Distribution and abundance of parasite nematodes: ecological specialisation, phylogenetic constraint or simply epidemiology?

We investigate the patterns of abundance-spatial occupancy relationships of adult parasite nematodes in mammal host populations (828 populations of ne matodes from 66 different species of terrestrial mammals). A positive relat ionship between mean parasite abundance and host occupancy, i.e. prevalence , is found which suggests that local abundance is linked to spatial distrib ution across species. Moreover, the frequency distribution of the parasite prevalence is bimodal, which is consistent with a core-satellite species di stribution. In addition, a strong positive relationship between the abundan ce (log-transformed) and its variance (log-transformed) is observed, the di stribution of worm abundance being lognormally distributed when abundance v alues have been corrected for host body size. Hanski et al. proposed three distinct hypotheses, which might account for t he positive relationship between abundance and prevalence in free and assoc iated organisms: 1) ecological specialisation, 2) sampling artefact, and 3) metapopulation dynamics. In addition, Gaston and co-workers listed five ad ditional hypotheses. Four solutions were not applicable to our parasitologi cal data due to the lack of relevant information in most host-parasite stud ies. The fifth hypothesis, i.e. the confounded effects exerted by common hi story on observed patterns of parasite distributions, was considered using a phylogeny-based comparison method. Testing the four possible hypotheses, we obtained the following results: 1) the Variation of parasite distributio n across host species is not due to phylogenetic confounding effects; 2) th e positive relationship between mean abundance and prevalence of nematodes may not result from an ecological specialisation, i.e. host specificity, of these parasites; 3) both a positive abundance-prevalence relationship and a negative coefficient of variation of abundance-prevalence relationship ar e likely to occur which corroborates the sampling model developed by Hanski et al. We argue that demographic explanations may be of particular importa nce to explain the patterns of bimodality of prevalence when testing Monte- Carlo simulations using epidemiological modelling frameworks, and when cons idering empirical findings. We conclude that both the bimodal distribution of parasite prevalence and the mean-variance power function simply result f rom demographic and stochastic patterns (highlighted by the sampling model) , which present compelling evidence that nematode parasite species might ad just their spatial distribution and burden in mammal hosts for simple epide miological reasons.