Interspecific Competition

1. Interspecific Competition Chapter 6

2. Interspecific Competition • Individuals of one species suffer reduction in fecundity, survivorship, or growth as a result of exploitation of resources or interference by individuals from another species

3. Interspecific Competition • May exclude one another from particular habitats (no coexistence) • Or may coexist (utilize slightly different habitats)

4. Interspecific Competition • Exploitation or interference

5. Exploitation - diatoms • OK when alone, but one disappears when together • More effective exploiters exclude less effective ones • Depresses resources so low other species can’t survive

6. Interference - barnacles One species pushes other off rock surfaces

8. Highly Assymetric • Consequences not same for both species • One harmed greatly, other less so or not much at all

9. More Effects • Competition for one resource affects competition for other resources • E.g., plants competing for light have ability to compete for nutrients affected

10. Competition Models • Lotka-Volterra model • Examines changes in abundance of one species while in competition with another species

11. Competition Models

12. Competition Models 4 possible outcomes

13. Competition Models • Species 1 is stronger competitor, so 1 wins • Species 2 is stronger competitor, so 2 wins • Both species are stronger competitors on each other than on themselves, so may exist in unstable coexistence • Both species are weak competitors, so exist in stable coexistence

14. Coexistence • Partitioning of resources, differentiation of realized niches • Species deprived of realized niche by a competitor is driven to extinction

15. Competitive Exclusion Principle • If two competing species coexist in stable environment, then they do so as a result of niche differentiation. • If there is no such differentiation, one competing species will eliminate the other.

16. Niche Differentiation • How much niche differentiation is necessary for species to coexist?

17. Niche Differentiation • Interspecific competition tends to increase separation along gradient (reduce overlap) to reduce the effects of the competition

18. Niche Differentiation • Intraspecific competition tends to decrease separation along gradient by expanding each species’ use of the gradient

19. Niche Differentiation • Niche differentiation results from interaction of the effects of intra- and interspecific competition

20. End result? • Environment may not be constant enough for effects of competition to reach final conclusion • Environment may change before one competitor wins

21. Evidence for interspecific comp. • Laboratory experiments • Controlled conditions • Simple environment, unlike natural

22. Evidence for interspecific comp. • Manipulative field experiments • Reduce density of one, see how other responds • Best for sessile organisms

23. Evidence for interspecific comp. • Natural experiments • Competitive release - expansion of niche in absence of competitor compared to size in presence of competitor

24. Evidence for interspecific comp. • Natural experiments • Character displacement - morphological difference between allopatric (species alone) and sympatric (species together) populations