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

• 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

N1

0 500 1000 1500 2000 2500

N2

0

500

1000

1500

2000

2500

dN2/dt = 0

N1

0 500 1000 1500 2000 2500

N2

0

500

1000

1500

2000

2500

dN1/dt = 0

dN1/dt > 0

dN1/dt < 0

dN2/dt > 0

dN2/dt < 0

K1

K1/a12

K2

K2/a21

Species 1 Species 2

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]

K is carrying capacity; dN/dt is population growth rate; a is competition coefficient with a12 being effect of sp. 2 on sp.1

Figures from Gotelli, “A Primer of Ecology”

Species 1 Species 2

...of species 2 by species 1 ...of species 1 by species 2

Competitive Exclusion

Figs from Gotelli

Equilibrium: stable coexistence vs. unstable competitive exclusion

coexistence “winner” depends on priority effects

Figs from Gotelli

Tilman’s mechanistic model

R2

R1

R1A R1B

R2B

R2A

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*

Tilman and Wedin 1991

Poor competitors remove less N

Good competitors remove more N

Roots are the foraging organ: mass correlated with N assimilation

Tilman’s field expt.

Wedin and Tilman 1993.

Chthamalus stellatus

Balanus balanoides

Nucella = Thais lapillus

Connell’s barnacle experiment

Hairston’s salamanders

Hairston 1980

Low overlap as a consequence of competition

Anoles in the Lesser Antilles

Anolis wattsi

Similar body size and perch height

Little overlap in size or perch height

Anolis gingivinus

Anolis bimaculatus

High body size and perch height overlap results in competition

Treatments with “W”: competing with A. wattsi

Pacala and Roughgarden 1982

Similar niche

Different niche

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

Strong et al 1979

But differences in bill size between co-occurring pairs no different than expected by chance??

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.

magnirostris introduced: has really large beak

Drought causes competition: selects for divergent (small) beaks in fortis

Drought selects for larger beaks in fortis (only large seeds available)

Grant and Grant 2006

Desert cats DO show character displacement when tested

against null models

Dayan et al 1990

Canine size for each species/sex in two locations

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

Webb 2000: evidence for habitat filtering in tropical trees

Two different estimates of relatedness

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 intenselyCahill 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

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

Complementarity: when positive (as here) means species have higher than expected yield in mixture (attributed to N-fixers and C4 species presence in high-diversity mixtures)

Fargione et al 2006

Cadotte et al 2009

Is it really about functional group diversity, or another diversity metric?

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

• 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