How parasites divide resources : a test of the niche apportionment hypothesis

1. Modelling species abundance patterns, i.e. the distribution of relative abundance among species within the same community, has become a common framework in community ecology. To describe species abundance patterns several statistical models have been proposed, e.g. the log-normal distribution, but these do not provide an ecological explanation of the underlying processes. 2. Tokeshi introduced (1990) and developed (1993, 1996, 1999) a series of niche-orientated stochastic models to fit species abundance patterns. They suggest that abundance of species are proportional to the resources they apportion. Following this hypothesis, Tokeshi defined some processes in resource partitioning to explain species abundance patterns. 3. To study the rules which govern species assemblages, parasite communities present advantages because each host harbours a replicate community, the total niche is limited to the body of the host and demographic processes are usually similar for all parasite species in a community. Using five Tokeshi models, we searched for common rules structuring the parasite communities of six species of marine fish from the coast of Chile. The biovolume of each parasite species was preferred as a measure of its ‘abundance’ over actual numerical abundance, to account for the considerable variation in body size among parasite species. 4. The random assortment model, which suggests a lack of competition and interaction and independent apportionment of resources between species in the community, was fitted successfully to the parasite communities of three fish species for both means and variances of abundance ranks using a new Monte-Carlo method. 5. These results are in accordance with current knowledge on parasite coexistence rules, i.e. that most parasite communities appear non-saturated with species, with many empty niches and with interspecific competition not playing a major structuring role.