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

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 a 12 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 R2R2 R1R1 R1AR1AR1BR1B R2BR2B R2AR2A

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 R2R2 R1R1

R2R2 R1R1

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

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 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 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 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 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 Phylogenetic Overdispersion in Floridian Oak Communities. Am Nat 163: Connell, J. H The influence of interspecific competition and other factors on the distribution of the barnacle Chthamalus stellatus. Ecology 42: Connell, J. H Diversity and the Coevolution of Competitors, or the Ghost of Competition Past. Oikos 35: Dayan, T., D. Simberloff. 2005Ecological and community-wide character displacement: the next generation. Ecology Letters 8: Dayan, T., D. Simberloff, E. Tchernov, Y Yom-Tov Feline canines: community-wide character displacement among the small cats of Israel. Am. Nat. 136: Fargione, J.; Tilman, D Predicting relative yield and abundance in competition with plant species traits. Functional Ecology 20: Gause G. F The struggle for existence. Williams & Wilkins. Grant, P.R. and B. R. Grant Evolution of character displacement in Darwin’s finches. Science 313: Hairston N. G. 1980a The experimental test of an analysis of field distributions: competition in terrestrial salamanders. Ecology 61: JF Cahill, SW Kembel, EG Lamb, and PA Keddy Does phylogenetic relatedness influence the strength of competition among vascular plants? Perspectives in Plant Ecology, Evolution, and Systematics 10: Macarthur R.H Population ecology of some warblers of northeastern coniferous forests. Ecology 39: Muchhala, N. And M. D. Potts Character displacement among bat-pollinated flowers of the genus Burmeistera: analysis of mechanism, process and pattern. Proc Roy Soc B 274: Schoener T. W Field experiments on interspecific competition. Am Nat 122: Strong, D. R., L. A. Szyska, D. Simberloff Tests of community-wide character displacement against null hypotheses. Evolution 33: Tilman, D. and D. Wedin Plant traits and resource reduction for five grasses growing on a nitrogen gradient Ecology 72: 72: Webb CO Exploring the phylogenetic structure of ecological communities: an example for rain forest trees. Am. Nat. 156:145– 55 Wedin D. And D. Tilman Competition Among Grasses Along a Nitrogen Gradient: Initial Conditions and Mechanisms of Competition. Ecological Monographs 6: