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Community EcologyCommunity - a group of organisms, of different species, livingin the same areaCommunity ecology is the study of the interactions betweenspeciesThe presence of one species may affect the ability of anotherspecies to live in the same area positively, negatively, or haveno effect.

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Type ofinteraction

Effect ofSpecies 1 on

Species 2

Effect ofSpecies 2 on

Species 1Competition - -Predator-Prey + -Parasite-Host + -Consumer-Consumee

+ -

Mutualism + +Commensalism + 0

Classification of types of interspecific interactions:

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Competition• among individuals of a single species is called intraspecific

competition - this results in density dependent effects on population growth

• between members of different species is called interspecificcompetition

Two species can have negative effects on each other throughmutual consumption of a limited resource

A resource is anything that if decreased will decrease the numbers or growth rate of a species in the area.

Competition for resources occurs if consumption by one individual reduces availability of the resource to others.

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Two types of interspecific competition:• Contest or Interference competition - one or both species

actively defend a resource against consumption by the otherspecies

• Exploitative competition - both species use the resource butneither defends it

Contest competition - involves active defense of a resourceexamples: Red cockaded woodpeckers defend nest holes from

occupation by squirrelsDamselfishes defend mats of algae from consumption by other

fishes Hummingbirds in tropics defend nectar producing trees from other nectar feeding birds and insects

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Exploitative competition - competition through consuming activities of an animal or plant

examples: Canopy trees consume light and limit its availabilityto understory plants

Many fish species feed on the same invertebratesExploitative competitors may not even encounter each other

because they feed at different times or on different life stagesof prey

Example: in the deserts the principal consumers of seeds are antsand birds

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The limiting resource concept -In natural systems it is common to find that one or at most a few resources are in limited supply for a speciesIn plants, it is commonly just one soil nutrient that is in limitedsupply.

Plant nutrients: N, P, K and micronutrients - all are required forplant growth. If any one becomes reduced below a critical valuethen growth of the plant or the plant population is limited.

Leibig’s Law of the Minimum - each population increases untilthe supply of some resource no longer satisfies the population’srequirement for it.

Which nutrient becomes limited first often depends up the rate ofconsumption of the nutrient by the species using it.

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N

P

dN/dt > 0

dN/dt < 0

Resource utilizationwill reduce both Nand P, but N will bereduced to limitinglevel before P.

N* limiting level of N

P*

limitinglevel ofP

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Competition Experiments - exclusion and coexistence

Early experiments (by Gause) with cultures of more than speciesof Paramecium showed that one species can cause the extinctionof another

Each species hadlogistic growthwhen culturedalone

When cultured in pairseither exclusion orcoexistence occurred

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Gause’s Competitive Exclusion Principle - when twocompetitors utilize the same required resource that is available inonly limited quantities, one species will cause the extinction ofthe other - “complete competitors cannot coexist”

Exclusion doesn’t happen if the two species are able to exploitportions of their niche that don’t overlap with the other’s

Gause found P. caudatum was able tofeed on bacteria at the top layers ofthe culture tube and excluded P.bursaria from that zone. P. bursariafed on yeast in the bottom layers ofthe culture and excluded P. caudatumfrom that zone.

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The outcome of competition oftendepends on the environment

If one environment is morefavorable to one species -allowing faster growth or a highercarrying capacity - it may be ableto out-compete a species thatwould replace it in anotherenvironmental situation.

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Fencedenclosuresconstructed toexcludeKangaroo ratsbut notexcludesmaller rodents

A larger number of smaller rodents could live inside theenclosure when Kangaroo rats were excluded -

They were each exploiting some of the same resources - afterremoval, the small rodents did not have Kangaroo rats ascompetitors

Competition in nature - many field experiments have shown thatcompetition is an important process in natural communities.

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Field experiments have demonstrated that the effects ofcompetition are often not symmetrical.

Sceloporus merriami has a negative effect on Urosaurus ornatusbut the reverse is not true. S. merriami is limited more by abioticfactors. U. ornatus is limited by biotic factors (competition).

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Classic example of competition in nature - interaction betweenbarnacles - Chthamalus and SemibalanusChthamalus lives in the intertidal

zoneSemibalanus lives in the subtidal

zoneIf Semibalanus is removed

Chthamalus also utilizes thesubtidal zone

Semibalanus is unable to live inthe intertidal zone due to desiccation

Semibalanus excludes Chthamalus from the subtidal zone throughits rapid growth - it undercuts or crushes Chthamalus

The realized niche is smaller than the fundamental niche forChthamalus

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Mathematics of Competition - the Lotka-Volterra equation

1 1 1 21

1

( )dN K N NrNdt K

α− −=

The logistic equation describes the growth of a single specieswhen there is a limited supply of resource.

This equation can be modified to incorporate the effect of asecond species:

( )dN K NrNdt K

−=

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The Lotka-Volterra equation incorporates the effect of a secondspecies on species 1.

α represents the effect of an individual of species 2 on species 1relative to the effect of an individual of species 1 on species 1.

1 1

1 1

2 2

2K

Here the effect of species 2 on species 1 is less than the effect ofspecies 1 on species 1: α < 1.

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1 1 1 21

1

( )dN K N NrNdt K

α− −=

N1

N2

K1

K1/αdN1/dt < 0

dN1/dt > 0

dN1/dt = 0

Any combinationof N1 and N2 thatfalls on the greenline represents apoint wherespecies 1 will notgrow or decline

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2 2 2 12

2

( )dN K N NrNdt K

β− −=

The effect of species 1 on species 2 can also be expressed by theLotka-Volterra Equation

Here, β represents the effect of species 1 on species 2 relative tothe effect of species 2 on species 2

18N1

N2

K2/β

K2

dN2/dt < 0

dN2/dt > 0

dN2/dt = 0

2 2 2 12

2

( )dN K N NrNdt K

β− −=

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N1

N2

K2/β

K2

dN2/dt = 0

K1

K1/α

dN1/dt = 0

When species 1 and species 2 are considered together 4different outcomes are possible

Case 1: Species 2 wins

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N1

N2

K2/β

K2

dN2/dt = 0

K1

K1/α

dN1/dt = 0

Case 2: Species 1 wins

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N1

N2

K2/β

K2

dN2/dt = 0

K1

K1/α

dN1/dt = 0

Case 3: Stable coexistence

Equilibrium population sizes

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N1

N2

K2/β

K2

dN2/dt = 0

K1

K1/α

dN1/dt = 0

Case 4: Unstable coexistence

Unstable equilibrium

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Predation is an important factorin community organization andcan reduce the effects ofcompetition. Predators cankeep each species below itscarrying capacity - allowingcoexistence, so long as thepredator is present.

Environmental disturbances anddisease can have similar effects- keeping species below thelimiting level of resources.

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When species coexist through utilization of different types ofresources or different ranges of a limiting resources it is called“Resource Partitioning” or “Niche Partitioning”

Anolis lizards in Cuba live in the same area but exploit differentmicrohabitats

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Species under continual competition will sometimes evolvedifferences because of selection against those individuals thatsuffer the most severe competition

Finches in the GalapagosIslands feed on seeds andtheir beak size determinesthe size seeds they can eat

When found on differentislands (allopatric) the beaksizes are similar

When found on the same island (sympatric) they have divergentbeak sizes - called “Character Displacement”

Competition for seeds is highest on intermediate sized seeds - onespecies specializes on large seeds, the other on small seeds