Abstract
Species coexistence attracts wide interest in ecology. Modern
coexistence theory (MCT) identifies coexistence mechanisms, one of
which, storage effects, hinges on relationships between fluctuations in
environmental and competitive pressures. However, such relationships are
typically measured using covariance, which does not account for the
possibility that environment and competition may be more related to each
other when they are strong than when weak, or vice versa. Recent work
showed that such ‘asymmetric tail associations’ (ATAs) are common
between ecological variables, and are important for extinction risk,
ecosystem stability, and other phenomena. We extend the MCT, decomposing
storage effects to show the influence of ATAs. Analysis of a simple
model and an empirical example using diatoms illustrate that ATA
influences can be comparable in magnitude to other mechanisms of
coexistence, and that ATAs can make the difference between species
coexistence and competitive exclusion. ATA influences are an important
new mechanism of coexistence.