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Chapter 45 ~ Community Ecology

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Outline

• 45.1 Ecology of Communities• 45.2 Community Development• 45.3 Dynamics of an Ecosystem

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45.1 Ecology of Communities

• Community Structure– A community is an assemblage of populations

interacting with one another within the same environment.

• The species composition (also called species richness) of a community is a listing of various species in the community.

• Species diversity includes both species richness and the abundance of different species.

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The Niche

• Ecological niche~ the sum total of an organism’s use of biotic and abiotic resources in its environment; its “ecological role”

– fundamental~ the set of resources a population is theoretically capable of using under ideal conditions

– realized~ the resources a population actually uses

• Thus, 2 species cannot coexist in a community if their niches are identical

Ex: Barnacle sp. on the coast of Scotland

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Competition: a closer look• Interference~ actual fighting

over resources

• Exploitative~ consumption or use of similar resources

• Competitive Exclusion Principle (Lotka / Volterra)~ 2 species with similar needs for the same limiting resources cannot coexist in the same place – Gause experiment

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Competition evidence• Resource partitioning~

sympatric species consume slightly different foods or use other resources in slightly different ways

• Character displacement~ sympatric species tend to diverge in those characteristics that overlap

Ex: Anolis lizard sp. perching sites in the

Dominican Republic Ex: Darwin’s finch beak size on the Galapagos Islands

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Interactions

• Interspecific (interactions between populations of different species within a community)

• Predation including parasitism; may involve a keystone species/predator

• Competition• Commensalism• Mutualism

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Heartworm

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Courtesy the University of Tennessee Parasitology Laboratory

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Clownfish AmongSea Anemone’s Tentacles

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© Dave B. Fleetham/Visuals Unlimited

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Cleaning Symbiosis

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© Bill Wood/Bruce Coleman, Inc.

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Predation defense• Cryptic (camouflage) coloration• Aposematic (warning) coloration• Mimicry~ superficial resemblance to

another species

– Batesian~ palatable/ harmless species mimics an unpalatable/ harmful model

– Mullerian~ 2 or more unpalatable, aposematically colored species resemble each other

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Succession

• Ecological succession~

transition in species composition over ecological time

• Primary~ begun in lifeless area; no soil, perhaps volcanic activity or retreating glacier

• Secondary~ an existing community has been cleared by some disturbance that leaves the soil intact

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45.3 Dynamics of an Ecosystem

• In an ecosystem, – Populations interact among themselves– Populations interact with the physical environment– The abiotic components of an ecosystem are the

nonliving components:• Atmosphere

• Water

• Soil

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Dynamics of an Ecosystem

– The biotic components of an ecosystem are living things that can be categorized according to their food source:• Autotrophs• Heterotrophs

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Dynamics of an Ecosystem

• Autotrophs– Require only energy and inorganic nutrients

• Generate the food necessary for the ecosystem

– Require only inorganic nutrients and an outside energy source to produce organic nutrients

– Photoautotrophs• Land plants and algae

– Chemoautotrophs• Some bacteria

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Dynamics of an Ecosystem

• Heterotrophs

– Need a preformed source of organic nutrients as they acquire food

– Consumers – consume food generated by a producer

• Herbivores - Feed on plants

• Carnivores - Feed on other animals

• Omnivores - Feed on plants and animals

• Detritivores – Feed on decomposing organic matter

• Decomposers – Break down dead organic matter

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Relationships, I• Trophic structure / levels~ feeding

relationships in an ecosystem• Primary producers~ the trophic level

that supports all others; autotrophs

• Primary consumers~ herbivores• Secondary and tertiary

consumers~ carnivores• Detrivores/detritus~ special

consumers that derive nutrition from non-living organic matter

• Food chain~ trophic level food pathway

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Relationships, II

• Food webs~ interconnected feeding relationship in an ecosystem

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Energy Flow, I• Primary productivity (amount of light energy converted to chemical energy by

autotrophs)– Gross (GPP): total energy – Net (NPP): represents the storage of energy available to consumers

– Rs: respiration

• NPP = GPP - Rs

• Biomass: primary productivity reflected as dry weight of organic material

• Secondary productivity: the rate at which an ecosystem's consumers convert chemical energy of the food they eat into their own new biomass

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Energy Flow, II• Ecological efficiency: % of E

transferred from one trophic level to the next (5-20%)

• Pyramid of productivity: multiplicative loss of energy in trophic levels

• Biomass pyramid: trophic representation of biomass in ecosystems

• Pyramid of numbers: trophic representation of the number of organisms in an ecosystem

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Dynamics of an Ecosystem

• Chemical Cycling– The pathways by which chemicals circulate through

ecosystems • Involve both living (biotic) and nonliving (geologic)

components

• Known as biogeochemical cycles– Water Cycle

– Carbon Cycle

– Phosphorus Cycle

– Nitrogen Cycle

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Dynamics of an Ecosystem

• Chemical Cycling– May involve:

• Reservoir - Source normally unavailable to producers– Ex: carbon present in calcium carbonate shells on ocean bottoms

• Exchange Pool - Source from which organisms generally take chemicals

– Ex: Atmosphere, soil

• Biotic Community - Chemicals remain in food chains, perhaps never entering a pool

– Human activities result in pollution because they upset the normal balance of nutrients

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Model for Chemical Cycling

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Reservoir• fossil fuels

• atmosphere• soil• water

Community

• mineral in rocks• sediment in oceans Exchange

Pool

producers

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Dynamics of an Ecosystem

• The Water (hydrologic) Cycle– Transfer rate

• The amount of a substance that moves from one component of the environment to another with a specified period of time

– Fresh water evaporates from bodies of water– Precipitation on land enters the ground, surface

waters, or aquifers– Water eventually returns to the oceans

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The Hydrologic (Water) Cycle

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net transport of water vapor by wind

Ocean

Ice

Groundwaters

lake

aquifer

freshwater runoff

H2O in Atmosphere

evaporationfrom ocean

precipitationto ocean

transpiration from plantsand evaporation from soil precipitation

over land

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Dynamics of an Ecosystem

• The Carbon Cycle– Atmosphere is an exchange pool for carbon dioxide

– In water, carbon dioxide combines with water to produce bicarbonate ions

– Bicarbonate in the water is in equilibrium with carbon dioxide in the air

– The total amount of carbon dioxide in the atmosphere has been increasing every year due to human activities such as fossil fuel combustion

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The Carbon Cycle

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Land plants

Soils

Ocean

combustion

photosynthesis

respiration

decay

runoff

diffusion

sedimentation

coal

oil

destructionof vegetation

CO2 in Atmosphere

dead organismsand animal waste

naturalgas

bicarbonate (HCO3–)

1

5

4

2

6

6

6

3

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Dynamics of an Ecosystem

• The Carbon Cycle (continued)– Greenhouse effect

• Carbon dioxide, nitrous oxide, methane• Allow sunlight to pass through atmosphere• Reflect infrared back to earth• Trap heat in atmosphere• Leads to global warming and climate change

– If Earth’s temperature rises• More water will evaporate• More clouds will form, and• Setting up a potential positive feedback loop

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Dynamics of an Ecosystem

• The Phosphorous Cycle– Phosphorous from ocean sediments moves on to land via

geologic activity– Weathering of rocks results in the deposition of phosphate

ions in the soil– Phosphate ions become available to plants– Animals obtain phosphate by consuming producers– Death and decay returns phosphate ions to the soil, and to

producers, again.– Some phosphate runs off into aquatic ecosystems

• Excessive phosphorous levels can lead to eutrophication– Over-enrichment of waterways

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The Phosphorus Cycle

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3

organisms

mineable rock

phosphate mining

detritus

Ocean

sedimentation

runoff

fertilizer

plants

animals

decomposers

geologic uplift

weathering8 2

sewage treatmentplants

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phosphatein soil

BioticCommunity

plant andanimal wastes

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4

5

1

phosphatein solution

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Dynamics of an Ecosystem

• Nitrogen Cycle– Atmospheric nitrogen is fixed (nitrogen fixation)

by bacteria• Made available to plants• Nodules on legume roots

– Nitrification - Production of nitrates, which plants can use as a source of nitrogen

– Assimilation-plants take up ammonium and nitrates from the soil and use them to produce proteins and nucleic acids

– Denitrification - Conversion of nitrate to nitrous oxide and nitrogen gas

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The Nitrogen Cycle

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plants

phytoplankton

decomposers

NO3

NO3

NO3

NH4+NH4

+

sedimentation

cyanobacteria

runoff

decomposers

denitrification

denitrifying bacterianitrification

denitrification

N2 fixation

nitrogen-fixingbacteria in nodules

and soil

plant andanimal waste

BioticCommunity

nitrifyingbacteria

denitrifyingbacteria

BioticCommunity

humanactivities

N2 fixation

N2 in Atmosphere

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3

2 4

5

4

3

2

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Dynamics of an Ecosystem

• Human activities and the Nitrogen Cycle– Acid Deposition

• Nitrogen oxides and sulfur dioxide are converted to acids when they combine with water vapor

• Affects lakes and forests• Reduces agricultural yield