Scale‐Dependent Shifts in Demographic and Environmental Control of Temporal β‐Diversity

Aim We investigated whether the relative influence of demographic stochasticity and environmental forcing on compositional dynamics shifts across scales. Location Riverine fish communities sampled across 39 regions in three biogeographic realms. Time Period 1981–2019. Major Taxa Studied Freshwater fish. Methods We first used simulations without environmental selection to assess whether temporal β‐diversity metrics capture demographic variability independent of community sizes. We then analysed 468 fish community time series, modelling local temporal β‐diversity and regional temporal changes in spatial β‐diversity as functions of community size, its temporal variability, species richness, rarity, and environmental variation and synchrony. Results Simulations showed that a rank‐change metric was not intrinsically biased by community size. Empirical analyses revealed scale dependence in the processes shaping compositional temporal variability. Locally, smaller median community size and higher abundance fluctuations increased temporal β‐diversity, consistent with stronger demographic stochasticity in small communities. Temperature seasonality emerged as the strongest climatic correlate of local temporal β‐diversity, with more seasonal environments exhibiting greater inter‐annual compositional change. Mean annual temperature and species richness also showed positive effects. In contrast, internal community properties did not predict regional dynamics. Instead, spatial synchrony of precipitation was the main predictor. More synchronized environments exhibited lower temporal variability in among‐site dissimilarity, consistent with Moran‐type environmental forcing constraining spatial β‐diversity. Main Conclusions Our findings reveal a scale‐dependent shift in the processes shaping compositional dynamics, with demographic stochasticity and climatic regime jointly structuring local compositional dynamics, whereas environmental synchrony dominates regional variability. These findings link demographic and environmental perspectives, clarifying how biodiversity change propagates across scales under declining populations and increasingly variable climates.