Demographic stochasticity alters the outcome of exploitation competition

Temporal variability in resource density is one of the mechanisms that facilitate coexistence between competitors. This study examines whether demographic stochasticity as a source of resource fluctuation can facilitate coexistence. The dynamics of a deterministic model (without demographic stochasticity) and a stochastic individual-based model (with demographic stochasticity) are compared. The deterministic model is an exploitation competition module consisting of two consumer species and one resource. The Gillespie algorithm is used to simulate demographic stochasticity in the corresponding individual-based model. The parameters of the models are chosen to represent cases where the deterministic model shows competitive exclusion according to the R⁎ rule and exhibits only stable equilibrium dynamics based on any combinations of the species. The analysis of the individual-based model shows that demographic stochasticity induces persistent population cycles between a consumer and the resource (i.e., when one of the consumers is absent), and this resource fluctuation allows the two consumers to coexist. Coexistence becomes possible through emerging tradeoffs that allow an inferior species (predicted by the deterministic model) to become competitively dominant (e.g., deviation of the R⁎ rule). These tradeoffs are useful for interpreting apparently contradicting empirical observations.