Background/Question/Methods Understanding mechanisms that generate range limits is central to knowing why species are found where they are, and how they will respond to environmental change. There is growing awareness that biotic interactions play an important role in generating range limits. However, 4 current theory and data overwhelmingly focus on abiotic drivers and antagonistic interactions. In this synthesis, we explore the effect that mutualists have on their partner’s range limits: the geographic “footprint” of mutualism. This footprint arises from two general processes: modification of a partner’s niche through environment-dependent fitness effects and, for a subset of mutualisms, dispersal opportunities. We develop a conceptual framework that organizes alternative footprints of mutualism and the underlying mechanisms that shape them, and evaluate supporting empirical evidence from the primary literature.
Results/Conclusions We find that the fitness benefits and dispersal opportunities provided by mutualism can extend species’ ranges; conversely, the absence of mutualism can constrain species’ from otherwise suitable regions of their range. We discuss these findings with respect to specialization, dependence, and intimacy of mutualism. Much remains unknown about the geographic footprint of mutualisms, leaving fruitful areas for future work. A particularly important future direction is to explore the role of mutualism in range shifts under global change.