The role of cheaters on the stability of mutualistic networks
To understand how species-rich mutualisms persist is an unsolved ecological problem. Some studies of mutualistic networks show the importance of their organization to persistence of populations and biodiversity maintenance. Nevertheless, the assumption that all the species in a mutualistic network are truly mutualistic is not supported by knowledge on the natural history of interacting species. There are cheater species that explore the mutualisms without providing any or few benefit to the partner in many systems, including cleaning interactions in coral reefs, pollination by animals, seed dispersal by vertebrates, and plant defense by ants. Cheating thus may play an important role on the ecological dynamics of mutualistic networks. Here, we combine ecological networks data and stability analysis to show that cheaters may promote stability in mutualistic networks. We considered cheaters in two different ways: species cheating on all its interactions and cheating only on some interactions. Then, we considered different proportions of animal cheaters and cheater interactions in each network. We elaborated theoretical networks using the same structural organization of natural mutualistic networks. We performed numerical simulations, to estimate how dynamics changed in the presence of cheaters. We also correlated network structure of these natural assemblages to the variations in stability.
The presence of cheaters, cheating in all its interactions as well in only some of its interactions, increases stability in mutualistic networks. However, these results depend on the network organization. Very nested networks acquire stability in the presence of cheaters faster than the modular networks do. Therefore, we show that the structure of mutualistic interactions seems to be a key ingredient to the stability increase. We hypothesize that evolutionary processes based on mutualisms affect community persistence in two distinct ways. While on the one hand, selection favoring mutualisms may destabilize communities, on the other hand the very existence of mutualistic interactions favors the evolution of cheaters leading to stabilizing effects. Indeed, by these theoretical results, we would expect that modular and persistent mutualistic networks should have a higher proportion of cheaters than nested and persistent networks. Cheaters are part of the natural history of most mutualisms and these results emphasize the importance of considering natural history aspects when inferring ecological networks dynamics.