Community Ecology

Key QuestionHow does Species Diversity Affect the

Sustainability of a Community

Organisms have a Habitat and a Niche in an Ecosystem

Example: Zebra MusselsHabitat: freshwater lakes, rivers and streams

Niche: feeds on green algae

Habitat vs. niche

• Habitat– Space an organisms

inhabits• Where it lives• Its address

– Several species can share the same habitat

– Based on biological requirements

• Niche– Functional role of an

organism• Its job

– unique to each organism– How it affects other

organisms and the environment

Key QuestionWhat role do species play and how do they

interact in a Community?

Niche Structure = how many occur, how unique are they, and how do species occupying different niches interact

Role of Species in an Ecosystem• Native: Normally live and thrive in a particular ecosystem

• Alien: Deliberately or accidentally introduced to an ecosystem - An invader

• Indicator: Marks a specific condition in the ecosystem or serves as an early warning to degradation

• Keystone: maintains diversity in the ecosystem, positively affects other organisms, top predator keystone species regulate populations

• Foundation species: Help create habitats and ecosystems

Organisms living in direct contact can benefit or harm each other as they interact

Relationships Between Organisms

• Symbiosis – Long term relationship between organisms of two different species living in direct contact– Mutualism – benefits both organisms

– Parasitism – benefits one and harms the other organism

– Commensalism – benefits one organisms while the other is not affected

• Trials of Life Pt1• Trials of Life Pt2• Trials of Life Pt3• Trials of Life Pt4

mutualism

Mutualism between acacia trees and ants

Ants feed on nectarand protein rich sacks

Attack and sting invadingInsects and eat fungal spores

Prune back nearing vegetation

Tree is protected from invasionProvides home and food for

the ant

Parasitism negatively effects population growthParasite = organism that takes nourishment from a host organism

sea lamprey

Parasitism – Misletoe

Commensalism

What type of relationship exists between these organisms?

Predator/Prey RelationshipsAre not symbiotic – One species (the predator) feeds

directly on another species (the prey) - they are not in long association with each other

Predation• Plays an ecological role in evolution by

natural selection– Predators kill less fit members of a

population– Survivors reproduce offspring with

adaptations that help them avoid predation

• Controls population levels– Adaptations to decrease the impact of

predators (protection mechanisms)• clumping• run, swim, fly• highly developed sight and smell• shells, bark, thorns, chemicals (stinging

nettles, skunk, octopus)• camouflage (insects, birds)• mimicry (viceroy butterfly)• Deceptive markings• Warning coloration

Predator – prey relationships are usually cyclical

Platable or harmless speciesLarval stage of hawkmouth moth Unpalatable or poisonous snake

Coral snake Milk Snake

Cross-mimicry: benefiting bothPredators learn to avoid coloration pattern

Peppered Moth

Interspecific Interactions – effects on population

Key Question

How do Communities Respond to Changing Environmental

Conditions?

Ecological Succession• Communities gradually change their structure and

species composition in response to environmental conditions(depend on biotic and abiotic factors)

- 2 types1. Primary succession – establishment of a community from barren ground – can take thousands of years to form fertile soils2. Secondary succession – in response to disturbance - can be positive as nutrients are cycled and new niches open up

Intermediate Disturbance Hypothesis - Fairly frequent but moderate disturbance leads to increased species biodiversity (richness and evenness

Primary succession – gradual establishment of community in a lifeless area

Secondary succession

Succession on Mt. St. Helens

Population

• A group of the same kind of organisms (of a single species) living in a given space at a given point in time

• Defined by:– type – time– Space

Key Question

What limits the growth of populations?

Populations establish various distribution patterns (dispersion)

• 3 types– Clumping

• Most common – example: schools of fish• Size / location varies with availability of resources

– Species clump toward resources• Provides protection from predators and therefore population

decline• Increases ability to catch prey• Groups form for mating and caring for young

– Uniform dispersion• Provides access to scarce resources

– Random dispersion (fairly rare)

Clumped dispersion: buffalo, swans, fish, lupine

Four rates determine change in population size

– Natality = birthrate: number of births per unit time

– Mortality = death rate: number of deaths per unit time

– Immigration: number of individuals moving into a population over time

– Emigration: number of individuals moving out of a population over time

Additions = removals

Effect onPopulation

size

positive

negative

positive

negative

Zerogrowth

Dispersal

• The ability of populations to spread from a central place– Active – animals– Passive – plants: move by other organisms or mechanisms– Barriers block dispersal

• Physical: mountains, water, land forms, food source• Behavioral: pack mentality, breeding preferences, territorialism,

tradition

Limiting Factor Principle– Any abiotic or biotic factor may be critical to the

success of organisms and affect the growth of a population (even if other factors are optimal)

The best soils for agriculture have no or few limiting factors.

Population Growth Limiting Factors• Density Dependent - Effect is independent of how big

population is• Weather: wind, rain, drought, fire, hurricane• Human activities: pesticides, clear cutting

• Density Dependent – Effect depends on the density of the population– 1. Competition for shared or limited resources

• Resources / food• Space - crowding stress

– Dense populations result in higher infant mortality

– 2. Predation– 3. Parasitism or diseases that spread

Stress due to crowding limits population

Predators – Density dependent limiting factor

Population Growth Curves

No population can grow indefinitely

Species have a “biotic potential” for population growth

Intrinsic rate of increase (r) = inherent reproductive capacity– unlimited resources– no limiting factors

• Examples: bacteria, geese, humans

– Results in exponential growth

• J-shaped growth curve

Exponential GrowthThe larger a population gets the faster it grows

Lag phase:Positive growth

starts slowly

Boom phase:Exponential growth

Example of exponential population growth in nature

Carrying Capacity (K)• The maximum population a habitat can support

without degrading the environment

– Abiotic and biotic factors limit population size• Competition• Scarce resources

– Environmental resistance – combination of all factors that limit population growth

– Carrying capacity is determined by the combination of biotic potential and environmental resistance

Homeostasis – A Stable equilibrium

• Growth rate decreases as a population reaches carrying capacity

• Growth rate averages zero• Population reaches equilibrium = Maintenance of stability in

numbers of individuals

• Viewed over a long period of time• Takes into account fluctuations in population density

• Populations are dynamic and changing

Homeostasis – maintaining stability

Populations usually carrying capacity and growth stabilizes

Logistic growth = growth with limits curve (s-shaped)

Homeostasis

Fig. 6-11, p. 119

Exponentialgrowth

Environmentalresistance

Population stabilizes

Carrying capacity (K)

Bioticpotential

Time (t)

Popu

latio

n si

ze (N

)

Fig. 6-12, p. 119

Population recoversand stabilizes

Carrying capacity

Populationruns out ofresources and crashes

Exponentialgrowth

Populationovershootscarryingcapacity

Year

Num

ber o

f she

ep (m

illio

ns)

Reproductive strategies and population fluctuations(most organisms have reproductive patterns between (r) and (K))

• 2 categories– K-strategists (Deer, lion, whale, human) = competitors

• Large organisms, long lives, produce few offspring, provide care for offspring

• Populations stabilize at carrying capacity• Controlled by density-dependent limiting factors

– r-strategists (insects, mice, fish) = opportunists• Small, short life, produce many offspring (high capacity for

population increase)• Produce many offspring but don’t reach carrying capacity• High mortality among young (little parental care)• Populations fluctuate wildly (boom and bust model)• Controlled by density-independent limiting factors

Growth Rate CurvesK-strategist vs. r-strategist

Predator – prey relationships cause population fluctuations

Class dataTreatment Ave K value Ave r value

control 5.3 .57

Saline - low 8 .44

medium 9 .3

high 8 .36

PO4 - low 12.7 .68

medium 9 .50

high 14.5 .52

NO3 - low 14 .26

medium 6.5 .30

high 7 .39

Shade - low 9.5 .287

medium 6 .219

high 5 .107

Ph 5 9 .288

Ph 6 22 .231

Ph 7 7 .357

Ph 8Ph 9

7.3

.92

.057