Positive species interactions shape species' range limits

Abstract

The relationship between niche and distribution, and especially the role of biotic interactions in shaping species' geographic distributions, has gained increasing interest in the last two decades. Most ecological research has focused on negative species interactions, especially competition, predation and parasitism. Yet the relevance of positive interactions – mutualisms and commensalisms – have been brought to the fore in recent years by an increasing number of empirical studies exploring their impact on range limits. Based on a review of 73 studies from a Web of Science search, we found strong evidence that positive interactions can influence the extent of species' geographic or ecological ranges through a diversity of mechanisms. More specifically, we found that while obligate interactions, and especially obligate mutualisms, tend to constrain the ranges of one or both partners, facultative positive interactions tend to widen ranges. Nonetheless, there was more variation in effects of facultative interactions on range limits, pointing to important context-dependencies. Therefore, we propose that conceptual development in this field will come from studying ecological interactions in the context of networks of many species across environmental gradients, since pairwise interactions alone might overlook the indirect and environmentally-contingent effects that species have on each other in communities of many interacting species. Finally, our study also revealed key data gaps that limit our current understanding of the pervasiveness of effects that positive interactions have on species' ranges, highlighting potential avenues for future theoretical and experimental work.