Sequential dispersal and productictivty drive beta diversity in space and time

Background/Question/Methods

A growing body of empirical evidence indicates that spatial variation in species composition (or “beta diversity”) can increase with increasing productivity. However, the ecological mechanisms that give rise to this relationship remain unresolved. Theory predicts that sequential species dispersal into local communities and stochastic dispersal history can interact with productivity to produce beta diversity. This can occur through two alternative pathways. First, sequential dispersal combined with increasing productivity could produce priority effects and alternative stable states among communities. In this hypothesis temporal change in composition should decline once communities attain their stable states. The second hypothesis predicts that stochastic sequential dispersal combined with increasing productivity drives compositional turnover in time (temporal beta diversity within communities), which produces spatial beta diversity (in this hypothesis temporal change in composition should increase with productivity and remain constant over time). We tested these hypotheses under controlled laboratory conditions using an experimental aquatic system composed of six species of zooplankton and a diverse phytoplankton community. Five levels of productivity (nutrient enrichment manipulations) were crossed with two zooplankton dispersal treatments (simultaneous dispersal of all species at once versus stochastic sequential dispersal in which dispersal history varied among local communities). The experiment ran for 224 days with response variables (compositional turnover in space and time) measured over the last 112 days after which all species had an opportunity to invade all local communities.

Results/Conclusions

Our results supported the hypothesis that sequential dispersal promotes temporal turnover in species composition and increasing spatial beta diversity along gradients of productivity. Communities that experienced simultaneous dispersal of all six zooplankton species through time showed no relationship between productivity and spatial beta diversity (measured using multivariate dispersion). Temporal turnover in composition (Bray-Curtis dissimilarity over time) also showed no relationship with productivity in these treatments. In contrast, communities that experienced stochastic sequential dispersal showed a significant increase in spatial beta diversity with increasing productivity. Moreover, temporal turnover in species composition also increased with productivity in these treatments such that spatial beta diversity and mean temporal turnover rates were significantly and positively correlated. Temporal turnover patterns remained relatively constant over time, indicating that beta diversity patterns could not be wholly attributed to the production of alternative stable states. Our results show that the stochastic nature of dispersal, when combined with system enrichment, can greatly reduce the predictability of community composition in both space and time.