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Biol Lett. Aug 23, 2010; 6(4): 494–497.
Published online Jan 20, 2010. doi:  10.1098/rsbl.2009.1021
PMCID: PMC2936203
Cheaters in mutualism networks
Julieta Genini,1,3* L. Patrícia C. Morellato,1 Paulo R. Guimarães, Jr,2 and Jens M. Olesen3
1Departamento de Botânica, Laboratório de Fenologia, Instituto de Biociências, UNESP Univ Estadual Paulista, CP 199, 13506-900 Rio Claro, SP, Brazil
2Departamento de Ecologia, Instituto de Biociências, Universidade de São Paulo, 05508-900 São Paulo, SP, Brazil
3Department of Biological Sciences, Aarhus University, Ny Munkegade 160, DK-8000 Aarhus C, Denmark
*Author for correspondence (julieta.genini/at/gmail.com).
Received December 8, 2009; Accepted December 18, 2009.
Abstract
Mutualism-network studies assume that all interacting species are mutualistic partners and consider that all links are of one kind. However, the influence of different types of links, such as cheating links, on network organization remains unexplored. We studied two flower-visitation networks (Malpighiaceae and Bignoniaceae and their flower visitors), and divide the types of link into cheaters (i.e. robbers and thieves of flower rewards) and effective pollinators. We investigated if there were topological differences among networks with and without cheaters, especially with respect to nestedness and modularity. The Malpighiaceae network was nested, but not modular, and it was dominated by pollinators and had much fewer cheater species than Bignoniaceae network (28% versus 75%). The Bignoniaceae network was mainly a plant–cheater network, being modular because of the presence of pollen robbers and showing no nestedness. In the Malpighiaceae network, removal of cheaters had no major consequences for topology. In contrast, removal of cheaters broke down the modularity of the Bignoniaceae network. As cheaters are ubiquitous in all mutualisms, the results presented here show that they have a strong impact upon network topology.
Keywords: cheaters, modularity, nestedness, network topology, pollination
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