Mutualisms impact species' range expansion speeds and spatial distributions.

Species engage in mutually beneficial interspecific interactions (mutualisms) that shape their population dynamics in ecological communities. Species engaged in mutualisms vary greatly in their degree of dependence on their partner from complete dependence (e.g. yucca and yucca moth mutualism) to low dependence (e.g. generalist bee with multiple plant species). While current empirical studies show that, in mutualisms, partner dependence can alter the speed of a species' range expansion, there is no theory that provides conditions when expansion is sped up or slowed down. To address this, we build a spatially explicit model incorporating the population dynamics of two dispersing species interacting mutualistically. We explore how mutualisms impact range expansion across a gradient of dependence (from complete independence to obligacy) between the two species. We then study conditions where magnitude of mutualistic benefits can hinder versus enhance the speed of range expansion. We show that either complete dependence, no dependence, or intermediate degree of dependence on a mutualist partner can lead to greatest speeds of a focal species' range expansion based on the magnitude of benefits exchanged between partner species in the mutualism. We then show how different degrees of dependence between species can alter the spatial distribution of the range expanding populations. Finally, we identify conditions under which mutualistic interactions can turn exploitative across space leading to the formation of a species' range limits. Our work highlights how couching mutualisms and mutualist dependence in a spatial context can provide insights about species range expansions, limits and ultimately their distributions.