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

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Presentation on theme: "R* & niches (and the meaning of everything) Ecology Club 11 Mar 10 Markus Eichhorn."— Presentation transcript:

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

2 Niches Revision –Classical theory –Modern objections Empirical niches –Tilmans 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 –Seldom observed Realised niche –What remains –Implies competition Dimension 1 Dimension 2 n-dimensional hypervolume

5 MacArthur & Levins (1967) Empirical frame –Gauses 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 Lets consider the concept of niche – If I knew what it meant Id be rich. Its 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? Liebigs Law (1840) –Most limiting GR –Animals – usually N Other forces –Main predators –Environmental stress Often few factors

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

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

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 Predator abundance (P) Per capita effects P*P*2

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

14 Niche features Zero net growth isocline (ZNGI) –Describes organisms response to environment –Equivalent to Hutchinsons niche Impact vectors (I) –Per capita effect of organism on the environment Supply vectors

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

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

17 Resource A Resource B Wins Coexist

18 Resource A Resource B Wins Either wins

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

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

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

22 Coexisting species 1. ZNGIs must intersect Otherwise one spp. always wins Each has an R* advantage 2. Impact vectors must α ZNGIs Stronger impact on most limiting R Likely for optimal foraging species Expend more effort on limiting R 3. 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 1. Spp. with lowest R* best competitor for that R 2. Dominance varies with ratio of 2 R 3. No. spp. no. limiting R 4. R supply vector outcome 5. Impact vectors outcome 6. Coexistence along a gradient through trade-offs 7. Highest SR at intermediate ratio of 2 R Few tests in animal systems Most in plants / microbes

25 R* evaluation 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) Supported?Producer1° consumerDetritivore Yes No Miller et al. (2007)

26 Tilman (1977) Cyclotella and Asterionella 2 essential Rs SiO 2 (μM) PO 4 (μM)

27 Tilman (1982) Park Grassland Experiment

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

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

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

31 Resource A Resource B

32 Resource A Resource B

33 Resource A Resource B Excluded speciesInvasive species

34 Predator A Predator B Coexistence through variable predator densities

35 Resource Predator

36 Resource Predator Gradient replacement due to either P or R

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

38 Nitrogen Light Pioneers CompetitorsThe successional niche

39 Nitrogen Light Facilitation

40 Nitrogen Light Increased light competition

41 New niche theory 1. Joint description of the environmental conditions that allow a local population to persist and the per capita effects on the environment 2. 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)


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