Geographic Gradients in Species Interactions: From Latitudinal Patterns to Ecological Mechanisms

The idea that species interactions are more ecologically and evolutionarily important toward lower latitudes underpins seminal theories in ecology and evolution. Recent global studies have found the predicted latitudinal gradients in interactions, particularly predation. However, latitudinal patterns alone do not reveal why interactions vary geographically and so do not provide strong predictions in space (e.g., for specific ecosystems) or time (e.g., forecasting responses to global change). Here, I review theory to identify a clearer, mechanistic, and testable framework for predicting geographic variation in the importance of species interactions. I review competing metrics of importance, proximate mechanisms that can increase interaction importance, and environmental gradients that could generate predictable geographic patterns (climate extremes and stability, warmth, productivity, and biodiversity). Strong empirical tests are accumulating thanks to the rise of global experiments and datasets; renewed focus on testing why interactions vary spatially will help move the field from identifying latitudinal patterns to understanding broader mechanisms.