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

Abstract

Seed-dispersal mutualisms have a fundamental role in regenerating natural communities1,2. Interest in the importance of seed dispersal to plant communities has been heightened by worldwide declines in animal dispersers3,4,5. One view, the ‘keystone mutualist hypothesis’, predicts that these human-caused losses will trigger a cascade of linked extinctions throughout the community6. 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 mutualism3,7, empirical evidence for community-level impacts of mutualists has remained anecdotal8,9, and the central role of mutualism, relative to other species interactions, has long been debated in the theoretical literature10,11. 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.

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Acknowledgements

Comments from A. Agrawal, E. Baack, E. Bruna, H. Cushman, A. Pauw, J. Rudgers, M. Stanton, S. Strauss and H. Robertson improved this manuscript. I thank E. Baack, W. Bond, D. Doak, M. and A. Johns, and P. Ward for their assistance and discussions during this project. Cape Nature Conservation and the South African Museum also provided support. This work was supported by grants from the National Science Foundation, Achievement Rewards for College Scientists Foundation, and the Center for Population Biology, UC Davis.

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Correspondence to Caroline E. Christian.

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Further reading

Figure 1: Comparison of the densities of four common, native fynbos ant species and the Argentine ant in invaded and uninvaded areas.
Figure 2: Bimodal distribution of seed size found in ant-dispersed Proteaceae.
Figure 3: Probability of dispersal for large- and small-seeded Proteaceae by four native fynbos ant species and the Argentine ant.
Figure 4: The effects of the Argentine ant on seed predation of large-seeded and small-seeded species.
Figure 5: Consequences of the Argentine ant invasion for post-fire seedling recruitment of small-seeded versus large-seeded Proteaceae.

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