Unlocking the mechanisms behind legume invasions: Are rhizobial mutualists the key?

Background/Question/Methods

Ecologists have a long history of examining the role of biotic interactions in determining invasion success. Currently, new molecular tools are driving a rapid understanding of the importance of feedbacks between plants and their associated microbial communities in determining species invasions. Invasive legumes present a particularly interesting case of plant-microbial feedbacks. A legume may successfully invade by obtaining atmospheric nitrogen to outcompete native plants, however this strategy depends upon being able to associate with compatible rhizobia in the exotic range. An invasive legume can either associate with the resident rhizobia in its invasive range or co-invade with rhizobia from its native range; however, both of these strategies depend upon the specificity between a legume species and its rhizobial associates. Of additional importance to the native system, co-invasion of an area by both invasive legumes and rhizobia may result in novel rhizobial associations for native legume species. Here, we examine legume-rhizobia specificity for three invasive legumes—gorse (Ulex europaeus), French broom (Genista monspessulana), and Scotch broom (Cytisus scoparius)—and several native legumes in the San Francisco Bay Area, as well as the incidence of spillover between native and invasive rhizobial populations through field collections and a greenhouse cross-inoculation experiment.

Results/Conclusions

The invasive legumes were less specialized than the native legumes in terms of their rhizobial associates. Additionally, the specificity for certain rhizobial associates varied among the invasive legumes, with French broom associating with the smallest subset of rhizobia tested and Scotch broom associating with the largest subset in greenhouse cross-inoculation trials. While French broom did not share any rhizobial associates with the native legumes, gorse and Scotch broom did exhibit some overlap. Specifically, native legumes associated with several phylotypes of Bradyrhizobium, Scotch broom associated with various Bradyrhizobium (some of which overlapped with the native associates), Rhizobium, and Mesorhizobium, gorse associated with various Bradyrhizobium (some of which overlapped with the native associates), and French broom nodulated with various Bradyrhizobium, Mesorhizobium, and Rhizobium that never associated with the native legumes. The availability of the preferred rhizobial symbionts of the invaders, as measured by their number of nodules and average nodule size, was positively correlated with the performance of the invasive legumes in the greenhouse, as measured by their biomass and growth rate. Overall, these results suggest that legume invasion success may depend on the ability of an invader to associate with resident rhizobial symbionts in its exotic range.