Non-stationarity in the co-occurrence patterns of species across environmental gradients

Quantifying the role of biotic interactions in driving community assembly often relies on analysing species co-occurrence patterns, where segregated patterns are taken to indicate antagonistic interactions such as competition. It is unknown, however, if co-occurrence patterns are stationary across environmental gradients, as it is possible that the strength of biotic interactions that drive these patterns also depends on the environment. In this study, we aim to understand how patterns of co-occurrence change when species move from their environmental range centre towards their range periphery while isolating the potential signal of biotic interactions from confounding factors. We used two separate statistical approaches (null models and joint species distribution models) to quantify pairwise co-occurrence patterns for tree species sampled in 9382 plots distributed across the conterminous US. We also analysed co-occurrence patterns that emerged from a simple meta-community model. We then assessed how patterns of species segregation and aggregation change in relation to habitat suitability while accounting for multiple factors known to confound co-occurrence analyses. We found strong non-stationarity in co-occurrences, with patterns shifting from segregated at the environmental range centre towards aggregated at range margins for the majority of tree species. Patterns were in full agreement between model simulations and both empirical analyses. Model simulations suggest that this pattern is at least partly driven by variation in the relative abundances of non-focal species even when no direct biological interactions are present. Synthesis. Patterns of tree species co-occurrence vary across environmental gradients, with increased segregation when environmental conditions are optimal and increased aggregation when the environment is less suitable. This pattern may originate from a trade-off between the abundance of the focal species pair, which decreases towards the environmental range margin, and the increasing abundance of non-focal species to which the environment is more suitable. The strong dependence of co-occurrence patterns on environmental conditions might limit the predictive ability of joint species distribution models, which couple species co-occurrences and their environmental responses, because co-occurrence patterns and environmental responses are confounded.