Positive species interactions and climate change at global scales

Background/Question/Methods: Positive species interactions (i.e. facilitation) have been recognized as a ubiquitous driver of community dynamics, and have been shown in hundreds of studies from around the globe to increase with increasing environmental stress (i.e. the stress-gradient hypothesis; the SGH). Most studies to date have focused on testing the SGH at local sites or within a certain type of ecosystem. Whether the SGH operates at larger spatial scales, such as along global climate gradients, is poorly understood. With accelerating climate change, understanding how competition and facilitation vary with climate at both local and global scales, nevertheless, is of fundamental importance to both ecological theory and applications. Here, we propose a global stress-gradient hypothesis (GSGH), where the original SGH is expanded from local to global scales for understanding how competition and facilitation vary with stress and climate globally.

Results/Conclusions: The GSGH predicts that positive and negative species interactions vary in a function of latitude and climate, with competition most frequent in temperate zones and facilitation more frequent in tropical and arctic zones. Facilitative interactions are likely frequent in the tropics due to strong biotic (e.g. consumer pressure in rain forests) or abiotic stresses (e.g. desiccation stress in tropical deserts and rocky intertidal shores), whereas in the arctic zones facilitative interactions are frequent mainly due to high abiotic stress such as cold. The GSGH also predicts that the impact of climate change on competition and facilitation vary globally in a function of ecosystem and climate background. These predictions appear to be supported by a preliminary, global meta-analysis of plant-plant interactions. To conclude, we advocate that a shift in the current, local focus of the SGH research to global scales is of paramount importance to advancing our understanding of the SGH and its practical values in conservation strategies and climate change predictions.