Abstract
Environmental fluctuations can mediate coexistence between competing species via the storage effect. This fluctuation-dependent coexistence mechanism requires three conditions: (i) a positive covariance between environment conditions and the strength of competition, (ii) species-specific environmental responses, and (iii) buffered population growth whereby species are less sensitive to competition in environmentally unfavorable years. In serially uncorrelated environments, the covariance condition (i) only occurs if favorable environmental conditions immediately and directly increase the strength of competition. However, for many demographic parameters such as maximal fecundity or adult survival, this direct link between favorable years and competition may not exist. Moreover, many environmental variables are temporal autocorrelated, but these correlations aren’t accounted for in the theory. To address these limitations, a theory for coexistence of competing species in autocorrelated environments is developed. This theory shows the positive autocorrelations in demographic rates that increase fitness (e.g. maximal fecundity or adult survival) produce the positive environment-competition covariance. Hence, when these demographic rates contribute to buffered population growth (e.g. maximal fecundity), positive temporal autocorrelations generate a storage effect, otherwise (e.g. fluctuations in adult adult survival) they destabilize competitive interactions. For negatively autocorrelated environments, this theory highlights an alternative stabilizing mechanism that requires three conditions: (i’) a negative environmental-competition covariance, (ii), and (iii’) species are less sensitive to competition in more favorable years. When the conditions for these stabilizing mechanisms are violated, temporal autocorrelations can generate stochastic priority effects or hasten competitive exclusion. Collectively, these results highlight that temporal autocorrelations in environmental conditions can play a fundamental role in determining ecological outcomes of competing species.
Footnotes
Minor corrections and additional references.