The influence of speciation on the evolution of complex food web structure

Background/Question/Methods

While once thought to be separated by profound differences in scale, evolution is increasingly recognized as being important to ecological dynamics.  Evolutionary processes along with community dynamics appear to be especially important determinants of ecosystem response to environmental change.  Recent studies of ecological networks such as food webs have advanced our understanding of how evolutionary processes and ecological interactions such as omnivory, competition, and mutualism affect ecological structure and stability. These and other advances such as those in understanding the role of biodiversity in ecosystem function have motivated the development of community evolution models such as ours that helps understand ecological change and unify ecological theory by integrating community and ecosystem ecology with evolutionary biology.

Our model starts with a relatively small (<20 species) food web and evolves larger networks through a process of stochastic speciation and deterministic population dynamics including extinction.  It is based on an allometric trophic network model that specifies food-web structure using a stochastic model of network architecture and community dynamics using a set of ordinary differential equations that govern the change in species’ biomass over time. Speciation proceeds by introducing new species at low population densities with traits slightly different from randomly chosen parent species. Traits include body size, trophic generality and diet based on location and feeding ranges within the one-dimensional community niche space.  Network and species properties such as diversity and biomasses are subsequently tracked through time.

Results/Conclusions

Remarkably realistic and highly diverse ecosystems emerge from our community evolution model.  They share many properties including network structure and body-size-abundance distributions with their empirical counterparts.  Others are relatively short and fat (many species at few levels) or tall and thin (few species at many trophic levels).  Including shared nutrient limitation among basal species increases the likelihood of obtaining more reasonable distributions of diversity among trophic levels.  Connectance varies during community evolution and helps predict when extinction cascades occur.  The generality, vulnerability and niche overlap of species and their neighbors within the network help determine which species persist and go extinct.  Whole network properties of the food webs evolve in response to speciation as webs increase in complexity.  This work shows how a few and relatively simple evolutionary and ecological assumptions and models can be integrated to help understand ecological change at multiple scales and unify ecological theory while yielding surprisingly complex and realistic ecosystem structures and dynamics through time.