Community size predicts temporal β-diversity at local but not regional scales

Cristina Mariana Jacobi , Tadeu Siqueira

Understanding what drives temporal changes in community composition is urgent given widespread population declines that increase vulnerability to demographic noise. We investigated how demographic stochasticity and environmental variability relate to compositional variability both locally (temporal β-diversity) and regionally (temporal changes in spatial β-diversity). To do this, we first simulated communities without environmental selection to test whether commonly used β-diversity metrics were robust to demographic stochasticity alone. In the absence of environmental forcing, a rank-change metric showed no consistent relationship with community size, confirming that observed size effects in empirical data would not be artifacts of the metric. We then analyzed riverine fish community time series collected between 1981 and 2019 across 39 regions spanning three biogeographic realms, modeling local and regional compositional variability against community size, its temporal variability, species richness, rarity, and environmental variation and synchrony. Empirical analyses revealed scale dependence in the processes shaping compositional change. At the local scale, internal community properties were more important. Smaller median community size and greater fluctuations in total abundance were related to higher temporal β-diversity, consistent with a stronger role of demographic stochasticity in small communities. Higher species richness also increased temporal β-diversity, likely by enlarging the pool of possible colonists and increasing the number of feasible community states. At the regional scale, however, these factors had little influence. Instead, the spatial synchrony of precipitation emerged as the main predictor of temporal changes in spatial β-diversity. Metacommunities embedded in more synchronized environments exhibited less temporal variability in among-site dissimilarity, suggesting that synchronized environmental forcing constrains spatial reshuffling of communities over time. Together, these results reveal a scale-dependent shift in the processes influencing compositional variability. Local temporal β-diversity can be primarily driven by demographic stochasticity and richness effects, whereas regional compositional dynamics by environmental factors. This integration of demographic and environmental perspectives highlights how the balance between internal and external processes shifts across scales. It is important to understand how such processes interact in a time when biodiversity is declining, populations are getting smaller and climates are becoming more variable. These trends could change how ecological communities vary over time.