The Princeton Guide to Ecology

Numerous species commonly compose natural plant assemblages from the poles to the equator, and a wealth of classic ecological experiments have demonstrated that these often compete strongly with one another for resources such as nutrients or light. Theoretical ecologists have demonstrated that competing species are only expected to stably coexist (i.e., coexist in the long run) when each is protected from local extinction by density- or frequency-dependent processes that benefit it when rare, or equivalently, when the net negative effects of intraspecific (within-species) competition exceed those of interspecific (between-species) competition. Competition for nutrients, such as nitrogen and phosphorus, is “size symmetric” because smaller and larger individuals are potentially equal competitors on a per-biomass basis. In contrast, shoot competition for light is “size asymmetric” because taller individuals are advantaged irrespective of biomass. We describe the different modeling approaches that this difference requires. However, in either case, stable coexistence requires trade-offs such that species that are better competitors for one limiting nutrient or in one light environment are necessarily worse competitors for a second limiting nutrient or in a second light environment. As one important example of how these models might account for the stable coexistence of numerous species across landscapes, we consider the effects of habitat heterogeneity in mean growing season temperature coupled with trade-offs in performance among species at different mean temperatures. Finally, given our theoretical understanding, we close with a discussion of the current challenges to and opportunities for advancing our empirical understanding of competition and coexistence in the real world.

coexistence. The indefinite persistence of two or more species. The empirically relevant sort of coexistence is termed “stable coexistence” in which species will continue to persist in the face of perturbations in their abundances. It is important to note that species that co-occur may or may not be stably coexisting; it is possible that one or more species are on their way to local extinction at a time scale that might appear slow to a casual observer.

competition. Most broadly, an interaction between individuals in which neither benefits. Here, we are considering exploitation competition for limiting resources in which the resource consumed or intercepted by one individual is no longer available to the second individual, thereby decreasing its fitness.

exclusion. A condition in which a species is driven to local extinction as a consequence of a competitive interaction.

founder control. A condition in which the dominant species in a competitive interaction is the species that is initially most abundant.

interspecific competition. Competition among individuals of different species.

intraspecific competition. Competition among individuals of the same species.

invader/invasion. In the context of theoretical ecology, an invader is a species introduced at arbitrarily small abundance to a habitat of a resident species at equilibrium. The question is asked: will the invader increase in abundance? Note that this use of the term is different from the sense in which an “invader”

-186-