Lecture 9: Interspecific Competition

1. Lecture 9: Interspecific Competition EEES 3050

2. Competition • In the past chapters, we have been discussing how populations grow and what factors determine that growth. • What happens when you put populations of more than one species together?

3. How do species interact? • Competition • Predation • Herbivory • Parasitism • Disease • Mutualism

4. Interspecific Competition • Competition • When two species use the same limited resource to the detriment of both species. • Assessment-some general features of interspecific competition • Competitive exclusion or coexistence • Tilman’s model of competition for specific resources (ZINGIs) • Coexistence: reducing competition by dividing resources

5. Assessment • mechanisms • consumptive or exploitative — using resources (most common) • preemptive — using space, based on presence • overgrowth — exploitative PLUS preemptive • chemical — antibiotics or allelopathy • territorial — like preemptive, but behavior • encounter — chance interactions

6. Modeling coexistence? • Can we model the growth of 2 species? • Remember logistic model? • What is K? • Now we add another factor that can limit the abundance of a species. • Another species.

7. Freshmen and donuts: an example • There is a room with 100 donuts – what does a typical male freshmen do? • First – eat several donuts. (A male freshman can eat 10 donuts) • Second – rapidly tell friends • But not too many! • Third – Room reaches carrying capacity at 10 male freshmen. • So K=10 for male freshmen.

8. Freshmen and donuts: an example • What happens if a male and female discover the room at the same time? • First – eat several donuts. (A female freshman can eat 5 donuts) • Second – rapidly tell friends • But not too many! • Third – Room reaches carrying capacity at ? males and ? females. • What is the carrying capacity? • It depends…

9. Lotka-Volterra • Need a way to combine the two equations. • If species are competing, the number of species A decreases if number of species B increases. • Such that: • Where alpha is the competition coefficient • Lotka-Volterra: A logistic model of interspecific competition of intuitive factors.

10. Freshman Example • In a room we have 100 donuts. • Need 10 donuts for each male freshmen. • So K1 = 10 • Need only 5 donuts for each female freshmen. • So K2 = 20 • If room is at K1 and 1 male leaves, how many females can come in? • So, , where α = 0.5 • And, , where B = 2

11. Possible outcomes when put two species together. • Species A excludes Species B • Species B excludes Species A • Coexistence

12. Changes in population 1:

13. Changes in population 1: Yellow: both increase White: both decrease

14. Changes in population 2: Yellow: both increase White: both decrease

15. Yellow: both increase White: both decrease Green: Sp 1 increase Brown: Sp 2 increase

16. Tilman’s model • Problems with Lotka-Voltera model? • No mechanism • Dr. Tilman developed a model based on resource use.

18. 1 – no species can survive 2 – Only A can live 3 – Species A out competes B 4 – Stable coexistence 5 – Species B out competes A 6 – Only B can live

20. Lab Experiments? • Gause using yeast and Birch using beetles. • Results show both exclusion and coexistence • It was hypothesized that the yeasts had enough differences to allow coexistence. • i.e. the requirements of the 2 species are slightly different.

21. Gause’s hypothesis: • As a result of competition two similar species scarcely ever occupy similar niches… • Also called the competitive exclusion principle: • “Complete competitors cannot coexist.” • Niche – still controversy about the definition. • 1) The role of a species in the community – Elton 1927. • 2) a subdivision of the habitat. - Grinnell 1917.

22. Back to Competition Coefficient • Competition coefficient: • Intensity of competition from species. • In our original donut example: • α= 1/ β • Also, can read about Gause’s yeast populations in book.

23. Changes in population 1: K2 K1 = 10 K2 = 20 α = 0.5 β = 2 K2/β

24. Back to Competition Coefficient • Competition coefficient: • Intensity of competition from species. • In our original donut example: • α= 1/ β • However: in systems that are more complex • the coefficients are not necessarily reciprocals. • And carrying capacity may not be purely determined by resource being competed for.

25. Changes in population 1: K2 K1 = 10 K2 = 17 α = 0.5 β = 3 K2/β

26. Niche: Hutchinson • Redefinition in 1958.

27. Niche: Hutchinson • Redefinition in 1958. • Two environmental variables, can produce an environmental space or a species niche. • Can add many other environmental factors. • n-dimensional hypervolume • Or a species Fundamental Niche • However, because competition can limit this fundamental niche, what we witness in nature is the: • Realized Niche

28. Can 2 species exist in the same niche?

29. Can 2 species exist in the same niche?

30. Can 2 species exist in the same niche?

31. Can 2 species exist in the same niche? • Observation: • Several types of warblers live in the same tree species. • Hypothesis based on competition theory: • Warblers will use different parts/areas of the trees. • Experiment: • No experiment conducted, but observations can be made to test hypothesis.

37. McArthur suggested competition to explain warbler patterns • Ghost of competition past. • So how do species coexist? • Different food resources, i.e. diet specialization • What about plants? • Plants usually need same resources, water, nutrients, light. • What about phytoplankton?

38. How do phytoplankton live in the same location? • Phytoplankton • Common pool of nutrients • Often large number of species • Same environment, i.e. amount of light, temperature. • In many bodies of water, nutrients are limited. • Reasons? • Environmental instability • Non-equilibrium system.

40. Assumptions of competition theory • life history characteristics of species are adequately summarized by the per capita growth rate of species; • deterministic equations are sufficient to model population growth, and environmental fluctuations need not be considered; • the environment is spatially homogeneous and migration is unimportant; • competition is the only important biological interaction; and • coexistence requires a stable equilibrium point.

41. How to determine if interspecific competition has occurred (or is occurring)? • From Wiens (1989) • Need a checkerboard distribution • Species overlap in resource use • Intraspecific competition occurs • Resource is limited • One or more species is limited • Other hypotheses do not fit.

42. Example where criteria 1 and 2 fit. Feeds far from shore Feeds near shore

43. Test theory with plants • Observation: • Plants all require light, nutrients and water. • Plant often found together. • Hypothesis: • Competition between plants ought to be common. • Plants do worse with other plants. • Experiment: • Results:

44. Experimental design. Compare the growth of annuals and shrubs in the Mojave Desert. 2 experiments Effects of annuals on shrubs Effects of shrubs on annuals How? Effects of annuals on shrubs Effects of shrubs on annuals

46. Results: Annual had positive benefits from shrubs. Shrubs had negative benefits from shrubs.

47. Facilitation: • Where one or both species benefit (have a positive effect) due to the presence of the other species.

48. Resource utilization curves • Species may evolve to minimize the impact of competition.

49. Character Displacement • Definition: • In areas where species overlap, there has been a divergence between the two species, supposedly as a result of competition.

50. Example: Galapagos finches • Theory: • According to displacement theory, species that are sometime found together will have a character that has changed compared to when the species are found by themselves. • Observation: • There are three species of finches in the Galapagos that are sometimes found together and sometimes separate. • Hypothesis: • There will be differentiation in bill size when species are on the same island: • Test (Not truly an experiment): • Examine the bill size of three species. • Results: