PS 32-194 - Modeling spatial outcomes of competitive and mutualistic three-species interactions

Abstract: Mutualisms are interspecific interactions that enhance the growth of species by an exchange of benefits or resources. In a mutualism between two species, a species' dependence on the beneficial resources or services provided by its mutualist partner can be viewed on a spectrum from completely obligate to complete independence from the partner. However, species interactions are rarely pairwise and often involve third-party interactions with other species. These interactions can directly affect the growth of the mutualist species or the stability of the mutualism, possibly resulting in species extinctions. Here, we study how mutualistic and competitive interactions can shape local and regional coexistence in a three-species community. We consider a spatially-explicit population dynamic model of three species interacting, in which two competing species share a mutualist partner with which they exchange benefits. With the competing species differing in their degree of dependence on the mutualist partner, we simulate the three species' growth and dispersal over many generations. We ask how competition and mutualism dependence shapes the range expansion and spread speed of each species. We identify the conditions under which the competing species coexist (regionally or locally) or competitively exclude each other. We find local coexistence when the negative effects of competition are low for both species. Regional coexistence can occur when competition has moderate negative effects on each competitor if the less dependent species is a slightly stronger competitor. We also find regional coexistence at high levels of competition when the more dependent species is the stronger competitor. Our work demonstrates that mutualism dependence is a mechanism that can allow competing species to coexist in a spatial landscape.