Associate Professor University of Minnesota - Twin Cities, United States
Species engage in mutually beneficial interspecific interactions (mutualisms) that shape their population dynamics in ecological communities. There is a vast diversity among mutualisms in the magnitude of benefits each partner species provides. However, there are two distinct gaps in our theoretical understanding of mutualisms. One, while past theory has extensively examined how mutualisms affect temporal species population dynamics, we still lack an understanding of how mutualisms shape species distributions across space. Second, current models of mutualism often assume the dependence of each species on its partner either as obligate or non-obligate (facultative). This assumption lumps together facultative mutualisms with different degrees of dependence and obscures their role in influencing species growth. We address these issues by building a spatially explicit ecological 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. Finally, we observe the population dynamics of the species at the core and the edge of the species’ expanding range and identify when mutualism can impose range limits.
Our work generates three main results. First, being more dependent on mutualisms can enhance range expansion speeds of species when the magnitude of benefits of mutualism are large. Conversely, being more dependent on mutualisms slows down range expansion speeds when the benefits are low. Second, intermediate dependence on their partner species can also result in a species’ greatest range expansion speeds provided the inter-specific benefits are asymmetric. Third, mutualisms influence species’ growth which in turn alters the population densities of the species across their respective ranges. This leads to differences in each species’ population density at the range core versus range edge. These differences across space arise due to the context-dependency of mutualisms wherein interactions that were beneficial for species at the center of the range end up becoming exploitative at the edge. In certain cases, mutualisms can even serve as a mechanism in the formation of range limits of the species. Our work highlights how considering the type of mutualistic interactions and couching it in a spatial context can lead to insights about species range expansions, limits and ultimately their distributions.