The main theories of biodiversity either neglect species interactions1,2 or assume that species interact randomly with each other3,4. However, recent empirical work has revealed that ecological networks are highly structured5,6,7, and the lack of a theory that takes into account the structure of interactions precludes further assessment of the implications of such network patterns for biodiversity. Here we use a combination of analytical and empirical approaches to quantify the influence of network architecture on the number of coexisting species. As a case study we consider mutualistic networks between plants and their animal pollinators or seed dispersers5,8,9,10,11. These networks have been found to be highly nested5, with the more specialist species interacting only with proper subsets of the species that interact with the more generalist. We show that nestedness reduces effective interspecific competition and enhances the number of coexisting species. Furthermore, we show that a nested network will naturally emerge if new species are more likely to enter the community where they have minimal competitive load. Nested networks seem to occur in many biological and social contexts12,13,14, suggesting that our results are relevant in a wide range of fields.
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Acknowledgments We thank P. Jordano and J. Olesen for providing data and insight, A. Ramirez Ortiz for discussions and P. Buston and D. Stouffer for comments on a previous draft. J. Olesen provided the drawings in Fig. 1. Funding was provided by the Spanish Ministry of Science and Technology (through a Ramon y Cajal Contract and a Consolider Ingenio Project to U.B., a PhD Fellowship to M.A.F. and a grant to B.L.) and by the European Heads of Research Councils, the European Science Foundation, and the EC Sixth Framework Programme through a European Young Investigator Award (J.B.). Research at the Centro de Biología Molecular Severo Ochoa is facilitated by an institutional grant from the Ramón Areces Foundation.
Author Contributions U.B., jointly with A.P.-G., A.F. and B.L., performed the analytical development. M.A.F. analysed the real data and, jointly with B.L., performed the simulations. J.B. compiled the real data and, jointly with U.B., designed the study and wrote the first version of the manuscript.
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Bastolla, U., Fortuna, M., Pascual-García, A. et al. The architecture of mutualistic networks minimizes competition and increases biodiversity. Nature 458, 1018–1020 (2009) doi:10.1038/nature07950
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