Does species-specific context-dependency in mutualisms explain community response to environmental change?

Background/Question/Methods

Mutualisms can affect the distribution and abundance of species. Notably, the fitness effects of mutualism are often context-dependent: mutualists increase performance in some environmental contexts and reduce performance in others. Furthermore, species vary in their response to mutualists in at least two ways: 1) allocation to mutualists and 2) ability to control allocation to mutualists in response to changes in the environment.  For the legume-rhizobium mutualism, in which plants exchange carbon resources for nitrogen fixed by rhizobia, interspecific variation in how legume species respond to mutualists may result in differences among species in response to nitrogen enrichment. However, few empirical studies have evaluated how variation in mutualistic interactions contributes to community responses to environmental change. Here we use the resource mutualism between legumes and rhizobia to test how legumes’ allocation to rhizobia and ability to regulate carbon allocation to rhizobia determine their persistence in plant communities in long-term fertilization experiments. We predicted that legume species that allocate less to rhizobia and those that are able to reduce allocation to rhizobia in high nitrogen or low light environments would experience a smaller decline in abundance in fertilized communities than other legumes.

Results/Conclusions

Legume species that allocate more to rhizobium mutualists were no less likely to be excluded from long-term fertilization treatments in the field; however, the ability to control allocation depending on environmental conditions did affect legume persistence in field nitrogen fertilization treatments. We found that a legume’s ability to reduce carbon allocation to rhizobia in response to high nitrogen in the greenhouse predicted species response to long-term fertilization in the field. Legume species that reduced allocation to rhizobia in response to nitrogen addition in the greenhouse showed minimal declines in abundance in field fertilization experiments. However, legumes that were better able to reduce allocation to rhizobia in greenhouse shade treatments had significantly larger declines in abundance in the field. These results indicate that community responses to fertilization may be driven in part by differences among legumes in their interactions with rhizobia. In particular, variation in legumes’ ability to reduce allocation to rhizobia in high nitrogen conditions may explain why some legumes persist in communities exposed to long-term nitrogen fertilization, while others do not.  Thus, species-specific differences in the context-dependency of mutualisms may help explain community response to environmental changes. It highlights the importance of mutualisms for determining species abundances and distributions.