Host-mediated site-segregation of ectoparasites: an individual-based simulation study

Coexisting ectoparasite species tend to evolve different site-specificities due to site-segregation. This may result either from a direct interaction between ectoparasite species such as competition for resources or from the selective pressure upon parasite populations exerted by host defence, i.e. a host-mediated interaction. Here we present an individual-based simulation model for the coevolution of one host and two parasite species to test this latter hypothesis. Parasite species are generalists at the start of the simulation process and develop site-specificities due to selective pressure exerted by the hosts. Hosts are selected to maximise the effectiveness of defence. We describe certain conditions that affect the emergence of site-segregation under the following assumptions: (1) parasite populations are not subject to resource limitations but are limited directly by host defence as ‘predation’; (2) hosts have two sites that need different defensive abilities to reduce their parasite burden; (3) parasites need to exhibit different evasive abilities to survive on different sites; (4) both the defensive abilities of the hosts and the evasive abilities of the parasites have a genetic background; (5) host grooming selects parasites for an increasing capability for evasion while parasites select hosts for an increasing efficiency of grooming, (6) parasite transmission is horizontal. Two trade-offs are incorporated into the model, a host trade-off between host defensive abilities on the two sites; and a parasite trade-off in parasite evasive abilities on the two sites. The following features of host-parasite coevolution are analyzed by simulation experiments: (a) persistence of coexistence of the two parasite species; (b) site-specialization of parasite species; (c) site-segregation of parasite species; (d) frequency distribution of parasites. We conclude that the optimization of host defence and parasite evasion strategies may select ectoparasites for site-segregation and this may stabilize the coexistence of parasite species. When both parasite species survived, complexity increased spontaneosly during the simulation runs: (i) frequency distribution of parasites changed from uniform to an aggregated one; (ii) generalist parasite species developed site-specificities and (iii) tended to segregate their specificities toward different sites. Thus a macroevolutionary trade-off emerged between the two parasite species.