Coexistence among symmetric competitors in fluctuating environments

Background/Question/Methods

The puzzling amount of diversity in communities with few limiting resources has long perplexed scientists. Classic theory requires one limiting resource for each competitor coexisting at equilibrium. Environmental fluctuations can slow the rate of competitive exclusions, but cannot induce stable coexistence without the presence of another coexistence mechanism, such as stronger intraspecific competition relative to interspecific competition. In this study we illustrate the mechanisms of coexistence in an environmentally heterogeneous community where species differ in their effectiveness of resource use at different temperatures. Species compete symmetrically according to Lotka-Volterra competition. Species' optimum temperatures are distributed along a temperature niche axis and growth declines asymmetrically at warmer and cooler temperatures, while niche area remains constant. We simulate large communities where daily temperature fluctuations are correlated and where they are randomized. We also simulate pairs of species and use analytic approximations to find the conditions for coexistence.

Results/Conclusions

Environmental heterogeneity can induce coexistence in a simple model of density dependent growth and symmetric competition.  Two types of coexistence occur: one maintained by the temporal niche differences created by temperature fluctuations and one maintained by synchronous dynamics with a similar, but more common competitor. A positive growth rate at low density requires a positive covariance in density (the effect of the environment) and overall competitive effect, which breaks down into conspecific and heterospecific contributions. Relatively common species have large conspecific contributions to competition, creating a positive density-competition covariance. Rare species use synchronous dynamics with similar species to create correlations between their density and interspecific competition. A concentration of intraspecific competition relative to interspecific competition is not required to generate stable coexistence in our model, but the correlations (either positive or negative) in species responses to temperature are key. Specifically the positive correlations that result in synchrony may provide a mechanism for the maintenance of high diversity in competitive communities where synchrony is common.