Competition
• Mutually negative interaction between two
species in the same guild or trophic level
• Changes in abundance, fitness, or some
fitness component (growth, feeding rate, body
size, survival)
Topics for today
• Mechanisms and models of competition
• Evidence for competition
– Experiments (lab, field)
– Observational (competitive exclusion, character
assortment or displacement)
• Latest advances in the study of competition
– Genetic diversity/distance and competition
– Functional trait complementarity and competition
– Habitat filtering vs. competitive exclusion using
phylogenetic methods
Schoener (1983): Mechanisms
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Consumption
Pre-emption
Overgrowth
Chemical interactions
Territoriality
Encounter competition
• Can we think of other sorts?
Schoener 1983
Lotka-Volterra models of competition
• Based on estimates of logistic population
growth, and how this differs in monoculture
vs. mixtures
Values of population sizes of two species, N1 and N2, that result in positive, negative, or
zero population growth for interacting species. The zero growth isoclines are shown as a
solid line for species 1 and a dashed line for species 2. [from Morin 1999]
Species 1
Species 2
2500
2500
dN1/dt < 0
2000
2000
K1/a12
1500
1500
N2
N2
dN1/dt = 0
1000
500
0
dN2/dt < 0
1000
K2/a21
dN2/dt > 0
dN1/dt > 0
500
dN2/dt = 0
500
K1
0
K2
0
1000
1500
N1
2000
2500
0
500
1000
1500
N1
K is carrying capacity; dN/dt is population growth rate; a is competition
coefficient with a12 being effect of sp. 2 on sp.1
2000
2500
Species 1
Figures from Gotelli, “A Primer of Ecology”
Species 2
Competitive Exclusion
...of species 2 by species 1
Figs from Gotelli
...of species 1 by species 2
Equilibrium: stable coexistence vs.
unstable competitive exclusion
coexistence
Figs from Gotelli
“winner” depends on priority effects
Tilman’s mechanistic model
R2
R2B
R2A
R1A
R1B
R1
Tilman’s model
• If ZNGI’s 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
Tilman’s model
R2
R1
Tilman’s model
R2
R1
Experimental evidence supporting Tilman’s model
Competitive ability can be measured by
species traits in monoculture
• 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
Evidence for competitive ability:
Tilman’s measure of R*
Poor competitors
remove less N
Good competitors
remove more N
Tilman and Wedin 1991
Roots are the foraging organ: mass
correlated with N assimilation
Tilman’s field expt.
Wedin and Tilman 1993.
Chthamalus stellatus
Connell’s barnacle
experiment
Nucella = Thais lapillus
Balanus balanoides
Hairston’s salamanders
Hairston 1980
Low overlap as a consequence of competition
Anoles in the Lesser Antilles
Similar body size
and perch height
Anolis gingivinus
Little overlap in
size or perch
height
Anolis wattsi
Anolis bimaculatus
Treatments with “W”: competing with A. wattsi
Similar
niche
Different
niche
Pacala and Roughgarden 1982
High body size and perch height overlap
results in competition
Patterns from field experiments
• Are there traits that predict who ‘wins’?
• Are there traits or patterns that predict where
competition is more intense?
Observational evidence for competitive
exclusion: MacArthur’s Warblers
Galapagos finch bill sizes differ more in
sympatry than allopatry
But differences in bill size between co-occurring pairs
no different than expected by chance??
Strong et al 1979
Yet the Grants showed that evolution
did indeed occur
Large-beaked G. fortis (A) and G.
magnirostris (B) can crack or
tear the woody tissues of T.
cistoides mericarps (D), whereas
small-beaked G. fortis (C)
cannot.
Drought selects
for larger beaks
in fortis (only
large seeds
magnirostris introduced:
available)
has really large beak
Drought causes
competition: selects
for divergent (small)
beaks in fortis
Grant and Grant 2006
Desert cats DO show character
displacement when tested
against null models
Canine size for each species/sex in two locations
Dayan et al 1990
And so do bat-pollinated Burmeistera
Muchhala and Potts 2007
All images from http://www.bio.miami.edu/muchhala/home.html
Current areas of inquiry
• Is competition stronger for closely related
species? And, can we infer whether
competitive exclusion has occurred using
phylogenetic methods?
• Is competition stronger for species in the
same functional group?
Darwin 1859
“As species of the same genus have
usually, though by no means invariably,
some similarity in habits and
constitution, and always in structure, the
struggle will generally be more severe
between species of the same genus, when
they come into competition with each other,
than between species of distinct genera.”
If habitats select for particular traits, and related
species share traits, expect phylogenetic clustering
(“habitat filtering”)
If competition or other density-dependent factors
are stronger between relatives than between distant
relatives, expect phylogenetic overdispersion
Two different
estimates of
relatedness
Webb 2000:
evidence for
habitat filtering
in tropical trees
NRI: species more related
than expected
(clustering); NTI: not
different from random (do
NOT see overdispersion)
Cavender-Bares et
al 2004: evidence
for competitive
exclusion in oaks
Using experimental
estimates of
competition, closely
related species do
not compete more
intensely
Cahill et al 2008
These are correlations between
competitive effect and phylogenetic
distance.
What do you expect this correlation
to be if closely related things
compete more intensely?
The Cedar Creek Biodiversity Plots
In general....
• Productivity is greater in plots with higher
species richness
• Is this because competition is lower in diverse
plots (more functional groups present)?
Two hypotheses
– Niche Complementarity: different functional
groups use resources differently (in
“complementary” ways), so greater efficiency
– Selection Effect: with higher diversity, greater
chance of “selecting” a competitive dominant in a
plot (ie species that grow large over time)
Fargione et al 2006
Net effect: Difference between
total biomass in a plot and
average biomass of monocultures
increases over time
Complementarity: when positive
(as here) means species have
higher than expected yield in
mixture (attributed to N-fixers and
C4 species presence in highdiversity mixtures)
Selection: if positive would mean
that species with high monoculture
biomass are competitive dominants,
and when present (by chance)
create more total biomass in diverse
plots
Fargione et al 2006
Is it really about functional group diversity,
or another diversity metric?
Cadotte et al 2009
Main points
• Models as a way to think about what we can
measure in the field
• Experiments can show patterns of functional
traits that are important
• Phylogenetic inference can provide new
insight for experimentally intractable systems,
and new interpretation of data from others
Reading for next week
• Connell 1961
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Booth, R. E., and J. P. Grime. 2003. Effects of genetic impoverishment on plant community diversity. Journal of Ecology
91:721–730.
Cadotte, M. W., J. Cavender-Bares, D. Tilman and T.H. Oakley. 2009. Using phylogenetic, functional and trait diversity to
understand patterns of plant community productivity. PLoS ONE 4(5): e5695.
Cavender‐Bares, J., D. D. Ackerly, D. A. Baum, F. A. Bazzaz. 2004. Phylogenetic Overdispersion in Floridian Oak Communities.
Am Nat 163:823-843
Connell, J. H. 1961. The influence of interspecific competition and other factors on the distribution of the barnacle
Chthamalus stellatus. Ecology 42: 710-723.
Connell, J. H. 1980. Diversity and the Coevolution of Competitors, or the Ghost of Competition Past. Oikos 35:131-138.
Dayan, T. , D. Simberloff. 2005Ecological and community-wide character displacement: the next generation. Ecology Letters
8: 875-894.
Dayan, T., D. Simberloff, E. Tchernov, Y Yom-Tov. 1990. Feline canines: community-wide character displacement among the
small cats of Israel. Am. Nat. 136: 39-60.
Fargione, J.; Tilman, D. 2006. Predicting relative yield and abundance in competition with plant species traits. Functional
Ecology 20:533-450.
Gause G. F. 1934. The struggle for existence. Williams & Wilkins.
Grant, P.R. and B. R. Grant. 2006. Evolution of character displacement in Darwin’s finches. Science 313: 224-226.
Hairston N. G. 1980a The experimental test of an analysis of field distributions: competition in terrestrial salamanders.
Ecology 61: 817-826.
JF Cahill, SW Kembel, EG Lamb, and PA Keddy. 2008. Does phylogenetic relatedness influence the strength of competition
among vascular plants? Perspectives in Plant Ecology, Evolution, and Systematics 10:41-50.
Macarthur R.H. 1958. Population ecology of some warblers of northeastern coniferous forests. Ecology 39: 599-619.
Muchhala, N. And M. D. Potts. 2007. Character displacement among bat-pollinated flowers of the genus Burmeistera:
analysis of mechanism, process and pattern. Proc Roy Soc B 274: 2731-2737
Schoener T. W. 1983. Field experiments on interspecific competition. Am Nat 122: 240-285.
Strong, D. R., L. A. Szyska, D. Simberloff. 1979. Tests of community-wide character displacement against null hypotheses.
Evolution 33: 897-913.
Tilman, D. and D. Wedin. 1991. Plant traits and resource reduction for five grasses growing on a nitrogen gradient Ecology
72: 72:685-700
Webb CO. 2000. Exploring the phylogenetic structure of ecological communities: an example for rain forest trees. Am. Nat.
156:145– 55
Wedin D. And D. Tilman. 1993. Competition Among Grasses Along a Nitrogen Gradient: Initial Conditions and Mechanisms
of Competition. Ecological Monographs 6:3 199-229