Competition and Niches: Modes, Models, and Interactions

1. Competition Chapter 10

2. Outline • Resource Competition • Modes • Niches • Mathematic and Laboratory Models • Lotka-Volterra • Competition and Niches • Character Displacement

3. Modes of Competition • Interference: • Direct aggressive interaction between individuals. • Intraspecific: • Competition with members of own species. • Interspecific: • Competition between individuals of two species - reduces fitness of both.

4. Intraspecific Competition Among Herbaceous Plants • Plant growth rates and weights have been found to increase in low density populations. • Competition for resources is more intense at higher population densities. • Usually leads to mortality among competing plants. • Self-Thinning

5. Intraspecific Competition Among Herbaceous Plants

6. Intraspecific Competition Among Planthoppers • Denno and Roderick attributed prevalence of competition among Homoptera to their habit of aggregating, rapid growth, and the mobile nature of their food supply. • Demonstrated intraspecific competition within populations of planthopper Prokelesisia marginata. • Probably result of limited resources.

7. 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.

8. 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.

9. 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.

10. Feeding Niches of Galapagos Finches

11. 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.

12. Lotka Volterra • Effect of interspecific competition on population growth of each species: • dN1 / dt = rm1N1 ((K1-N1-12N2) / K1) • dN2 / dt = rm2N2 ((K2-N2- 21N1) / K2) • 12: Effect of individual of species 2 on rate of pop. growth of species 1. • 21: Effect of individual of species 1 on rate of pop. growth of species 2.

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

14. Lotka Volterra • Predict population growth for the two species will stop when: • N1=K 1-  12 N2 and N2 = K 2-  21 N1 • Zero Growth Isoclines • Above: Population decreasing • Below: Population increasing • Coexistence of two species is only possible when isoclines cross.

15. Lotka Volterra

16. 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.

17. Paramecia Lab Experiments

18. Flour Beetle Experiments • Tribolium beetles infest stored grain products. • Park studied interspecific competition between T. confusum and T. castaneum under varied environmental conditions. • Growing the two species together suggested interspecific competition restricts the realized niches of both species to fewer environmental conditions.

19. 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.

20. 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.

21. Niche Overlap and Competition Between Barnacles • Connell discovered interspecific competition in barnacles. Balanus plays a role in determining lower limit of Chthamalus within intertidal zone. • Did not account for all observed patterns.

23. 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.

24. Competition and Niches of Small Rodents

25. 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.

26. Character Displacement

27. 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.

28. 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.