Recipient host-parasitoid communities provide weak biotic resistance to range-expanding gall wasp (Hymenoptera: Cynipidae) despite evidence of host switching by generalist parasitoids

Anthropogenic change is causing species to expand their ranges into new locations. As species move, they will lose interactions with species that fail to follow and gain new interactions with species that are not coevolved or coadapted. A loss of or less effective interactions with competitors and enemies provide open niche opportunities or weak biotic resistance that may result in “ecological release.” Niche opportunities are determined by traits of interacting species and are facilitated by non-overlapping traits in recipient communities. Here, we study a poleward range-expanding oak gall wasp, Neuroterus saltatorius (Hymenoptera: Cynipidae), which occurs on Quercus garryana, and experiences ecological release in western oak ecosystems. We created quantitative host-parasitoid interaction networks at 10 sites and compared network diversity, structure, and function between the native and expanded range. We also measured morphological traits of hosts and parasitoids and calculated peak attack timing of parasitoids on hosts in both regions. Interaction networks were less diverse, more connected, and more generalized in the expanded range. More generalized interactions result in strong morphological trait matching in the expanded range. Despite evidence that parasitoids in recipient communities readily attack the novel host, we found lower potential for apparent competition between the novel host and co-occurring hosts and lower parasitoid attack rates of the novel host in the expanded range. Phenological mismatching between parasitoids attacking the novel and co-occurring hosts was greater in the expanded range and could be one mechanism by which the range-expanding host is experiencing weak biotic resistance and ecological release.