Parasite species richness and its effect on persistence in food webs.

Parasitic species are likely to have a significant effect on the stability of ecosystems. However, little is known of the nature of this effect, with debate over whether it is positive or negative. In previous work it was observed that a mixture of interaction types increases the local stability of a network. Following this, we investigate the consequences for species persistence of replacing host species with parasitic species. We consider systems with varying mixtures of mutualistic and antagonistic interactions, showing that the effect of parasitic interactions on a system depends on both the interaction types present and the levels of parasitism considered. Higher levels of mutualism make a system vulnerable to destabilisation on the addition of parasite species. However, for systems with antagonistic interactions, persistence in the system decreases primarily due to the failure of parasite species to persist. This increases with increasing proportions of parasite species, leading to a peak number of parasite species able to persist. Increasing parasite species richness does not have as significant an effect on host species richness as we might expect; although parasites have an important role to play in ecological networks, their effect on persistence is seen primarily through their own self-limitation.