PARASITE-MEDIATED COMPETITION - A MODEL OF DIRECTLY TRANSMITTED MACROPARASITES

There is growing interest in host-parasite ecology, particularly with regard to the effect of parasites on host community structure, but few studies have explored the theory of parasite mediation of host compet ition. I developed a model to examine the impact of directly transmitt ed macroparasites on the competition and population dynamics of two sy mpatric host species. Several important features of macroparasite-medi ated competition are considered in the model: the two host species com pete in a classical Lotka-Volterra manner, the parasite influences the survivorship and reproduction of both hosts, and parasites exhibit ag gregated distribution among host populations. The conditions for host coexistence were derived explicitly. The results suggest that host int rinsic growth rate, host carrying capacity, susceptibility, parasite p athogenicity, and the magnitude of parasite aggregation are important components influencing host coexistence. Parasites may enhance or disr upt host coexistence, depending on the relationships of these paramete r values. If the individuals of the inferior species are less suscepti ble to infection than the individuals of superior species and/or exhib it less pathology, the inferior competitor will bear a competitive adv antage, resulting in either stable coexistence or competitive exclusio n. In contrast, parasites may cause more rapid extinction of the infer ior species if the inferior species is more susceptible to and/or less tolerant of parasite infection. When two host species are equally sus ceptible to infection and parasites are equally pathogenic, parasites may still have significant consequences on the competitive outcome. Se nsitivity analysis suggests that an increased degree of parasite aggre gation among hosts reduces the ability of parasites to regulate host p opulations and decreases parasite equilibrium density. Finally, micro- and macroparasite models are compared.