Species ranges are changing in response to anthropogenic change, yet interacting species are not likely moving in concert. As a result, species interactions are being disrupted, formed a new, or are occurring in different contexts. Understanding the complex ways in which networks of interacting species are altered, and the outcome of altered interactions on range-expanding species and recipient communities is a pressing question. We leverage a recent, poleward range expansion of an oak gall-wasp species, Neuroterus saltatorius (Hymenoptera: Cynipidae) that is outbreaking in its expanded range. It shares its host plant with several other cynipid wasp species that are attacked by a suite of parasitoid wasps. We performed standardized sampling of cynipid galls, including our focal species, on 400 trees spread throughout four sites in the native and six in the expanded region over the development time of the focal species. We reared out and identified gall emergents to the lowest taxonomic unit. We created quantitative host-parasitoid networks and compared community and network metrics between regions to examine i) how network structure changes between native and expanded regions, ii) how parasitoids assemble onto a range-expanding host, iii) and if changes in network structure provide biotic resistance or facilitate ecological release.
Results/Conclusions
We found 15 cynipid species, with higher richness in the native region. Stem cynipid species were more common and abundant in the native region, and leaf cynipid species, including our focal species, were more abundant in the expanded region. We identified 63 parasitoid wasp species, 12 inquiline species (i.e., wasps that inhibit but don’t produce gall tissue), and revealed 183 unique host-parasitoid interactions. Parasitoid species richness and host-parasitoid interactions were higher in the native region. Despite lower richness, networks had higher connectance in the expanded region. More connected networks are often more generalized, and we found that networks in the expanded region had a lower proportion of specialist parasitoids, lower network specialization (H2), and fewer compartments. By comparing interactions between native cynipids and our focal cynipid in both regions, we inferred how parasitoids assembled onto the range-expanding host. N. saltatorius likely lost four specialist parasitoids, with two specialists “hitching a ride” from the native region. Host “switching” of generalists from other cynipid species in the expanded region was common. Lower attack rates suggest that the generalized networks in the expanded region are not providing biotic resistance and that altered host-parasitoid interactions are likely facilitating ecological release.