MODELING COEVOLUTION IN MULTISPECIES COMMUNITIES

We introduce the Webworld model, which links together the ecological m odelling of food web structure with the evolutionary modelling of spec iation and extinction events. The model describes dynamics of ecologic al communities on an evolutionary time-scale. Species are defined as s ets of characteristic features, and these features are used to determi ne interaction scores between species. A simple rule is used to transf er resources from the external environment through the food web to eac h of the species, and to determine mean population sizes. A time step in the model represents a speciation event. A new species is added wit h features similar to those of one of the existing species and a new f ood web structure is then calculated. The new species may (i) add stab ly to the web, (ii) become extinct immediately because it is poorly ad apted, or (iii) cause one or more other species to become extinct due to competition for resources. We measure various properties of the mod el webs and compare these with data on real food webs. These propertie s include the proportions of basal, intermediate and top species, the number of links per species and the number of trophic levels. We also study the evolutionary dynamics of the model ecosystem by following th e fluctuations in the total number of species in the web. Extinction a valanches occur when novel organisms arise which are significantly bet ter adapted than existing ones. We discuss these results in relation t o the observed extinction events in the fossil record, and to the theo ry of self-organized criticality. (C) 1998 Academic Press