Shared parasitoids in a meta-community: Indirect interactions inhibit herbivore membership in local communities

To date, we lack field experiments that assess the importance of apparent competition to metacommunity structure. Here, I conducted a series of field experiments to test whether apparent competition, mediated by shared egg parasitoids (Anagrus spp.), occurs between two planthopper species (Delphacodes scolochloa and Prokelisia crocea) of the North American Great Plains. The two planthoppers feed on different plant species within prairie potholes and, thus, do not interact directly. At the scale of individual potholes, a five-fold increase in Delphacodes density (relative to control potholes) resulted in a steady decline in Prokelisia density over two generations. Concurrently, Prokelisia eggs in these potholes suffered twice the level of parasitism as Prokelisia eggs in control potholes. In contrast, a six-fold increase in Prokelisia density had no effect on Delphacodes density or parasitism in those potholes. The superiority of Delphacodes over Prokelisia likely can be attributed to two main factors, a larger source population size and the presence of a phenological refuge from parasitism for Delphacodes. In another experiment, in which small populations of Prokelisia were established either in close proximity to Delphacodes or in isolation, I found that the likelihood of Prokelisia persistence was 36% lower in the former than the latter populations. This difference was attributable to very high rates of parasitism of Prokelisia when adjacent to Delphacodes. These two experiments provide strong evidence that the two planthopper species engage in apparent competition and that the shared parasitoids can play a significant role in limiting membership in local communities.