Divergent biodiversity change within ecosystems

The Earth's ecosystems are under unprecedented pressure, yet the nature of contemporary biodiversity change is not well understood. Growing evidence that community size is regulated highlights the need for improved understanding of community dynamics. As stability in community size could be underpinned by marked temporal turnover, a key question is the extent to which changes in both biodiversity dimensions (temporal alpha- and temporal beta-diversity) covary within and among the assemblages that comprise natural communities. Here, we draw on a multi-assemblage dataset (encompassing vertebrates, invertebrates, and unicellular plants) from a tropical freshwater ecosystem and employ a cyclic shift randomization to assess whether any directional change in temporal alpha-diversity and temporal beta-diversity exceeds baseline levels. In the majority of cases, alpha-diversity remains stable over the 5-y time frame of our analysis, with little evidence for systematic change at the community level. In contrast, temporal beta-diversity changes are more prevalent, and the two diversity dimensions are decoupled at both the within-and among-assemblage level. Consequently, a pressing research challenge is to establish how turnover supports regulation and when elevated temporal beta-diversity jeopardizes community integrity.