Abstract
In an assemblage of multiple predators sharing a single prey species, the combined effects of the component species may scale unpredictably because of emergent interspecific interactions. Prior studies suggest that chaotic but persistent community dynamics are induced by intra-/interspecific interactions between native and nonnative parasitoids competing over a shared host. Here, we test the impact of the nonnative parasitoid Heterospilus prosopidis (Hymenoptera: Braconidae) on the intraspecific interference and offspring sex ratio of the native parasitoid Anisopteromalus calandrae (Hymenoptera: Pteromalidae). We found that the nonnative parasitoid reduced intraspecific interference among native parasitoids and decreased the proportion of female offspring produced by the native parasitoid (predicted under conditions of reduced host availability). At higher host densities, the nonnative parasitoid contributed less to the total proportion of hosts parasitized, as its innate saturating Type II response changed to a dome-shaped Type IV response with increasing density of the native parasitoid, while the native parasitoid retained its increasing Type I response. This inverse host-density-dependent response between the two parasitoids and associated competitive superiority can explain the observed changes in parasitism; at high host densities, the searching efficiency of the native parasitoid increases via host feeding while the nonnative parasitoid experiences egg limitation. These results highlight the importance of the complementary top-down effects of multiple consumers on a single resource.
Highlights
In an assemblage of multiple predators sharing a single prey species, the combined effects of the component species may scale unpredictably because of emergent interspecific interactions
There was a significant interaction between A. calandrae density and the presence of H. prosopidis (Table 1), which indicates a negative effect of the nonnative parasitoid on mutual interference among A. calandrae
Mutual interference among the native parasitoid A. calandrae was reduced by the presence of the nonnative parasitoid H. prosopidis independent of host density, supporting our first hypothesis
Summary
In an assemblage of multiple predators sharing a single prey species, the combined effects of the component species may scale unpredictably because of emergent interspecific interactions. This inverse host-density-dependent response between the two parasitoids and associated competitive superiority can explain the observed changes in parasitism; at high host densities, the searching efficiency of the native parasitoid increases via host feeding while the nonnative parasitoid experiences egg limitation These results highlight the importance of the complementary top-down effects of multiple consumers on a single resource. In communities that contain multiple species of predators or parasitoids, the combined effects of each individual prey–predator (or host–parasitoid) interaction may be nonlinear rather than additive because of indirect trophic effects or the modification of direct interspecific interactions (which may be inherently nonlinear)[3] These and other emergent properties affect the structure of multispecies assemblages and result in complex population dynamics[4,5,6]. Parasitoid interacts with its hosts by modifying the mass action term independent of host density (directly or indirectly13–15)—each individual parasitoid attacks fewer hosts at higher parasitoid densities, which lowers the number of parasitoid offspring per parent recruited to the generation
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