Changes in community size affect the outcome of competition.

We examine the role of stochasticity and competitive ability in affecting competition between two species using models derived for population genetics. Just as changing population size affects the fixation of a new mutation, we show that changing the total number of competitors (i.e., community size) can alter the course of competitive exclusion across a wide range of initial starting densities of the two competing species. Shifts in competitive exclusion occur because changes in community size affect the relative importance of competitive ability and stochasticity in affecting the outcome of competition, potentially allowing inferior invaders to usurp superior residents. By shifting the role of stochasticity and competitive ability, any process that changes the total number of competitors in a habitat (e.g., disturbance, eutrophication, fragmentation, predation) may lead to shifts in competitive exclusion and the composition of communities.