Invasive herbivores can lead to the exclusion of other herbivores in a host-parasitoid web through apparent competition

Background/Question/Methods

Upon arrival in a new habitat, invasive species can impact directly the population levels of their prey, predators or native competitors. Invasive species can also influence indirectly other species of the same trophic level by interacting with a prey or a predator they share, but demonstration of such apparent competition effects is difficult. Here we study the impact of an invasive herbivorous pest on the dynamics of another local herbivorous insect, both attacked by the same parasitoids using two models of differential equations. The first model represents the dynamics of local hosts and parasitoids and the second model includes a third species, the invading host. Both hosts follow a logistic growth whereas parasitoids follow a negative exponential growth. Parasitoids display a functional response of Holling II on the local host, and of either type II or type III on the invading host. We calculate conditions for existence and stability of the different equilibriums of the two systems, with and without invading hosts.

Results/Conclusions

Without invasive hosts, local hosts always persist with, or without parasitoids. By contrast local hosts can disappear in the presence of invading hosts. Furthermore, invasive hosts reduce the ability for the local host to reach its carrying capacity, independently of the functional response used on the invasive host. Conditions for extinction of local hosts are less restrictive with a functional response of Holling type II than with a functional response of Holling type III on the invasive host.

Thus, the invasion by a new herbivore can lead to the extinction of local herbivores through apparent competition. The issue is function of the mortality rate of the parasitoids and the carrying capacity of invading hosts.