R* & niches (and the meaning of everything)

1. R* & niches (and the meaning of everything) Ecology Club 11 Mar 10 Markus Eichhorn

2. Niches • Revision • Classical theory • Modern objections • Empirical niches • Tilman’s R* • ZNGIs • Impact vectors • Supply points • Coexistence criteria

3. Parallel definitions • Species requirements for survival • Grinnell (1917), Hutchinson (1957) • Impacts on the environment • Elton (1927), MacArthur & Levins (1967)

4. Dimension 2 Dimension 1 Hutchinson (1957) • Fundamental niche • Seldom observed • Realised niche • What remains • Implies competition n-dimensional hypervolume

5. MacArthur & Levins (1967) • Empirical frame • Gause’s principle • Lotka-Volterra models • Maximum overlap • Niche packing • Little support • Not falsifiable • Requires evidence of trade-offs • Predation & stress not included

6. What they say… No concept in ecology has been more variously defined or more universally confused than “niche” Real & Levin (1991) I believe that community ecology will have to rethink completely the classical niche-assembly paradigm from first principles Hubbell (2001)

7. Let’s consider the concept of niche – If I knew what it meant I’d be rich. It’s dimensions are n But a knowledge of Zen Is required to fathom the b***h Cottam & Parkhurst in Hurlbert (1981)

8. Reductionism • Plant coexistence • 3 main resources • High local SR • How to differentiate? • Liebig’s Law (1840) • Most limiting → GR • Animals – usually N • Other forces • Main predators • Environmental stress • Often few factors

9. Birth rates Resource availability (R) Per capita effects Death rates Predator density (P)

10. R* (Tilman 1982) Per capita effects R* R*2 Resource availability (R)

11. R* definition • Minimum R level • Birth rate = death rate • dN/dt = 0 • Population persists • Competition • Lower R* wins • Reduces resources • Other factors • Predation (P*) • Stress (S*)

12. Predation Per capita effects P* P*2 Predator abundance (P)

13. Resource B Predator B Predator A Resource A Predator (P) Stress (S) Resource (R) Resource (R)

14. Niche features • Zero net growth isocline (ZNGI) • Describes organism’s response to environment • Equivalent to Hutchinson’s niche • Impact vectors (I) • Per capita effect of organism on the environment • Supply vectors

15. Resource B Predator B Predator A Resource A Predator (P) Stress (S) Resource (R) Resource (R)

16. Resource B Predator B Predator A Resource A Predator (P) Stress (S) Resource (R) Resource (R)

17. Wins Coexist Resource B Wins Resource A

18. Wins Either wins Resource B Wins Resource A

19. Each species has a stronger impact on the predator to which it is most vulnerable Predator B Wins Coexist Wins Predator A

20. Better defended species (P*↑) must be a poorer resource competitor (R*↓) Predator (P) Wins Coexist Wins Resource (R)

21. More efficient competitor (R*↑) more affected by stress Stress (S) Wins Wins Resource (R)

22. Coexisting species • ZNGIs must intersect • Otherwise one spp. always wins • Each has an R* advantage • Impact vectors must α ZNGIs • Stronger impact on most limiting R • Likely for optimal foraging species • Expend more effort on limiting R • Intermediate supply vector • Depends on position of supply point • Intraspecific competition > interspecific

23. Implications • No. spp. = no. limiting resources / predators • Local coexistence only • –ve feedback between requirements & impacts • Regional coexistence through habitat heterogeneity

24. Predictions • Spp. with lowest R* best competitor for that R • Dominance varies with ratio of 2 R • No. spp. ≤ no. limiting R • R supply vector → outcome • Impact vectors → outcome • Coexistence along a gradient through trade-offs • Highest SR at intermediate ratio of 2 R Few tests in animal systems Most in plants / microbes

25. R* evaluation Miller et al. (2007) • Plant v. animal ecologists • Difference largely due to tradition & inertia • Predictions supported but more evidence needed • 41 R* tests → 39:1:1 (Wilson et al. 2007)

26. Tilman (1977) 5 Cyclotella and Asterionella 2 essential Rs 4 3 PO4 (μM) 2 1 0 20 40 60 80 100 SiO2 (μM)

27. Tilman (1982) Park Grassland Experiment

28. Grasshopper diets Same diet, different optima Behmer & Joern (2008)

29. Serengeti browsers Topi v. Wildebeest – unstable equilibrium Leaf Stem

30. Serengeti ungulates Large species win when lots of cell wall Small species when high quality forage Cell contents biomass Murray & Baird (2008) Cell wall biomass

31. Resource B Resource A

32. Resource B Resource A

33. Resource B Resource A Excluded species Invasive species

34. Predator B Predator A Coexistence through variable predator densities

35. Predator Resource

36. Predator Resource Gradient replacement due to either P or R

37. No effect of varying R Stress Resource e.g. rocky shore seaweed species & desiccation

38. Light Nitrogen Pioneers Competitors The successional niche

39. Light Nitrogen Facilitation

40. Light Nitrogen Increased light competition

41. New niche theory • Joint description of the environmental conditions that allow a local population to persist andthe per capita effects on the environment • The ZNGI of an organism, combined with the impact vectors on the ZNGI in the multivariate space defined by the environmental factors Chase & Leibold (2003)