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CompetitionChapter 13

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Modes of Competition

• Interference: –

• Intraspecific: –

• Interspecific: –

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Bart’s hay field…

http://www.gardenseeker.com/herbs/images/alfalfa_sprouted.jpg

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Intraspecific Competition Among Herbaceous Plants

• Plants compete for…• Competition is more intense at …

• Self-Thinning (-3/2 or -1/2 slope rule)

¿How does this explain influence spacing in plants?

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Niches

• Niche: Summarizes environmental factors that influence growth, survival, and reproduction of a species.

• Gause: Principle of Competitive Exclusion– Two species with identical niches cannot coexist

indefinitely.• One will be a better competitor and thus have higher

fitness and eventually exclude the other.

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Niches

• Hutchinson defined niche as:– n-dimensional hyper-volume• n equates the number of environmental factors

important to survival and reproduction of a species.

– Fundamental niche - hypervolume– Realized niche includes interactions such as

competition that may restrict environments where a species may live.

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Feeding Niches of Galapagos Finches

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Feeding Niches of Galapagos Finches

– Grant found differences in beak size among ground finches translates directly into diet.• Size of seeds eaten can be estimated by measuring

beak depths.– Individuals with deepest beaks fed on hardest seeds.

• After 1977 drought, the remaining seeds were very hard. Thus, mortality was most heavy in birds with smaller beaks.

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Mathematical and Laboratory Models

• Metz summarized models:– Abstractions and simplifications, not facsimiles of

nature.– Man-made construct; partly empirical and partly

deductive.– Used to provide insights into natural phenomena.

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Lotka Volterra

• In general, LV predicts coexistence of two species when, for both species, interspecific competition is weaker than intraspecific competition.

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Paramecia Lab Experiments

What happened when grown together?

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Paramecia Lab Experiments

• Gause demonstrated resource limitation with Paramecium caudatum and Paramecium aurelia in presence of two different concentrations of Bacillus pyocyaneus.– When grown alone, carrying capacity determined

by intraspecific competition.– When grown together, P. caudatum quickly

declined.• Reduced resource supplies increased competition.

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Competition and Niches

• Competition can restrict species to their realized niches.– But if competitive interactions are strong and

pervasive enough, they may produce an evolutionary response in the competitor population.• Changes fundamental niche.

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Niches and Competition Among Plants

• Tansley suggested interspecific competition restricts realized niche of each of two species of bedstraw (Galium spp.) to a narrower range of soil types.

• Can you think of other variables/resources that could be partitioned?

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Competition and Niches of Small Rodents

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Competition and Niches of Small Rodents

• Brown studied competition among rodents in Chihuahuan Desert.– Predicted if competition among rodents is mainly

for food, then small granivorous rodent populations would increase in response to removal of larger granivorous rodents.• Insectivorous rodents would show little or no response.• Results supported hypothesis.

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Character Displacement

• Because degree of competition is assumed to depend upon degree of niche overlap, interspecific competition has been predicted to lead to directional selection for reduced niche overlap.

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Character Displacement

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Character Displacement

• Taper and Case: Necessary criteria:– Morphological differences between sympatric

species are statistically greater than differences between allopatric populations.

– Differences between sympatric and allopatric populations have genetic basis.

– Differences between sympatric and allopatric populations evolved in place, and are not derived from different founder groups already differing in the character.

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Taper and Case: Characteristics

– Variation in the character must have a known effect on use of resources.

– Must be demonstrated competition for the resource and competition must be directly correlated with character similarity.

– Differences in character cannot be explained by differences in resources available to each of the populations.

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