Modelling coevolution in multispecies communities

We introduce the Webworld model\ which links together the ecological modelling of food web structure with the evolutionary modelling of speciation and extinction events[ The model describes dynamics of ecological communities on an evolutionary time!scale[ Species are defined as sets of characteristic features\ and these features are used to determine interaction scores between species[ A simple rule is used to transfer resources from the external environment through the food web to each of the species\ and to determine mean population sizes[ A time step in the model represents a speciation event[ A new species is added with features similar to those of one of the existing species and a new food web structure is then calculated[ The new species may "i# add stably to the web\ "ii# become extinct immediately because it is poorly adapted\ or "iii# cause one or more other species to become extinct due to competition for resources[ We measure various properties of the model webs and compare these with data on real food webs[ These properties 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 the fluctuations in the total number of species in the web[ Extinction avalanches occur when novel organisms arise which are significantly better adapted than existing ones[ We discuss these results in relation to the observed extinction events in the fossil record and to the theory of self-organized criticality