Demographic and environmental stochasticity alter spatial coexistence strength via stabilizing mechanisms

Background/Question/Methods

A large body of both empirical and theoretical work has focused on the difference between niche and neutral communities, and particularly on the effects of drift and the role of stochasticity in community structure. However, demographic and environmental stochasticity may affect more than just abundance patterns in communities. It is well known that stochasticity can lead to extinction events in small populations, and we therefore hypothesize that stochasticity alters coexistence strength via effects on species’ low-density growth rates. To test this hypothesis, we created both deterministic and stochastic metacommunity models and solve for Chesson’s spatial coexistence mechanisms in each model. By comparing coexistence mechanisms and coexistence strength in deterministic models to models with demographic stochasticity, environmental stochasticity, or both, we isolated the role of different forms of stochasticity in altering coexistence strength. We additionally compared how stochasticity affects both species’ abundance patterns and coexistence strength across a variety of simulations ranging from neutral to niche-structured communities. Using this approach we identify a previously unappreciated role of stochasticity in maintaining niche-structured communities. 

Results/Conclusions

As is well established in the literature, we saw that neutral community abundance patterns are structured by temporal drift via stochasticity. We additionally show that stochasticity has no effect on mean coexistence strength or individual coexistence mechanisms in neutral communities. Rather, stochasticity in neutral communities induces temporal variability in coexistence strength. In comparison, niche-structured abundance patterns are affected primarily by selection mechanisms, and stochasticity has only minimal effects on species’ abundances.  However, we show that both demographic and environmental stochasticity alter overall coexistence strength in niche-structured metacommunities via the storage effect and fitness-density covariance. Demographic stochasticity tends to decrease total coexistence strength, as it makes species’ abilities to respond to environmental heterogeneity less predictable. Environmental stochasticity, on the other hand, can increase coexistence strength in niche-structured communities by magnifying the potential for stabilizing coexistence mechanisms. While stochasticity is most commonly associated with neutral communities due to its influence on temporal abundance patterns, we show that it is equally important for niche communities. Stochasticity directly alters the strength of stabilizing coexistence mechanisms in niche-structured communities, thus altering the niche structure itself.