R* & niches
(and the meaning of everything)
Ecology Club
11 Mar 10
Markus Eichhorn
Niches

Revision
– 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
– Seldom observed

Realised niche
Dimension 2
– What remains
– Implies competition
Dimension 1
n-dimensional hypervolume
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
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 nicheassembly 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
– 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
Per capita effects
Birth rates
Resource availability (R)
Death rates
Predator density (P)
Per capita effects
R* (Tilman 1982)
R*
R*2
Resource availability (R)
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*)
Per capita effects
Predation
P*
P*2
Predator abundance (P)
Resource B
Predator B
Predator A
Stress (S)
Predator (P)
Resource A
Resource (R)
Resource (R)
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
Resource B
Predator B
Predator A
Stress (S)
Predator (P)
Resource A
Resource (R)
Resource (R)
Resource B
Predator B
Predator A
Stress (S)
Predator (P)
Resource A
Resource (R)
Resource (R)
Wins
Resource B
Coexist
Wins
Resource A
Wins
Resource B
Either wins
Wins
Resource A
Predator B
Each species has a stronger
impact on the predator to
which it is most vulnerable
Wins
Coexist
Wins
Predator A
Predator (P)
Better defended species
(P*↑) must be a poorer
resource competitor (R*↓)
Wins
Wins
Coexist
Resource (R)
Stress (S)
More efficient competitor (R*↑)
more affected by stress
Wins
Wins
Resource (R)
Coexisting species
ZNGIs must intersect
1.
•
•
Otherwise one spp. always wins
Each has an R* advantage
Impact vectors must α ZNGIs
2.
•
•
•
Stronger impact on most limiting R
Likely for optimal foraging species
Expend more effort on limiting R
Intermediate supply vector
3.
•
•
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
1.
2.
3.
4.
5.
6.
7.
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
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)
Tilman (1977)
5
Cyclotella and
Asterionella
4
PO4 (μM)
2 essential Rs
3
2
1
0
20
40
60
SiO2 (μM)
80
100
Tilman (1982)
Park Grassland Experiment
Grasshopper diets
Same diet, different optima
Behmer & Joern (2008)
Serengeti browsers
Leaf
Topi v. Wildebeest – unstable equilibrium
Stem
Cell contents biomass
Serengeti ungulates
Large species
win when lots
of cell wall
Small species
when high
quality forage
Cell wall biomass
Murray & Baird
(2008)
Resource B
Resource A
Resource B
Resource A
Resource B
Resource A
Excluded species
Invasive species
Predator B
Predator A
Coexistence through variable predator densities
Predator
Resource
Predator
Gradient replacement
due to either P or R
Resource
Stress
No effect of
varying R
Resource
e.g. rocky shore seaweed species & desiccation
Light
Pioneers
Competitors
Nitrogen
The successional niche
Light
Facilitation
Nitrogen
Light
Increased light
competition
Nitrogen
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)