Responses to changing environment: nonstationary environments could promote coexistence

Background/Question/Methods

Most models of interacting species assume a stationary environment (the environment may fluctuate, but with constant mean, variance, and other statistical moments). However, stationarity is just an approximation of convenience to a long-term changing situation. Coexistence is predicted to occur as a result of stationary environmental change when species differ in their temporal patterns of resource consumption, but can this prediction be upheld in a nonstationary environment? Key to coexistence from environmental fluctuations is coupling of resource consumption and resource availability. High resource consumption rates, due to favorable environmental conditions, are expected to draw down resources, causing high competition.  The phenomenon is called covariance between environment and competition (covEC), and is key to the storage effect coexistence mechanism. Can this mechanism still function under nonstationary environmental change? To answer this question, we added nonstationary environmental variation to the MacArthur consumer-resource model and developed quantitative measures of factors contributing to covEC, and ultimately species coexistence. 

Results/Conclusions

Nonstationary trends can contribute to many aspects of the interactions between two or more species, but these can be analyzed in terms of their quantitative effects on long-term population growth through average fitness effects and stabilization from covEC. At first sight, divergence of the resource maintenance requirements of two species due to nonstationary change must undermine coexistence through its effect on fitness differences. However, decreasing average maintenance requirements can counteract this trend by strengthening covEC. CovEC stabilizes coexistence when covEC is weaker for a species perturbed to low density (an “invader”) compared to an unperturbed species (a “resident”). This difference helps the invader recover from low density and promotes coexistence. Strengthening covEC  overall strengthens this difference, and therefore strengthens coexistence under this form of nonstationary change. Although intuition suggests that nonstationary environmental change must often be disruptive to natural communities and promote exclusion, our results show that sometimes the opposite outcome occurs, and indeed, nonstationary environments are compatible with the rich species diversity actually observed in the natural world.