How indirect interactions shape long-term invasion dynamics in complex ecological communities

Abstract Many facets of ecological theory rely on the analysis of invasion processes, and general approaches exist to understand a species’ short-term ability to invade a community. However, predicting the long-term transformations of communities following an invasion remains –even in theory– a challenging endeavour. We propose a simple method to predict these impacts, as a function of community structure and invader dynamical characteristics, applicable to a vast class of dynamical models. Our approach reveals that short-term invasion success and long-term consequences are two independent axes of variation controlled by different properties of both invader and resident community. Whether a species can invade or not is controlled by its invasion fitness, which depends on environmental conditions and direct interactions between the invader and other species. But, whether this invasion will cause extinctions, or a even regime shift, depends on a specific measure of indirect interaction feedbacks that may involve the entire resident community. As an illustration of the new insights obtained by this method, we explore the interplay of invader specialization and community complexity, and discuss how asymmetrical invader-resident and resident-resident interactions can make communities less vulnerable to invasions.