Disentangling How Climate and Dispersal Drive Temporal Trends in Synchronous Population Dynamics.

Spatially synchronised population dynamics are driven by a combination of shared environmental conditions among sites and the movements of individuals between sites. Untangling the drivers of population synchrony requires investigation of how populations are correlated across space and time in relation to climate and mobility-related attributes. Here, we use species survey data from over four decades to investigate average levels and temporal trends in population synchrony for 58 British bird and butterfly species. We first show that population synchrony is significantly associated with synchrony in seasonal climatic variables. After accounting for spatiotemporal climatic patterns, we determine whether temporal trends in population synchrony are shaped by mobility-related attributes. We test this through an interspecies comparison using three variables correlated with mobility: biotope specialism, estimated species mobility, and local abundance change, which is known to affect emigration rate. We find that temporal trends in population synchrony are most marked for generalist butterfly species, butterflies with high estimated mobility, and butterflies that had changed in their mean abundance. For birds, we find changes in population synchrony are associated with specialist bird species and those that increased in abundance over time. Our results reveal a widespread effect of mobility attributes and abundance patterns on population synchrony over time, suggesting that variation in dispersal is a key factor determining the extent to which population dynamics are synchronised.