Adaptive dynamics of trait diversification in mutualistic networks

Introduction

Species have the faculty to adapt to their environment in order to survive. This adaptive behavior has shaped the evolution of phenotypic traits in mutualistic networks such as in plant-pollinator or frugivory networks, leading to trait  convergence or diversification and the appearance of new functional groups. The evolutionary dynamics of phenotypic characters in turn affects the way species interact and consequently the complex assemblage of mutualistic networks. In this study, the interplay between the effect of mutualistic interactions on the coevolution of phenotypic traits and how phenotypic character change influences the way species interact were explored.

Methodology

A model of mutualistic network shaped by both the evolutionary dynamics of species traits and the ecological dynamics of species population was built. The population dynamics were modeled by a first order ordinary differential equation composed of a demographic term plus the supply given by the mutualistic interaction. Parameters of the population dynamics are influenced by the considered evolving traits that are representative of each species population. To link the dynamics of the populations and the evolutionary dynamics of the traits, tools provided by adaptive dynamics theory were used. Evolutionary dynamics of species traits were explored by studying the evolution of resident traits into mutant traits and the condition for both the residents and the mutants to coexist. The so-called canonical equation of adaptive dynamics was used to model the evolutionary dynamics of the system. The dynamics of the interaction networks was based on species' foraging behavior according to the benefits they gain from the interactions. Metrics of network architecture such as connectance or the nestedness were measured to analyze the organization of mutualistic networks.

Results

The appearance of evolutionary branching points leading to the emergence of traits diversification is favored when intraspecific competion for resources is strong and when interacting partner species do not have a strong selective behavior based on phenotypical trait complementarity. The strength of coevolutionary selection enhances the connectance and nestedness of the resulting networks because of a reciprocal specialization of the interacting functional groups. Moreover, the strength of intraspecific competition seems not to explicitly affect the structure of the networks.

Conclusion

For a given set of parameters, the evolution of a phenotypic trait such as  body masses for frugivores or fruit sizes for plants, interacting with ecological changes driven by resources competition and mutualism, suffices for a polymorphic population to emerge from the system. The resulting interaction networks have complex structures for the intermediate strength of the coevolutionary selection.

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Last edited: 23 March 2015