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Lecture 7

Lecture 7.doc

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University of Toronto St. George
Spencer Barret

Lecture 7: Species interactions Competition (-/-) whereas mutualism (+/+) Lotka-Volterra equations for two species competing for resources. Logistic equation has a braking term for Intraspecific competition within a species. Add a second braking term for interspecific competition between species. aij per capita effect on I by j = competition coefficient. 2 species can coexist or species 1 always win (N1=K1, N2=0) or species 2 always win (N2=K2, N1=0) or identity of winners may depend on initial N. Coexistence requires both species to inhibit their own growth more than they inhibit each others. Law of competitive exclusion: Lotka-volterra model outcomes became widely interpreted as unstable since complete competitors cannot coexist. This is true in math models, but not true in natural. There are external factors preventing even unstable competition from going to completion. Gause’s experiments saw predators eat all prey and then starve. Predator-prey strongly tended to be unstable unless there is habitat complexity. Natural selection may select for higher competitive ability or for reducing resource use overlap. On intertidal zone, upper limits set by desiccation/dryness/abiotic factor, lower limits set by competition for space on rock with barnacles. Limiting similarity: niches must be sufficiently different to allow coexistence Resource partitioning: organisms that are similar dividing up resources Assembly rules: if species A is established
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