How do life history traits influence the environment's effect on population synchrony? Insights from European birds and insects

Populations closer together in space are more likely to experience shared environmental fluctuations. This correlation in experienced environmental conditions is the main driver of spatial population synchrony, defined as the tendency for geographically separate populations of the same species to exhibit parallel fluctuations in abundance over time. Morans theorem states that spatially distinct populations are expected to show the same synchrony in their population dynamics as the synchrony in their environment. However, this is rarely the case in the wild, and the population synchrony of different species inhabiting the same area is rarely similar. These species-specific differences in how the environment synchronizes populations can be due to life history traits that make some species more susceptible to environmental stochasticity, such as reduced mobility or faster pace of life. In this study, we compiled long-term annual abundance datasets on European birds and insects (Lepidoptera sp. and Bombus sp.) to identify how environmental synchrony (i.e., positively spatially correlated fluctuations in the environment, also called the Moran effect) affects species population synchrony. As expected, the environment synchronized populations of both birds and insects. Populations experiencing correlated fluctuations in precipitation or temperature had higher synchrony in annual population growth rates. Birds were more strongly synchronized by temperature, while precipitation was a stronger driver of synchrony in insects. In birds, species with short generation times had a stronger synchronizing effect of the environment compared to species with long generation times. Moreover, in birds the effects of synchrony in the environment also depended on movement propensity, with a positive impact for resident and short-distance migration species. In insects, annual population synchrony was affected by species movement propensity and dietary niche breadth, but these traits did not modify the effects of environmental synchrony. Our study provides empirical support for the prediction that spatial correlation in population dynamics is more influenced by environmental stochasticity for life histories with lower mobility and faster pace of life, but only in birds. By quantifying spatial population synchrony across different levels of environmental synchrony and life history traits, our study improves the understanding of the Moran effect as well as factors that drive population persistence in the face of environmental change.