Stochastic sequential dispersal and nutrient enrichment drive beta diversity in space and time

Nutrient pollution constitutes a major threat to biodiversity, one whose magnitude is predicted to increase greatly in the near future. While the negative effects of excessive nitrogen and phosphorus loading on local species diversity are widely appreciated, a growing body of evidence indicates that increases in productivity (a correlate of nutrient supply) can also reduce predictability in community composition by driving community divergence and increases in beta diversity (or compositional dissimilarity among communities). Stochastic variation in dispersal history has frequently been cited as a cause of such patterns. However, underlying mechanisms have not received strong experimental scrutiny. I present results of a microcosm experiment testing the effects of enrichment and dispersal mode on zooplankton community structure. I show that beta diversity increases with enrichment but only in the presence of sequential dispersal and variation in dispersal history. Sequential dispersal and enrichment enhanced beta diversity by driving increases in temporal compositional turnover (or beta diversity in time). These results suggest that nutrient enrichment and dispersal may have interactive effects on community organization by facilitating persistent compositional flux and reducing our ability to predict the structure of communities in both space and time.