Consequences of a biological invasion reveal the importance of mutualism for plant communities.

Seed-dispersal mutualisms have a fundamental role in regenerating natural communities. Interest in the importance of seed dispersal to plant communities has been heightened by worldwide declines in animal dispersers. One view, the 'keystone mutualist hypothesis', predicts that these human-caused losses will trigger a cascade of linked extinctions throughout the community. Implicitly, this view holds that mutualisms, such as seed dispersal, are crucial ecological interactions that maintain the structure and diversity of natural communities. Although many studies suggest the importance of mutualism, empirical evidence for community-level impacts of mutualists has remained anecdotal, and the central role of mutualism, relative to other species interactions, has long been debated in the theoretical literature. Here I report the community-level consequences of a biological invasion that disrupts important seed-dispersal mutualisms. I show that invasion of South African shrublands by the Argentine ant (Linepithema humile) leads to a shift in composition of the plant community, owing to a disproportionate reduction in the densities of large-seeded plants. This study suggests that the preservation of mutualistic interactions may be essential for maintaining natural communities.