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

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

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

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

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

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)

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)

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

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

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

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*)

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

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

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

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

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

Wins Coexist Resource B Wins Resource A

Wins Either wins Resource B Wins Resource A

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

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

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

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

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

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

R* evaluation Supported? Producer 1° consumer Detritivore Yes 22.5 5.5 3 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)

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)

Tilman (1982) Park Grassland Experiment

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

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

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

Resource B Resource A

Resource B Resource A

Resource B Resource A Excluded species Invasive species

Coexistence through variable predator densities Predator A Predator B Coexistence through variable predator densities

Resource Predator

Resource Predator Gradient replacement due to either P or R

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

The successional niche Nitrogen Light Pioneers Competitors The successional niche

Nitrogen Light Facilitation

Increased light competition Nitrogen Light Increased light competition

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)