The effect of environmental variation on stable coexistence of competitors: experimental evidence from zooplankton ( <i>Daphnia magna</i> and <i>D. pulex</i> )

Abstract Coexistence of competing species may be influenced by environmental variation. Specifically, theory suggests that short-term environmental variability can contribute to long-term coexistence among competitors. Here, we address the role of environmental variation on competitive interactions between two zooplankton species (Daphnia magna and D. pulex) which are found sympatrically, but where mechanisms allowing for such coexistence remain unclear. Using competition experiments, we show that under constant temperature conditions, one of the species (D. magna) was greatly outnumbered by their competitor (D. pulex). Furthermore, population simulations showed a significant possibility for extinction of the inferior competitor, and distributions of estimated niche differences and relative fitness differences included parameter sets that precluded stable coexistence. Under fluctuating temperature conditions, however, the numerical dominance by D. pulex was considerably reduced. Moreover, under these conditions the occurrence of extinction of D. magna in the simulations became negligible, and all parameter sets drawn from the estimated distributions of niche differences and relative fitness differences met the requirements for stable coexistence. Our results provide empirical support for previous model results showing how short-term variation in temperature can promote species coexistence.