Competition

1
Competition

• Interaction between individuals, brought about by
  a shared requirement for a resource in limited
  supply
• Leads to a reduction in survivorship, growth,
  and/or reproduction of at least some of the
  competing individuals

2
What are competitive species?

• Grime species with capacity for resource
  capture and rapid growth, when resources are
  readily available defined by mechanism
• Tilman species which have a low equilibrium
  resource requirement and maintain growth when
  resources are limited defined by outcome

3
Quantifying competition

• Competitive effect of a plant on its neighbors
• Competitive response of a plant to its
  neighbors
• Competitors as defined by Grime, Tilman usually
  focus on whether there is a competitive effect

4
Competition and the Niche

• Competition should be most severe when
  individuals are similar in resource requirements
  or life form
• Predict intraspecific competition very high
• Predict that interspecific competition may lead
  to niche divergence

5
Intraspecific Competition

• The law of constant yield

6
Intraspecific Competition

• Skewed size distributions

7
Intraspecific Competition

• Emergence order and skewed size distribution

8
Intraspecific Competition

• Self thinning rule, W c N (-3/2)
• Growth occurs only as individuals die but
  growth faster than death

9
Intraspecific Competition

• Slope approaches 1 when death balances growth
  (e.g. when growth is slow, as in shade)

SUN SHADE
In Lolium perenne
10
Intraspecific Competition

• Variation in allocation strategy with increased
  density

In Foxglove Digitalis purpurea
11
Intraspecific Competition

• Variation in allocation strategy with increased
  density

In corn Zea mays
12
Interspecific competition

• Studied both in the field (removal, addition
  experiments) and in the greenhouse (additive and
  replacement designs)
• Some controversy about methodology
• Greenhouse expts have greater control
• Difficult to extrapolate from greenhouse to nature

13
Interspecific competition

• Partial additive vs. replacement designs

Additive Replacement
14
Interspecific competition

• Partial additive inappropriate cant
  disentangle effects of total density and relative
  proportions
• Replacement allows comparison of inter- and
  intra-specific competition, but results depend
  entirely on density chosen

15
Interspecific competition

• Best design multiple replacement series, also
  called complete additive series
• What we used in labmanipulate both density and
  proportion, so can infer relative importance of
  both inter- and intra-specific competition

16
DeWit Plots
Yield is normally biomass, absolute (not
relative) values are presented
Species I J
Yield
Dashed expected line if interspecific and
intraspecific competition are equal
Species I
Species J
0.5
1
0
Proportion species I
17
DeWit Plots
18
DeWit Plots

• Model 1 effect of sp I on sp J and vice versa
  equivalent to intraspecific competition
• Interpretation size may differ between the
  species but they have similar niches, ability to
  capture and use resources

density
19
DeWit Plots

• Model 2 Asymmetric competition. One species is
  a superior competitor, and has a greater
  interspecific than intraspecific competitive
  effect

20
DeWit Plots

• Model 3 Mutual antagonism. Interspecific
  competition greater than intraspecific for both
  species.

21
DeWit Plots

• Model 4 Interspecific competition less than
  intraspecific competition for both species. May
  result from niche divergence.

22
Interspecific competition

• Grime thought competition unimportant in
  low-resource environments or highly disturbed
  environments
• Expect stress tolerators or ruderals
• Tilman argued that competition important in all
  environments, but resource being competed for
  differed
• High resource envts, compete for light low
  resource envts, compete for nitrogen

23
Tilmans model

• Zero net growth isoclines (ZNGI) minimum
  resource requirements for species
• ZNGI relatively lower species a better
  competitor for that resource (requires LESS of
  it)
• ZNGI relatively higher resource can be
  considered a limiting factor for the species

24
Tilmans model
R2B
R2A
R1A
R1B
25
Tilmans model

• If ZNGIs overlap, we add consumption vectors (C)
  to illustrate how each species uses resources
• If each species consumes more of the resource
  that limits itself, get coexistence
• If each species uses more of the resource that
  limits the other species, outcome is unstable

26
Tilmans model
27
Tilmans model
28
Tilmans model

• Unstable outcome unpredictable which species
  will exclude the other
• Depends on initial conditions relative amount
  of each resource, which species present first,
  etc.

29
Tilmans experiments

• Start by measuring R the amount of resource
  left when a population of a single species
  reaches equilibrium density
• Species with lowest R should competitively
  exclude all others
• Focus Nitrogen. In grasslands, below-ground
  competition considered more important

30
Tilmans measure of R
Poor competitor
Good competitors
Tilman and Wedin.1991. Ecology 72685-700
31
Tilmans experiments

• Introduce seeds into field plots with known
  initial N level
• Best competitors will exclude others, as
  predicted by R

32
Tilmans field expt.
Tilman and Wedin.1991. Ecology 721038-1049
33
Other factors in competition

• Often diffuse rather than pairwise
• Indirect effects from other species may be
  important

34
Competition and indirect effects 1

• Dodder (Cuscuta) in salt marsh changed
  competitive relationships

35
Competition and indirect effects 1
Callaway and Pennings. 1998. Oecologia 114
100-105.
36
Competition and indirect effects 2

• Mixtures of rice have reduced disease
  transmission

Zhu et al. 2000. Nature 406718-722
37
Competition summary

• Local density may be more important than total
  density
• Small initial differences in size can have large
  consequences later
• We can (sometimes) predict the outcome of
  competitive interactions, but indirect effects
  complicate things (more when we do multitrophic
  interactions)