Combined effects of intraspecific competition and parasitoid attacks on the dynamics of a host population: a stage-structured model

Although it is well known that intraspecific competition affects host-parasitoid dynamics, previous studies have adopted oversimplified descriptions of competition. We developed and analysed a host-parasitoid stage-structured model in which host intraspecific competition is accounted for by a flexible density dependence function. We focused particularly on the influence of two factors generally neglected in previous studies: the strength of competition and possible differences in the effect of intraspecific competition between healthy and parasitized hosts. We showed, by means of a combination of analytical studies and simulations, that the presence of parasitoids in the system markedly changes the effects of competition on population dynamics. In particular, position in the contest-scramble spectrum, which plays a crucial role in single-population dynamics, may have a smaller effect in populations subjected to parasitism. We also showed that stability is increased when healthy hosts are more susceptible to competition than parasitized hosts. The effects of host intraspecific competition and its characteristics on dynamics strongly depend on the characteristics of parasitism and on the distribution of attacks in particular. Moreover, interactions between top-down (parasitism) and bottom-up (intraspecific competition) effects may be crucial for the regulation of host-parasitoid systems.