Microbial mutualist distribution limits spread of the invasive legume Medicago polymorpha

Abstract

The spread of invasive species can be limited or promoted by the distributions of mutualists, which presents an opportunity for managing biological invasions. However, the ways in which the geographical distributions of mutualists shape the range limits and invasion dynamics of introduced species remains unclear. Legumes include noxious invaders whose success is tied to their mutualism with nitrogen-fixing rhizobium bacteria. We examine whether the availability of compatible rhizobia limits the spread of an invasive legume at both small (50 m) and large (United States) spatial scales across natural invasion fronts. Using greenhouse experiments we simulated the dispersal of Medicago polymorpha into soils at increasing distances from a host patch. We find that rhizobial mutualist availability rapidly declines to almost zero within 20 m of established legume patches and legume fitness and the percentage of leaf nitrogen derived from symbiotic nitrogen fixation show a concomitant decline. The decline of fitness due to the lack of rhizobium mutualists differs among M. polymorpha genotypes. Our findings support the missed mutualist hypothesis whereby mutualist limitation reduces invasiveness. As M. polymorpha colonizes novel areas, seeds following either short- (50 m) or long-range seed dispersal will be mutualist-limited and exhibit low fitness and nitrogen fixation. Without co-introduction via transport of soil containing both mutualists, the patchy distribution of E. medicae may limit M. polymorpha invasion. Plant lineages exhibit genetic variation upon which selection could act to reduce dependence on rhizobia, thus a scarcity of symbionts could cause plant symbiosis traits to evolve during invasion.