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

2. Niches Revision Empirical niches Coexistence criteria 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. Hutchinson (1957) Fundamental niche Realised niche Seldom observed
What remains
Implies competition
Dimension 1
Dimension 2
n-dimensional hypervolume

5. MacArthur & Levins (1967) Empirical frame Little support
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 Liebig’s Law (1840) Other forces
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 Competition Other factors
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) Impact vectors (I)
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 Impact vectors must α ZNGIs
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 Supported? Producer 1° consumer Detritivore Yes 22.5 5.53 No
8.5
1.5 1
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)
Miller et al. (2007)

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

27. Tilman (1982)
Park Grassland Experiment

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

29. Topi v. Wildebeest – unstable equilibrium
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. Coexistence through variable predator densities
Predator A
Predator B
Coexistence through variable predator densities

35. Resource
Predator

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

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

38. The successional niche
Nitrogen
Light
Pioneers
Competitors
The successional niche

39. Nitrogen
Light
Facilitation

40. Increased light competition
Nitrogen
Light
Increased light competition

41. New niche theory
Joint description of the environmental conditions that allow a local population to persist and the 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)