kill the plant, rather just removes part of it.

response to the other species population change, we often see predator-prey cycles, or regularly

spaced intervals of increases and decreases in the population sizes of the predator and prey.

Herbivory Animals such as grazers (which eat herbaceous plants)

and browsers (which eat woody plants) are herbivores,

animals that consume part of living plants or algae, often

feeding on many individuals at a time. The process of

herbivory differs from predation on animals in that

herbivory usually does not kill the plant, rather just

removes part of it. The removal of plant tissue may

impact the individual plant’s ability to survive or put it

at a competitive disadvantage with the surrounding

vegetation. Since most plants cannot move to escape

herbivory, many have evolved other strategies to protect

their tissues from herbivory. Some plants have structural

defenses, such as thorns, hairs, or spines to discourage

consumption, and others have chemicals to deter

animals or insects from consuming them. Both predation

and herbivory can have profound effects on the

distribution and abundance of their prey species, often

altering the structure and composition of ecological

communities and even sometimes causing shifts in

community types.


Interspecific competition occurs when there is a limiting resource that both species require. Lions and hyenas occupy the same ecological niche and compete for the same prey.

Competition Interspecific competition negatively impacts two or more species. Interspecific competition

occurs when there is a limiting resource that both species require, such as water, nutrients, or

light, that is in short supply. There are two major types of competition: interference competition

and exploitation competition.

Interference competition occurs when one species actively attempts to exclude another species,

as when a blue jay chases other birds from a birdfeeder. Exploitation competition occurs when

one species more efficiently utilizes a limiting resource than another species does. Some

common examples of exploitation competition include lion and cheetah populations competing

for gazelles at the same time, or two species of shrubs competing for soil nutrients and water.

Interspecific competition, particularly exploitation competition, can alter the structure of the

community and can temporarily or sometimes permanently impact the availability of the limiting

resource.

Parasitism At least half of the species on our planet are symbionts, or organisms that live in conjunction with another organism. The majority of these relationships are parasitic, where the parasite consumes

the tissues or robs the resources of the host organism. In some cases, the parasite is a pathogen,


Clownfish are protected from predator fish by the stinging tentacles of the sea anemone. In return, the clownfish chases away polyp-eating fish and fertilizes the anemone.

causing disease in the host species. There are many common examples of parasites impacting a

human host such as head and body lice, tapeworms, scabies, and ticks. Parasitic species that live

on the surface of their host are called ectoparasites. Many fungal and insect ectoparasites attack

crop and horticultural species, causing significant amounts of damage. Athlete’s foot, mites,

fleas, and ticks are commonly found living on the surface tissues of animals.

Endoparasites are parasitic species that live inside their host organism and feed on the host

organism or rob the host of nutrients. Many human bacterial diseases are the result of parasites,

including tuberculosis, which is caused by the species Myobacterium tuberculosis, and the

bubonic plague, caused by the bacterium Yersinia pestis. Endoparasites are mostly protected

from the external environment by their host species. Although they are provided with a constant

source of food (usually the host’s tissue), they are at constant risk from attack by the host organism’s immune system. Both types of parasitism and any accompanying disease can greatly impact the ecological

function of a species. Parasites can serve as population regulation controls, removing the weakest

members in a population, and can weaken the defenses of an organism such that it loses a

competitive advantage or can make an organism, population, or species more vulnerable to

predation or additional disease.


A common obligate mutualistic relationship is the interaction between a fungus and algae that form a lichen.

Amensalism A relationship between organisms of two different species in which one is unaffected and the

other is negatively impacted by the association is called amensalism. Amensalisms can occur

either through competition, when a larger or stronger organism eliminates a smaller or weaker

organism from obtaining necessary resources, or by antibiosis, when one organism is damaged

or killed by a chemical that the other organism secretes. An example of antibiosis is the black

walnut tree, which secretes a toxin called juglone that injures or kills many other plants that grow

in the root zone of the tree.

POSITIVE SPECIES INTERACTIONS

So far we have been discussing interactions in which one or more species are negatively

impacted by another; however, other symbiotic relationships have evolved such that one species

is positively impacted by the other, or even both species are positively affected by the

relationship.

Mutualism

A relationship between members of two species in

which both members benefit from the association is

known as mutualism. We can find examples of these

relationships in plants and animals (including humans!).

There are two major types of mutualism. One type is

known as obligatory mutualism, where one species

cannot survive without the other species. A common

obligate mutualistic relationship is the interaction

between a fungus and algae that form a lichen. The

fungus would not survive without its algal partner,

which makes food for the fungus. The fungus in turn

provides protection for the algae and obtains nutrients

and water for the lichen. The second type of mutualism

is facultative relationships, where the organisms both

benefit from being together, but it is not required for

their survival. One of the most commonly used

examples demonstrating a facultative mutualistic

relationship is that of the clownfish and the sea

anemone. Clownfish are protected from predator fish by

the stinging tentacles of the sea anemone. In return, the clownfish chases away polyp-eating fish

and fertilizes the anemone.

Commensalism

A relationship between two organisms of different species in which one organism benefits from

the association and the other is neither benefitted nor harmed is known as commensalism. The

relationship between cattle egrets and cattle (or other domestic livestock) is one of the most

frequently used examples: Cattle egrets forage for insects and other prey items in fields where

cattle are grazing. As cattle move around, they stir up the insects, making it easier for the cattle

egrets to capture their prey. The cattle egrets benefit from this relationship, but the cows are not

impacted either positively or negatively.


Commensalism in India: The Trickster in Folklore and in Nature Indian folklore tells the story of a hungry jackal that went out one night in search of food in a

local village near his jungle home. A pack of village dogs chased the jackal away, and he fell

into a tub of indigo dye, turning himself blue. The blue jackal continued into the jungle and

encountered a lion, the King of the Jungle. The jackal introduced himself as Chandru, the

protector of all jungle animals. He told the lion that he would only protect the jungle in exchange

for food and shelter. For a time, all of the jungle animals brought

him the best of food, but when the monsoon rains came, they

washed all of the blue dye off of Chandru, and the animals saw that

they had been tricked and that he was just a simple jackal. The

animals chased the trickster into the jungle, and he was never seen

again.

As in the story, in India, the golden jackal functions as a bit of a

trickster in the commensal relationship it shares with the tiger. The

jackal often lives just outside of towns and villages and only is active at night. Lone jackals are

scavengers and will trail a single tiger and feed on the remains of the tiger’s prey. The tigers

generally ignore the jackals and neither benefit nor are negatively impacted by the jackal’s

presence.

COMMUNITY ORGANIZATION AND STRUCTURE The processes of predation, herbivory, parasitism, mutualism, and commensalism are strategies

that an individual organism uses to acquire the resources that it needs for metabolism, growth,

and reproduction. All of the interactions among species in an ecological community serve to

provide order to the community. In this section, we will look at some of the ways that a

community is structured by transferring this energy among the organisms in the ecological

community.

Trophic Cascades

We can look at the way that energy flows through an ecosystem by two major pathways. Energy

flow may be controlled by the rate at which energy enters an ecosystem and is converted to

organic matter (“bottom-up” control) or can be controlled by the rate at which top predators in an

ecosystem consume organic matter, influencing the composition and abundance of organisms

lower on the food chain (“top-down” control). Changes in the distribution or abundance of

organisms at one trophic level can have profound impacts on organisms at other trophic levels.

The reintroduction of wolves (the top predator) to Yellowstone National Park in 1995 allowed

researchers to study the trophic cascade involving wolves, elk, and aspen. In the absence of

wolves (before 1995), elk population numbers in the park had been at very high levels, resulting

in high herbivory of the aspen stands. When wolves were released back into the park, they began

to regulate numbers of elk, thus releasing some of the browsing pressure on the aspen.2


FIGURE 2.3

Trophic levels of a terrestrial and an aquatic food web.

The complex, interwoven set of relationships between trophic levels within an ecosystem is

referred to as a food web.

Food Webs

The example just discussed described one set

of relationships between aspen, elk, and

wolves, but in reality, the ecosystem is much

more complex. There are other plants that elk

browse on, such as willows and cottonwoods,

other animals that browse on aspen (deer), and

other animals that wolves prey upon (moose,

deer). This more complex, interwoven set of

relationships between trophic levels within an

ecosystem is referred to as a food web. The

species in a food web are either a basal species,

an intermediate species, or a top predator. A

basal species is one that does not feed upon

any other species but is eaten by other species.

These species include plants and algae. An

intermediate species is both a food source for

others as well as one that eats other organisms,

such as herbivores. A top predator eats other

species but is not a food source for any other

species. In the Yellowstone example, the

aspen, willows, and cottonwoods are basal

species, the elk, deer, and moose are

intermediate species, and the wolves are the

top predators.

Interactions between species in a food web are

often more complex than simply the predator-

prey relationship. When a predator consumes

one species of prey, this may alter the

competitive relationships between that type

of prey and other associated species. These

indirect effects of predation may be very

important to ecosystem dynamics and should

be considered when trying to understand the

complex interactions at play.

Keystone Species

Some species fill roles in a community that

are critically important to the functioning of

the community. These keystone species

largely determine ecosystem structure, and in

their absence the ecosystem would

dramatically change.3 Keystone species are

often not particularly common in the system,

but their impact on system structure and


Sea stars were one of the first organisms to be recognized as a keystone species.

function is disproportionately large. Sea stars were one of the first organisms to be recognized as

a keystone species. They are critical to the survival of many other species in the intertidal zone

because they are predators of the mussels and barnacles that would otherwise eat most of the

organisms in the tide pools.

DISTURBANCES

In the past several years, we have seen unusually high numbers of large wildfires in the western

United States. Periods of prolonged drought, a legacy of suppression of fire, and a warming and

drying climate have brought large fires to the forefront of the media. Because of the negative

attention that these extreme fire events are getting in recent years, it is easy to forget that fire and

other disturbances are natural parts of the way that ecosystems function and are often required

for the persistence of some species in an ecological community. More generally, a disturbance

is a discrete event in time that disrupts ecosystem, community, or population structure and

changes substrate and resource availability as well as changes the physical environment.

Types of Disturbances

Disturbance can be natural or anthropogenic (human-caused) and is either exogenous

(originating outside of the ecosystem), such as fire, drought, or wind, or endogenous (originating

inside the ecosystem), such as native diseases and pathogens. Fire is probably what most of us

think of first when we mention disturbance, but wind, flooding, insect herbivory, disease,

earthquakes, and volcanic eruptions are also common natural disturbance processes. In addition,

humans can initiate disturbances that have dramatic impacts on natural ecosystem functioning.

Some that are particularly destructive include logging, industrial pollution, livestock grazing,

introduction of invasive species, and conversion of land for urban areas or agriculture.


A ponderosa pine ecosystem is characterized as having a frequent, low intensity fire regime.

Measures of Disturbance

We can characterize a disturbance by both spatial and temporal components including size, type

(e.g., flood, fire), frequency (how often the disturbance returns; e.g., 100-year flood, 30-year fire

return interval), intensity (degree of impact to the community; e.g., high-intensity fire),

seasonality (e.g., summer wildfire, winter floods), etc. The disturbance system, or regime,

describes the way that these components combine to create the natural patterns of disturbance in

a particular ecological community. For example, a ponderosa pine ecosystem is characterized as

having a frequent, low-intensity fire regime. In this ecosystem, fire typically returns about every

ten to twenty years and consumes only the understory vegetation, leaving the trees intact. These

low-intensity surface fires maintain the ponderosa pine forests in an open park-like ecosystem,

with large canopy trees, little understory vegetation, and a litter layer of pine needles.

Adaptations to Disturbance

Plants and animals that share a long evolutionary history with the disturbance regime of their

ecosystem have adaptations to survive or regenerate following a disturbance event. Plants are

either adapted to survive a disturbance event, or to quickly colonize after the disturbance has

passed. As we just discussed, ponderosa pine are adapted to survive low-intensity surface fire.

They have very thick bark, needles with high moisture content, and insulated buds, allowing

them to persist though repeated fire events. Many cacti have evolved to sustain periods of

prolonged drought. Their tissues are capable of storing water for long periods of time, and they


The seeds of the ohia tree are very small and can travel by wind very long distances and germinate in small cracks in the bare lava where pockets of moisture are trapped.

have very shallow roots that enable them to utilize very

small amounts of precipitation. Other plants can quickly

germinate after a disturbance. The native Hawaiian Ohia tree

is one of the first species to colonize the new lava flows on

the active Kilauea volcano. The seeds of this tree are very

small and can travel by wind very long distances and

germinate in small cracks in the bare lava where pockets of

moisture are trapped.

Animals similarly can survive the disturbance regime of an

area by utilizing one or more mechanisms. Many animals

simply flee the disturbance area, by moving away from a

burn area or upland from a flood zone. Burrowing animals

can survive a fire, windfall, or tornado by staying beneath a

protective layer of soil. Lungfish can survive periods of

drought by burrowing in the mud and secreting a mucus coat

that hardens and protects them until there is water available

again.

Stability and Resilience

The adaptations that plants and animals have evolved that

allow them to persist in areas where disturbance is a natural

part of the ecosystem contribute to the stability of the

biological system. Stability is the ability of an ecosystem to recover after a disturbance. It is

usually thought that increased biodiversity leads to increased stability (though there are

exceptions), and thus stability is often measured in terms of numbers or biomass. However, it can

also be measured by an ecosystem’s resistance to change, or resilience. A resilient community is

capable of undergoing disturbance and change and will still maintain the same function,

structure, diversity, and ecological interactions that were present before disturbance.

Plantago or Indian Psyllium Desert plants have unique strategies that allow them to survive and persist

in very harsh arid environments. Species of Plantago or Indian psyllium are

native to the arid regions of North India. They have evolved a special

mucilage (a thick, gelatinous substance) that makes up about 30 percent of

their seed coat. This coating swells when wetted and retains water,

protecting the seed from desiccation while germinating.

People in India often combine the seeds with fruit juice or stewed fruit.

Crushed seeds are added to oil and vinegar as a treatment for rheumatism or

other problems with swelling. The seeds are commonly exported for manufacturing of Metamucil, a fiber

laxative. The same water-holding mucilage that protects desert seeds also absorbs water and toxins in the

human digestive tract and acts as a soothing lubricant.


FIGURE 2.4

This diagram depicts forest succession. Succession tends to move from small-statured, short-lived herbaceous

plants toward taller, longer-lived woody species.

SUCCESSION

Disturbance events are often the starting point for ecological succession, the directional change

in species composition, structure, and resource availability over time that is driven by biotic

activity and interactions as well as changes in the physical environment.

Community Change

At the core of succession is change. Changes can be compositional as the relative numbers of

species present changes, and structural, as forests slowly take over areas that were previously

grassland. The mechanisms driving that change are the colonization and extinction of species in

response to abiotic processes and biological interactions. The theoretical endpoint of succession

is a climax community. It is the stable endpoint that experiences very little ongoing change,

until a large disturbance resets the successional clock. In reality, however, at any given time

there are areas on a landscape that are in a climax stage and others at different stages of

succession.

Very generally, succession tends to move from small-statured, short-lived herbaceous plants

(often called pioneer species, the first species to establish after a disturbance) toward taller,

longer-lived woody species. FIGURE 2.4 depicts a landscape with areas at different times since

fire. The newly burned areas show the starting point for succession. In the early post-fire

environment, grasses, wildflowers, and weedy species often establish within the first year or two.

Several years after a fire, longer-lived species such as shrubs and perennial forbs may start to

dominate the plant community, and seedlings of the canopy species will begin to germinate.

Mechanisms of Succession

Species occurrence following succession is a result of 1) the species that can get there and

establish first (dispersal and colonization) and 2) species that can persist and reproduce

(competition). In addition, there is an element of chance as to which species will start the

succession, depending on which species got there first.


FIGURE 2.5

Diagram depicted forest succession.

Ecological Climax, Stability, and Alternative Stable States

Classic ecological theory says that succession is moving directionally toward a stable climax

community. In nature, we see this occurring in many areas, where a disturbance resets the

successional clock. The ecosystem moves through different successional stages in a predictable

way from grassland to shrubland to hardwood forest to evergreen forest. The directional process

of succession toward a climax community assumes that succession is predictable and repeatable.

However, in reality, sometimes different combinations of species develop in the same area under

similar environmental conditions. We call these other situations alternative stable states. Multiple

stable states can occur when there is an addition or exclusion of a dominant or strongly

interacting species during any phase of succession. In many cases, human actions have been the

cause of the shift toward an alternative stable state.

Gap Dynamics

We often think of disturbances as being large-scale events, but it is important to recognize that

they can also occur on much smaller scales, such as when a heavy wind event knocks down one

or a few trees, or when heavy rainfall causes erosion on a part of a hill slope. These small-scale

disturbances create gaps in the climax vegetation, and succession occurs within these gaps as it

does on a larger scale. These small scale successional dynamics are often referred to as gap

phase dynamics and occur between larger disturbance events, allowing increased light to enter

the ecosystem and serving to diversify the community.


Section II Summary

A community is a unique collection of species that interact with one another and with their

environment in the same place and at the same time. These organisms may compete with one

another for limiting resources such as light, nutrients, food, and water; may rely on each

other for food; or may interact in a way that is mutually beneficial. Collectively, these

interspecific interactions form the field of ecology known as community ecology.

Species diversity, the variety of species found in an ecological community, is one type of

biodiversity. While we generally think of biodiversity and species diversity as being

synonymous, it is important to recognize that the term biodiversity can refer to variability at

multiple scales, from genes to species to ecosystems.

There is a major gradient of diversity from the very diverse equatorial tropics to the species-

poor polar regions, as well as other generalized patterns of diversity. Community ecology

seeks to understand these patterns and generally attributes them to many interacting causal

factors: evolutionary speed, geographic area, interspecific interactions, ambient energy,

productivity, and patterns of disturbance.

Interactions among species can benefit one, both, or neither of the species involved.

Predation occurs when one species (a predator) kills and eats another organism, its prey.

Herbivory is the process of an animal or insect consuming part or all of a plant, and is

sometimes considered to be a special type of predation. Competition occurs when there is a

limiting resource that both species require, such as water, nutrients, or light that is in short

supply, and both species are negatively impacted. Parasitism occurs when one species lives

in or on another species and feeds on its host or uses its resources.

Mutualism is a relationship between members of two species in which both members benefit

from the association. Commensalism is a relationship between two organisms of different

species in which one organism benefits from the association and the other is neither

benefitted nor harmed. All of the interactions among species in an ecological community

serve to provide order to the community.

Energy flow may be controlled by the rate at which energy enters an ecosystem and is

converted to organic matter (“bottom-up” control) or can be controlled by the rate at which

top predators in an ecosystem consume organic matter, influencing the composition and

abundance of organisms lower on the food chain (“top-down” control). Changes in the

distribution or abundance of organisms at one trophic level can have profound impacts on

organisms at other trophic levels.

The complex, interwoven set of relationships between trophic levels within an ecosystem is

referred to as a food web.

Keystone species fill roles in a community that are critically important to the functioning of

the community. These species may not be particularly numerous, but they largely determine

ecosystem structure, and in their absence the ecosystem would dramatically change.


Disturbances can be natural or anthropogenic and are characterized by their type, size,

frequency, intensity, and seasonality. Plants and animals that share a long evolutionary

history with a disturbance regime have adaptations to survive or regenerate following a

disturbance event. The adaptations that plants and animals have evolved that allow them to

persist in areas where disturbance is a natural part of the ecosystem contribute to the stability

of the biological system.

Disturbance events are often the starting point for ecological succession, the directional

change in species composition, structure, and resource availability over time that is driven by

biotic activity and interactions as well as changes in the physical environment. Generally,

succession tends to move from small-statured, short-lived herbaceous plants toward taller,

longer-lived woody species.

Species occurrence following succession is a result of 1) the species that can get there and

establish first and 2) species that can persist and reproduce. Multiple stable states can occur

when there is an addition or exclusion of a dominant or strongly interacting species during

any phase of succession, altering community dynamics and the successional pathway. In

many cases, human actions have been the cause of the shift toward an alternative stable state.

Small-scale disturbances create gaps in the climax vegetation, and succession occurs on them

as it does on a larger scale. These small-scale successional dynamics are often referred to as

gap phase dynamics and occur between larger disturbance events.


FIGURE 3.1

The sun’s heat warms the atmosphere, drives the Earth’s water cycle, moves air

and water currents, and is transformed into chemical energy that drives the

production of carbohydrates and other carbon-based compounds that are the

building blocks of organic life.

SECTION III: ECOSYSTEM, LANDSCAPE, AND

GLOBAL ECOLOGY

Ecosystem ecology studies the links between multiple organisms and their physical environment

as an integrated system. An ecosystem level approach is critical in the management of the

planet’s natural resources since it considers the interconnected relationships between multiple

biotic systems, including humans and their abiotic environment. An ecosystem approach takes a

big picture view of multiple plant and animal communities and of how energy and material

cycles through and between them.

Ecosystem level studies

often seek to understand

the factors that determine

the pools (amount or

quantity) and fluxes

(transfers or flows) of

material and energy in an

ecosystem. Some of the

materials that move

through ecosystems

include naturally derived

materials such as carbon,

water, nutrients, and

minerals as well as

materials derived by

humans such as

pesticides, herbicides, and

chemical contaminants.

Ecosystem processes

(such as photosynthesis

and decomposition) are

ways that energy and materials are transferred from one pool to another.

Energy Cycling

Sunlight is the ultimate source of energy that keeps Earth’s ecosystems functioning. The sun’s

heat warms the atmosphere, drives the Earth’s water cycle, moves air and water currents, and is

transformed into chemical energy that drives the production of carbohydrates and other carbon-


FIGURE 3.2

The flow of energy through an ecosystem is the story of how carbon cycles through the food chain.

based compounds that are the building blocks of organic life. We can think about energy cycling

in an ecosystem largely as the movement of these carbon-based compounds through the tissues

of live and dead plants and animals, from the microscopic to the macroscopic.

There are two forms of energy: kinetic energy, the energy of an object in motion, and potential

energy, stored energy that is available for performing work. There are two fundamental laws that

govern the use and storage of all energy. The first law of thermodynamics states that matter

cannot be created nor destroyed. Matter can change forms—as, for example, when a solid

changes to a liquid with the addition of heat, or to a gas with the addition of even more heat—but

it never disappears completely. The second law of thermodynamics says that energy disperses

from being localized to spread out unless it is prevented from doing so. For example, when you

turn off the burner on a stove, it is still very hot, but without the continuous source of energy

(heat), the warmth present in the burner will begin to disperse, warming the area around it, and

the burner will begin to cool down.

These same principles apply to our ecological systems. Energy and matter are not created out of

thin air, nor are they destroyed. They simply change form. A plant is consumed and an herbivore

grows. An herbivore dies, decomposes, and replenishes the organic matter in the soil. As energy


FIGURE 3.3

a) A trophic pyramid; b) A food web

is transferred from one organism to another, a portion of that energy is stored in living tissue, and

a part is dissipated as heat to the surrounding air. In this way, the flow of energy through an

ecosystem is the story of how carbon cycles through the food chain.

Primary Production

You may remember from earlier that the conversion of sunlight into carbon compounds by

photosynthesis drives the production of energy for use by biological organisms. Photosynthesis

has two major steps: the first is the harvesting of energy from sunlight and the second is the

fixing of carbon to generate carbohydrates. The rate of photosynthesis determines the supply of

energy available to organisms.

The rate that sunlight is converted by autotrophic organisms (organisms which produce organic

compounds; for example, plants on land and algae in water) via photosynthesis into organic

compounds is referred to as primary productivity. Gross primary productivity (GPP) is the

total rate of photosynthesis, or the total energy obtained by autotrophs. However, autotrophs

must use some energy in the process, reducing the total productivity rate. The rate of energy

stored after accounting for the energy expended is referred to as net primary productivity

(NPP). This productivity of an ecosystem is also sometimes referred to as a rate of production

and is measured in units of energy per unit area per unit time (such as grams per square meter per

year). The stored energy found at a given area at a given time is often referred to as biomass,

which is simply the amount of organic material that can be found at a given area at any given

time. We can measure the total amount of biomass in an agricultural pasture or in a body of

water if we are interested in the amount of organic matter that is stored in that area.

In terrestrial ecosystems, temperature, heat, and nutrients control rates of primary productivity.

Generally, NPP increases with increasing mean annual precipitation, mean annual temperature,


and a longer growing season. Places that are very warm and moist, such as tropical rainforests,

have extremely high rates of NPP. In contrast, places that are warm but dry, such as deserts, have

low rates of productivity.

In aquatic ecosystems, the major controls on primary productivity are temperature, light, and

nutrient availability. As we discussed earlier, light availability decreases with depth in the water

column; therefore, primary production also decreases with depth. Microscopic phytoplankton

(free-floating algae, protists, and cyanobacteria) perform the majority of the ocean’s primary

production, form the basis of the oceans’ food web, and fix large amounts of carbon. Oceanic

plants, like their terrestrial counterparts, need nutrients for growth. The two most important

nutrients for phytoplankton growth are nitrate and phosphate.

In some conditions, there is no sunlight available to support photosynthesis. Certain bacteria

have evolved to these conditions and are able to synthesize energy from the oxidation of

inorganic materials. We find this method of energy production, called chemosynthesis, in many

places where there is no light availability, such as deep sea communities, in hot springs, in the

soil, and in mammalian intestines. Some scientists have hypothesized that chemosynthesis may

have played a more significant role early in Earth’s evolution than what we see today.

Primary production is not constant over time. Rates of photosynthesis and plant growth can vary

greatly with season and plant age. Places with distinct cold seasons or dry periods have a period

of plant dormancy, where primary productivity pauses. In warmer temperate and moist tropical

regions, there is little difference in primary productivity between seasons. Differences in annual

temperature and precipitation can affect the rates of plant growth as well. As we discussed,

temperature and moisture are major controls on primary productivity, so we expect warm years

with increased moisture to be more productive than years that are cold and dry. In forested

ecosystems, productivity is impacted by the age of the dominant trees. Young trees

photosynthesize and grow very rapidly, but older trees put more of their energy into the

structural components of the stem and branches and reduce growth rates overall.

NUTRIENT CYCLING

We talked about the way that carbon is fixed by primary producers and travels through the food

chain. Similarly, other essential elements are incorporated into living tissues and move through

an ecosystem. These cycles of nutrient movement through ecosystem components are called

biogeochemical cycles. The origin of these elements is either the atmosphere (like in the carbon

cycle) or from the weathering of rocks and minerals. They are incorporated into soil or water and

are taken up by plants, thus becoming fixed in living tissues and traveling a path through the

food chain. When plants and animals die, the elements are returned to the soil as dead organic

matter and are used by various decomposers, which in turn return the elements to their mineral

form, where they again become available for plant uptake. In this way, minerals are recycled

through an ecosystem.

Decomposition

The key to nutrient recycling through an ecosystem is decomposition, the breakdown of organic

material by decomposer organisms and the release of simple, soluble organic and inorganic

nutrients by those organisms as waste materials. Decomposers are bacteria and fungi that feed on

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http://oceanexplorer.noaa.gov/explorations/04deepscope/background/deeplight/media/diagram3.html

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FIGURE 3.5

The cycles of nutrient movement through ecosystem components are called biogeochemical cycles.

dead organic matter or detritus. Bacteria are the dominant decomposers of dead animal material,

and fungi are the dominant decomposers of decaying plants. The undecomposed organic matter

on the soil surface is known as litter and is abundant in most ecosystems. Large soil animals

such as earthworms, nematodes, and insects break up this litter into progressively smaller

particle sizes, increasing the surface area available for further chemical breakdown. This

chemical process, known as mineralization, breaks the bonds between carbon molecules and

inorganic nutrients, making them available for uptake by plant roots and starting the nutrient

cycle again. Like most biological processes, rates of decomposition are greatly impacted by

climate. Decomposition proceeds faster in areas with warmer temperatures and greater moisture

availability.

HUMAN ECOLOGY An exponentially growing human population is currently impacting our planet more than any

other species has done in Earth’s history. Our hunter-gatherer ancestors depended on plant

productivity and an abundance of animals for subsistence. With the initial development of

agriculture about 12,000 years ago, our dependence on planetary resources shifted from natural

systems to agricultural systems. In the mid-eighteenth century, humans’ relationship with

natural resource use changed dramatically with the Industrial Revolution. A significant shift in

the use of human and animal labor to mechanized labor provided the means for large-scale

production of food, but also a much larger and more destructive ecological footprint. Our current

system of agriculture, manufacturing, transport, and storage of the food needed for a human

population that exceeds 7 billion has had many profound impacts on our planet.


Our current system of agriculture, manufacturing, transport, and storage of the food needed for a human population that exceeds 7 billion has had many profound impacts on our planet.

Human Population Growth

The root cause of all of the adverse impacts that humans are having on ecosystems is population

growth—there are simply too many of us for the resources available, and every day over 228,000

more people are added.4 At the time when agriculture was first initiated, the planet supported

about 5 million people and was growing at a rate of about 0.05 percent per year. During the mid-

1700s, the Industrial Revolution caused increases in global population growth rates. Around

1800, world population reached one billion people. The second billion was reached 130 years

later (1930), the third 30 years later (1959), the fourth 15 years later (1974), the fifth 13 years

later (1987), the sixth in 12 years (1999), and the seventh 12 years later (2011). We are expected

to reach 8 billion people in the year 2030 and nine billion by 2050. You can see that in recent

years the growth rate has slowed considerably, as people have become more aware of human

impact on the environment, yet our population continues to grow rapidly.

Endangered Species and Ecosystems

The field of conservation biology is concerned with the decline of biological populations and

the critical ecosystems in which they occur. In most cases, their decline is a direct result of

human impacts to habitats due to fragmentation, land cover conversion, and depletion or

pollution of resources such as food and water. Conservation biologists identify species or

populations that are already at a small population size or that are at risk of becoming endangered

in the future due to declining numbers. Once a species is recognized as at risk of becoming

endangered or extinct, the potential causes of its decline must be recognized and remedied.

Aggressive actions need to be taken to reverse the factors driving species to extinction.

http://www.theglobaleducationproject.org/earth/images/final-images/g-pop-growth-chart-map.gif

http://www.theglobaleducationproject.org/earth/images/final-images/g-pop-growth-chart-map.gif


A male Asiatic lion. The Asiatic lion as well as the Bengal tiger and leopards are in rapid decline in India due to

frequent poaching.

Threats to Biodiversity

Overkill Ongoing fishing, hunting, or gathering beyond a rate from which a species can rebound is referred to as overkill. Overkill and can quickly threaten a species, particularly one that has a low rate of natural increase. Due to frequent poaching, the Asiatic lion, Bengal tiger, and leopard are in rapid decline in India and are found in restricted habitat patches. Similarly, the Hawaiian sandalwood tree has been overharvested across the island chain and some species are now endangered, and all are in decline. We see this story repeated in commercial fisheries around the globe. Marine fishery numbers are difficult to estimate, and overharvesting has caused the collapse of many fish stocks. Habitat Fragmentation and Land Cover Change In several of the previous sections we

have discussed how human activities

have threatened ecosystems, often

causing the extinction of some of the

organisms that depend on them for

habitat. Habitat destruction is the

leading cause of species extinctions.

The primary reason for these habitat

transformations is to expand

agricultural land to meet the dietary

needs of a rapidly expanding human population. The tropical biomes are often the focus of

discussions on habitat loss as the most rapid rates of forest conversion for agriculture still occur

in these areas.

Biotic Invasions Humans have intentionally or unintentionally introduced countless species of plants and animals

into areas outside of their historical range. Not all introduced species will do well in a new

environment, but the few that do are often superior competitors because they are no longer

limited by their native competitors, predators, and diseases. Animal invaders impact native

species through direct competition, grazing on native plants, and alteration of habitat. Plant

invaders outcompete native plants and alter nutrient cycling, hydrology, and fire regimes, further

degrading an area. One of the most problematic plant invaders in the American West is

cheatgrass, an annual grass that has spread extensively over sagebrush steppe ecosystems,


replacing vegetation. Cheatgrass is extremely flammable and has decreased the time between

successive fire events in many areas.

In aquatic ecosystems, invasive species also have profound impacts. The Great Lakes region has

been particularly impacted due to its function as a major shipping port. A notorious invader to

this region has been the zebra mussel, which has been linked to the decline of native clams and

mussels by competition and to the demise of many thousands of sea birds, as they carry avian

botulism.

Pollution

Pollution is present in terrestrial, marine, and aquatic systems across the globe. In many cases

pollutants cause physiological stress to individual plants and animals as well as degrade habitats.

Pollutants can overload nutrient levels in rivers and streams, lead to respiratory, heart, and lung

disease, and can be found in the bloodstream and organs of animals that live far from the original

source. There are several major types of pollution: air pollution (e.g., emissions of harmful gases

or particles), water pollution (e.g., dumping of waste materials, chemicals, or microorganisms

into a water body), and land pollution (e.g., improper storage and leaching of domestic or

industrial waste materials). Many times the various types of pollution interact and occur

simultaneously, such as the dumping of solid waste and subsequent leaching into nearby

waterways and groundwater.

Climate Change

Global climate change has become perhaps the greatest ecological challenge of our time. Over

geological time, changes in the planet’s climate have profoundly impacted the types of biological

organisms that have existed in different time periods. Currently, we are undergoing a period of

change in our climate that is unprecedented. While historical warming and cooler patterns have

occurred, they were driven by variability in the tilt of the Earth. The changes that we are

experiencing today are a direct result of human activities. The burning of fossil fuels is adding

heat-trapping greenhouse gases to the atmosphere, causing warmer temperatures, stronger

storms, melting of polar regions, a rise in the sea level, and longer periods of drought. These

changes are impacting the environment as well as human quality of life in many ways.

Water shortages are becoming common. In places where water is limited, agriculture suffers and

hydroelectric plants have a reduced capacity to generate electricity. Plants and animals that live

in and near water bodies are not always able to adapt to the reduced flow levels and warmer

water temperatures. Increased incidences of drought and flooding damage crops and food storage

capabilities. Disease may spread more readily as temperatures increase. Many disease-carrying

insects such as mosquitos and ticks cannot survive very cold periods. As winters get warmer,

these insects’ capacity to survive year round is increased. This short list gives just a few of the

projected results of a warming climate. As you can imagine, they are certain to be detrimental to

humans as well as the other species with which we share the planet. It is critical that we take

rapid and deliberate steps to slow or reverse these trends and build a future based on

sustainability of our natural resources.


Sustainability

Sustainable stewardship of planetary resources requires that humans do not use resources at a

faster rate than they can be replaced by biological processes. This requires us to understand

ecological processes such as groundwater recharge to prevent our exhaustion of the water

supply; to understand the complex successional patterns of regrowth of woody species after a

timber harvest; and for us to reduce fossil fuel extraction and use fossil fuels at a level that no

longer results in an increase in atmospheric greenhouse gases.

You have probably seen the common bumper sticker or heard the slogan “Think Globally, Act

Locally,” which has been used in many ways to urge people to make small decisions daily on a

personal level that consider the health of the planet. On an individual level, this can mean

making decisions to reduce water use, purchasing things that are made from recycled content and

minimal packaging materials, choosing foods grown without agricultural chemicals, walking,

bike, or take public transportation rather than drive, and to elect policymakers who share a vision

for environmental sustainability.

Globally, large changes will need to be made in the coming years to reduce emissions, shift to

alternative sources of energy, and cut waste in energy, water, and other resource use. Individual

decisions, community adaptations, and international cooperation will all be necessary to mitigate

the human-caused damage that has already been done to planetary natural resources and to

conserve them for future generations.

Section III Summary

Ecosystem ecology studies the links between multiple organisms and their physical

environment as an integrated system. An ecosystems approach takes a big picture view of

multiple plant and animal communities and examines the ways that energy and material cycle

through and between them.

Ecosystem level studies often seek to understand the factors that determine the pools (amount

or quantity) and fluxes (transfers or flows) of material and energy in an ecosystem.

Ecosystem processes (such as photosynthesis and decomposition) are ways that energy and

materials are transferred from one pool to another.

Sunlight is the ultimate source of energy that keeps Earth’s ecosystems functioning. Energy

cycling in an ecosystem is the movement of carbon-based compounds through the tissues of

live and dead plants and animals.

There are two forms of energy: kinetic energy, the energy of an object in motion, and

potential energy, stored energy that is available for performing work.

There are two fundamental laws that govern the use and storage of all energy. The first law

of thermodynamics states that matter cannot be created nor destroyed. The second law of

thermodynamics says that energy disperses from being localized to spread out unless it is

prevented from doing so.


The rate that sunlight is converted by autotrophic organisms via photosynthesis into organic

compounds is referred to as primary productivity. Gross primary productivity (GPP) is the

total rate of photosynthesis, or the total energy obtained by autotrophs. The rate of energy

stored after accounting for the energy expended is referred to as net primary productivity

(NPP).

In terrestrial ecosystems, temperature, heat, and nutrients control rates of primary

productivity.

In aquatic ecosystems, the major controls on primary productivity are temperature, light, and

nutrient availability. Microscopic phytoplankton perform the majority of the ocean’s primary

production, form the basis of the oceans’ food web, and fix large amounts of carbon.

Chemosynthesis is the synthesis of organic compounds using the energy released from

chemical reactions instead of the energy of sunlight, and it occurs where there is no light

available, such as in deep sea communities, in hot springs, in the soil, and in mammalian

intestines.

Rates of photosynthesis and plant growth can vary greatly with season and plant age. Places

with distinct cold seasons or dry periods have a period of plant dormancy, where primary

productivity pauses. In warmer temperate and moist tropical regions, there is little difference

in primary productivity between seasons. Differences in annual temperature and precipitation

can affect the rates of plant growth as well.

Organisms that cannot make their own food rely on primary producers as their energy source.

All animals, fungi, and most bacteria are heterotrophs, consuming plant or animal material

for maintenance and growth. The generation of biomass of heterotrophic organisms is called

secondary production.

Cycles of nutrient movement through ecosystem components are called biogeochemical

cycles. The origin of these nutrients is either the atmosphere or from the weathering of rocks

and minerals. These nutrients are then incorporated into soil or water and are taken up by

plants, thus becoming fixed in living tissues and traveling a path through the food chain.

The key to nutrient recycling through an ecosystem is decomposition—the breakdown of

organic material by decomposer organisms, and the release of simple, soluble organic and

inorganic nutrients by those organisms as waste materials.

An exponentially growing human population is currently impacting our planet more than any

other species has done in Earth’s history.

The field of conservation biology is concerned with the decline of biological populations and

the critical ecosystems in which they occur. In most cases, their decline is a direct result of

human impacts to habitats due to fragmentation, land cover conversion, and depletion or

pollution of resources such as food and water.


The major threats to biodiversity include overkill, habitat fragmentation, biological

invasions, pollution, and climate change. Global climate change has become perhaps the

greatest ecological challenge of our time. Currently, we are undergoing a period of change in

our climate that is unprecedented. While historical warming and cooling patterns have

occurred, they were driven by variability in the tilt of the Earth. The changes that we are

experiencing today are a direct result of human activities. It is critical that we take rapid and

deliberate steps to slow or reverse these trends and build a future based on sustainability of

our natural resources.

Sustainable stewardship of planetary resources requires that humans do not use resources at a

faster rate than they can be replaced by biological processes. Individual decisions,

community adaptations, and international cooperation will all be necessary to mitigate the

human-caused damage that has already been done to planetary natural resources and to

conserve them for future generations.


CONCLUSION

“When we try to pick out anything by itself, we find it hitched to everything else in the universe.”

~John Muir (1838–1914)

Often new advances in the science of ecology reveal a new linkage between organisms and their

abiotic environment. The more we learn, the more we are forced to appreciate that no species

goes unaffected by outside processes, but each species is interdependent upon the world around

it. In this resource guide, we have discussed the way that climate, soils, water, nutrients, and

topography define the bounds of organisms’ distribution. Within those bounds, the relationships

between individuals of the same species determine which ones will persist and provide the

genetic material for the next generation of organisms. Interactions between members of different

species along with abiotic factors determine the geographic distribution of a species.

Once we start to understand the biotic and abiotic relationships of multiple ecosystems, and

understand how energy, water, and nutrients flow through and between them, we gain a

landscape-level perspective on the interconnectedness of the different communities and

ecosystems that a landscape encompasses. Globally, the same pattern persists. Multiple

landscapes are pieced together to form a planet that is connected by flows of energy, water,

carbon, and other elements and is variable in space and time. These connections are not simple—

they are generally far more complicated than we yet understand. To begin to understand them, it

is important that ecologists study the planet as an integrated whole, and one in which humans are

just a single species among the many others with which we coexist.


GLOSSARY

A horizon – the topsoil; this layer of soil is made up primarily of weathered parent material, but

is relatively rich in organic materials due to leaching from the O horizon above it

Abiotic – of or pertaining to the physical environment

Abundance – the number of individuals of a species in a given area, or the relative amount of

species in a particular ecosystem

Age structure – the proportion of the population in each age class

Allele – one of two or more different possible forms of the same gene, resulting in genetic

variability within a population

Ambient energy hypothesis – says that where there is more energy, there will be greater

biodiversity

Amensalism – a relationship between organisms of two different species in which one is

unaffected and the other is negatively impacted by the association

Antibiosis – a relationship between organisms of two different species in which one is

negatively impacted by a substance produced by the other; a specific type of amensalism

Autotrophic organisms – organisms that produce their own nutritional organic compounds

B horizon – sometimes called the subsoil, contains limited organic matter and often has

accumulated mineral particles due to leaching from the topsoil

Basal species – species that do not feed upon any other species but are eaten by other species

Benthic zone – the ecological zone at the bottom of a body of water

Biodiversity – the variation of life

Biodiversity hotspots – places of high biodiversity that have been greatly threatened by human

activity


Biogeochemical cycles – cycles of nutrient movement through ecosystem components

Bioluminescence – the biochemical emission of visible light by organisms

Biomass – the amount of organic material that can be found at a given area at any given time

Biomes – large regions dominated by similar types of ecosystems

Biosphere – the highest level of biological organization encompassing all of the world’s

ecosystems

Biotic – of or having to do with life or living organisms, particularly in their ecological

relationships

C horizon – the soil layer that lies beneath the subsoil and is comprised of unconsolidated

materials; there is low biological activity in this layer, and it retains many characteristics of the

parent material.

Carrying capacity – the maximum population size of a species that a given ecosystem can

sustain

Chemical weathering – the breakdown of rock materials by water, oxygen, and acids

Chemosynthesis – the process by which microbes create energy by converting carbon molecules

and nutrients into organic matter in the absence of sunlight

Climate – the long-term average weather pattern for an area, which places constraints on the life

that can successfully endure, grow, and reproduce in an area

Climax community – the theoretical stable endpoint in the successional trajectory that

experiences very little ongoing change until disturbance resets the successional clock

Commensalism – a relationship between two organisms of different species in which one

organism benefits from the association and the other is neither benefitted nor harmed

Community – two or more interacting populations of plants and animals of different species in

the same area

Community ecology – the study of the interactions within and among ecological communities

Conservation biology – the study of the decline of biological populations and the critical

ecosystems in which they occur

Continental climates – climates characterized by large seasonal and daily variation in

temperature


Deciduous – describes a type of woody vegetation that loses its leaves in the fall, allowing much

more light to penetrate the canopy than in the summer months

Decomposition – the breakdown of complex organic compounds into simpler ones, which add

nutrient materials to the soil

Density-dependent factors – factors that impact growth, death, and birth rates differentially

depending on the initial size of the population for a given area

Density-independent factors – factors that impact birth and death rates proportionally with

population size, regardless of how many individuals were initially present

Disturbance – a discrete event in time that disrupts ecosystem, community, or population

structure and changes substrate and resource availability and changes the physical environment

Ecological niche – a set of optimal environmental conditions that are most conducive to the

successful photosynthesis, growth, survival, and reproduction of a species, and that defines the

way that a species fits into an ecological community or ecosystem

Ecology – the scientific study of the relationships between organisms and their environment

Ecosystem – a community of organisms and the physical environment in which they live

Ecosystem diversity – the diversity of ecological communities that are found within a fixed area

Ecosystem ecology – the study of the way climate alters the distribution of biotic communities,

the rates at which nutrients and water enter a community, and the way that soils and precipitation

patterns alter the relationships between plant communities and other organisms

Ecosystem processes – the ways that energy and materials are transferred from one pool to

another

Ectoparasites – parasitic species that live on the surface of their host

Endogenous – originating inside the ecosystem

Endoparasites – parasitic species that live inside their host organism and feed on the host

organism or rob the host of nutrients or other resources

Environmental studies – a field of study that examines human impacts on physical, biological,

and chemical processes

Estuary – the ecosystem where the river meets the ocean

Evapotranspiration – the sum of the water lost from evaporation plus transpiration


Evenness – the percentage that the individuals of each species contributes to the total number of

organisms of all species present

Evolution – directional change in populations over time

Evolutionary speed hypothesis – says that there are more species in some areas because

speciation happens faster in these areas or has been happening longer

Exogenous – originating outside of the ecosystem

Exploitation competition – occurs when one species utilizes more of a limiting resource or uses

a limiting resource more efficiently than another species

Facultative mutualism – a type of mutualistic relationship where the organisms both benefit

from being together, but it is not required for their survival

First law of thermodynamics – states that matter cannot be created nor destroyed

First-order streams – the smallest and highest elevation streams

Food web – an interwoven set of relationships between trophic levels within an ecosystem

Fundamental niche – niche based on environmental factors

Gap phase dynamics – small-scale successional dynamics that occur between larger disturbance

events, allowing increased light to enter the ecosystem and serving to diversify the community

Genetic diversity – variation in alleles present in a population that results in individual

differences in appearance, function, and behavior

Genetic drift – change in the genetic composition of a population due to chance or random

events rather than by natural selection, resulting in changes in allele frequencies over time

Genotype – the genetic makeup of a particular organism

Geographic area hypothesis – states that large areas have more diversity than smaller land

areas

Greenhouse gases – atmospheric gases that absorb and reradiate the earth’s radiation

Gross primary productivity (GPP) – the total rate of photosynthesis or the total energy

obtained by autotrophs

Heterotrophs – organisms that consume plant or animal material for maintenance and growth

Horizon – each layer within the soil profile


Hydrologic cycle – the process by which water travels from the air to the Earth and back to the

atmosphere

Hypothesis – an educated guess based on prior experience and knowledge about what is driving

an observation that is phrased in such a way that it is scientifically testable

Interference competition – occurs when one species actively attempts to exclude another

species

Intermediate disturbance hypothesis – says that in the absence of disturbance, a few most

competitive species become dominant; when there are small periodic disturbances, biological

diversity is greater.

Intermediate species – a species that is both a food source for others as well as a consumer of

other species

Interspecific interactions – biotic interactions between members of different species

Interspecific interactions hypothesis – high species diversity in the tropics is associated with

greater competition and higher predation rates

Intraspecific interactions – biotic interactions between members of the same species

Keystone species – species that fill roles that are critically important to the functioning of the

community

Kinetic energy – the energy of an object in motion

Landscape ecology – a field of ecology that examines large-scale spatial patterns and their

relationship to ecological functioning

Landscapes – patchworks of multiple communities and ecosystems, typically studied at larger

spatial scales

Litter – the fresh, undecomposed organic matter on the soil surface

Mangroves – coastal ecosystems inhabited by salt-tolerant trees and shrubs

Maritime climates – climates characterized by high humidity with little daily or seasonal

fluctuations in temperature

Marsh – an ecosystem type that is formed when upland sediment is carried downriver and

accumulates near ocean shorelines; dominated by grasses, rushes, and forbs that are rooted

underwater


Mechanical weathering – the physical breakdown of parent material by processes such as

freeze/thaw and wet/dry cycles that cause parent material to expand and contract, slowly

breaking the rock into smaller pieces over time and increasing the surface area

Metapopulations – interacting groups of populations of the same species that are dispersed

among patchy habitats but have occasional migration and interaction

Mineralization – the process where bonds between carbon molecules and inorganic nutrients are

broken, making them available for uptake by plant roots

Mutualism – a relationship between members of two species in which both members benefit

from the association

Natural selection – the process by which individuals with better adapted heritable characteristics

tend to survive and reproduce more successfully than other individuals

Net primary productivity (NPP) – the rate of energy stored after accounting for the energy

expended

Niche – a set of optimal environmental conditions that are most conducive to the successful

photosynthesis, growth, survival, and reproduction of a species and that define the way that the

species fits into an ecological community or ecosystem

Nitrogen fixation – the assimilation of nitrogen into organic compounds

O horizon – the surface layer of soil, consisting of organic material that accumulates from

decomposing plant materials

Obligatory mutualism – a type of mutualistic relationship whereby one species cannot survive

without the other species

Orographic or rain shadow effect – A moving air mass picks up moisture as it travels over a

body of water. As the air mass moves up a mountain range, it cools and condenses, releasing

much of its moisture as rain or snow. The drier air mass then warms as it travels down the inland

side of the mountain range.

Overkill – ongoing fishing, hunting, or gathering beyond a rate from which a species can

rebound

Parasite – an organism that consumes the tissues of the host organism or robs it of its food or

other resources

Parent material – the rock or mineral substrate that underlies the soil profile

Pathogen – an organism that causes disease in a host species


Pelagic zone – the open ocean

Phenotype – the physical manifestation of an organism’s genetic makeup

Photosynthesis – the conversion of sunlight into carbon compounds, which drives the

production of energy for use by primary producers

Pioneer species – the first species to establish after a disturbance

Pools – 1) deep, slow-moving stretches of streams with fine sediment; 2) the amount of material

or energy in an ecosystem

Population – a group of individuals of the same species that live in a particular area and interact

with one another

Population dynamics – ways in which organisms of the same species interact with each other in

the same geographical area

Population ecology – the study of populations of organisms, which often seeks to find answers

to questions about how and why the locations and numbers of populations change over time

Population size – the number of individuals in the population

Potential energy – stored energy that is available for performing work

Predation – an interspecific relationship by which one species, a predator, kills and eats another

organism, its prey

Predator-prey cycles – regularly spaced intervals of increases and decreases in the population

sizes of the predator and prey

Primary productivity – the rate that sunlight is converted by autotrophic organisms via

photosynthesis into organic compounds

Production – the rate of generation of biomass in an ecosystem

Productivity hypothesis – says that greater production should result in greater biological

diversity

Range – the geographical area in which a particular species can be found

Realized niche – a niche based on environmental factors and the presence of other species

Resilience – an ecosystem’s resistance to change


Respiration – the process by which organisms take up oxygen (O2) and produce carbon dioxide

(CO2)

Riffles – fast-moving portions of a stream that are flowing over coarse substrate

Riparian – the narrow ecosystem that parallels streams, rivers, and other water channels

Rocky intertidal – the rocky zone occupying the area between high and low tide

Salinity – a measure of the dissolved salt content in water

Savannas – a grassland ecosystem occurring in tropical and subtropical ecosystems with trees

and shrubs intermixed with a dense grass understory

Scientific method – the process by which scientific inquiry takes place, including the following

defined steps: ask a question, state a measureable hypothesis, conduct an experiment, analyze the

results, and make a conclusion; based on these conclusions, a hypothesis is then either rejected or

not.

Second law of thermodynamics – energy disperses from being localized to spread out unless it

is prevented from doing so

Secondary production – generation of biomass of heterotrophic organisms

Soil profile – vertical layering in the soil column

Soil texture – the relative proportions of different sized sediment grains

Speciation – the formation of new and distinct species by evolution

Species diversity – a measure of the variability and variety of living organisms in an ecosystem

Species richness – the number of different species occurring in a defined geographical area

Stability – ability of an ecosystem to recover after a disturbance

Stress – an environmental condition that constrains physiological processes, lowering an

organism’s rate of growth, survival, or reproduction

Succession – the directional change in species composition, structure, and resource availability

of an area over time that is driven by biotic activity and interactions as well as changes in the

physical environment and the dominating species

Succulent – plants such as cacti that have thick, heavy foliage for water storage

Surface runoff – overland flow of excess water


Symbiont – organism that lives in association with another organism

Theory – formed to collectively explain the results of a large number of experimental

observations

Top predator – eats other species but is not a food source for any other species

Upwelling – periods when deep, cold, nutrient-rich ocean waters are driven to the surface to

replace the warmer, nutrient-poor surface waters


NOTES

1. Turner, M.G. 1989. Landscape Ecology: The effect of pattern on process. Annual Review of

Ecology, Evolution and Systematics. Vol. 20: 171–197.

2. Ripple, W.J. and R.L. Beschta. 2012. Tropic cascades in Yellowstone: The first 15 years after

wolf reintroduction. Biological Conservation. Vol. 145(1): 205–213.

3. Mills, L.S., M.E. Soule, and D.F. Doak. 1993. The keystone species concept in ecology and

conservation. Bioscience Vol43(4): 219.

4. Population Reference Bureau.

BIBLIOGRAPHY

Cain, M.L., W.D. Bowman & S.D. Hacker, 2011. Ecology. 2nd

Edition. p. 648.

Currie, D.J. 1991. Energy and large-scale patterns of animal and plant species richness.

American Naturalist Vol. 137: 27–49.

Krebs, C. 2008. The Ecological World View. p. 574.

Mills, L.S., M.E. Soule, and D.F. Doak. 1993. The keystone species concept in ecology and

conservation. Bioscience Vol43(4): 219.

Ripple, W.J. and R.L. Beschta. 2012. Tropic cascades in Yellowstone: The first 15 years after

wolf reintroduction. Biological Conservation. Vol. 145(1): 205–213.

Smith, T.M. and R.L. Smith. 2009. Elements of Ecology, 7th

Edition, p. 649.

Turner, M.G. 1989. Landscape Ecology: The effect of pattern on process. Annual Review of

Ecology, Evolution and Systematics. Vol. 20: 171–197.

Willig, M.R., D.M. Kauffman, and R.D. Stevens. 2003. Latitudinal gradients of biodiversity:

pattern, process, scale, and synthesis. Annual Review of Ecology, Evolution, and Systematics.

Vol. 34: 273–309.

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