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
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
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