COS 278-5 - Biogeographical patterns in competitor-enemy interactions suggests release from antagonistic interactions for a range-expanding insect

Background/Question/Methods

Species are expanding their ranges poleward as a consequence of anthropogenic change. As interacting species are not likely to move in concert, range-expanding species will likely move into less diverse communities and experience ‘high niche opportunities,’ due to reduced competition or predation. Range-expanding species may be released directly from enemies if specialists fail to follow from the native range or indirectly from weak apparent competition if generalist enemies fail to switch from competitors or if predators or competitors are not present in the expanded range. We study a recent poleward expansion by oak gall wasp Neuroterus saltatorius that occurs on a dominant oak in western North America, Quercus garryana. In its expanded range, it is outbreaking and threatening oak ecosystems. We examine how interactions with enemies (parasitoids) and competitors (co-occurring gall wasps) change over the range of Q. garryana and in the native and expanded range of N. saltatorius, and if changes create high niche opportunities. We systematically sampled gall wasps on 540 trees at 18 sites from northern California to Vancouver Island, BC, and identified emergent parasitoid wasps to morphospecies. We created quantitative host-parasitoid networks and compared metrics across latitude and in the native and expanded range.

Results/Conclusions

We found a negative relationship between latitude and gall wasp richness, parasitoid richness, and network diversity, suggesting that this community follows a latitudinal diversity gradient. We found a similar pattern between latitude and the number of interactions, suggesting that the range-expanding species might experience weaker interactions (i.e., high niche opportunities) at the poles. Networks were more generalized at higher latitudes in that they were more connected. N. saltatorius had a different composition of parasitoid enemies in its expanded range that resulted from a change in the frequency of generalist parasitoids but not due to a loss of specialists. Potential for apparent competition was lower at the poles, with lower overlap in frequency of parasitoids attacking shared hosts. Low apparent competition resulted from a loss of apparent competitors that shared parasitoids with N. saltatorius in the native range but were not present in the expanded range, such as Andricus kingi. Apparent competitors were also weaker in the expanded range, such as A. opertus that shared fewer enemies with N. saltatorius in the expanded range. Taken together, poleward networks may provide high niche opportunities by having fewer and weaker apparent competitors.