  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 timescale. 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.
