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 a and temporal b diversity) covary within and amongst the assemblages that comprise natural communities. Here, we draw on a new multi-assemblage data set (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 a diversity and temporal b diversity exceeds baseline levels. In the majority of cases a diversity remains stable over the 5 year time frame of our analysis, with little evidence for systematic change at the community level. In contrast, temporal b diversity changes are more prevalent, and the two diversity dimensions are de-coupled at both the within- and among assemblage level. Consequently, a pressing turnover supports regulation, and when elevated temporal b diversity jeopardizes community integrity.