The modularity of seed dispersal: differences in structure and robustness between bat– and bird–fruit networks

Abstract

In networks of plant–animal mutualisms, different animal groups interact preferentially with different plants, thus forming distinct modules responsible for different parts of the service. However, what we currently know about seed dispersal networks is based only on birds. Therefore, we wished to fill this gap by studying bat–fruit networks and testing how they differ from bird–fruit networks. As dietary overlap of Neotropical bats and birds is low, they should form distinct mutualistic modules within local networks. Furthermore, since frugivory evolved only once among Neotropical bats, but several times independently among Neotropical birds, greater dietary overlap is expected among bats, and thus connectance and nestedness should be higher in bat–fruit networks. If bat–fruit networks have higher nestedness and connectance, they should be more robust to extinctions. We analyzed 1 mixed network of both bats and birds and 20 networks that consisted exclusively of either bats (11) or birds (9). As expected, the structure of the mixed network was both modular (M = 0.45) and nested (NODF = 0.31); one module contained only birds and two only bats. In 20 datasets with only one disperser group, bat–fruit networks (NODF = 0.53 ± 0.09, C = 0.30 ± 0.11) were more nested and had a higher connectance than bird–fruit networks (NODF = 0.42 ± 0.07, C = 0.22 ± 0.09). Unexpectedly, robustness to extinction of animal species was higher in bird–fruit networks (R = 0.60 ± 0.13) than in bat–fruit networks (R = 0.54 ± 0.09), and differences were explained mainly by species richness. These findings suggest that a modular structure also occurs in seed dispersal networks, similar to pollination networks. The higher nestedness and connectance observed in bat–fruit networks compared with bird–fruit networks may be explained by the monophyletic evolution of frugivory in Neotropical bats, among which the diets of specialists seem to have evolved from the pool of fruits consumed by generalists.

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Acknowledgments

We are especially grateful to all people who made their data available in papers, monographs or databases. Carsten Dormann, Mário Almeida-Neto, Nico Blüthgen, Roger Guimerà and Vladimir Batagelj helped us use their network software. Theodore Fleming made invaluable suggestions on an early version of this manuscript. Diego Vazquez created the Interaction Web Database (http://www.nceas.ucsb.edu/interactionweb/), from which we took some datasets. Many colleagues, especially Wibke Thies, helped to complement the ‘Kalko BCI dataset’ after its first publication in 2004. Fundação de Amparo à Pesquisa do Estado de São Paulo (Brazil) funded this project by giving Grants to M.A.R.M. (2006/00265-0), F.M.D.M., M.A.M.A. and P.R.G. M.A.R.M. was also supported by the Alexander von Humboldt Foundation (Germany, 1134644). E.K.V.K. was supported by the Deutsche Forschungsgemeinschaft (Germany), and P.J. by the Consejo Superior de Investigaciones Cientificas (Spain). Experiments comply with the current laws of Germany, Brazil and Spain.

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Correspondence to Marco Aurelio Ribeiro Mello.

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Communicated by Steven Johnson.

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Mello, M.A.R., Marquitti, F.M.D., Guimarães, P.R. et al. The modularity of seed dispersal: differences in structure and robustness between bat– and bird–fruit networks. Oecologia 167, 131 (2011). https://doi.org/10.1007/s00442-011-1984-2

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Keywords

  • Complex networks
  • Ecosystem services
  • Food webs
  • Guilds
  • Mutualisms