Biogeographical patterns in competitor-enemy interactions suggest release from apparent competition in a range-expanding insect

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Rachel Chen, Dylan G. Jones, Aly K. Milks and Kirsten M. Prior, Department of Biological Sciences, Binghamton University (SUNY), Thomas H.Q. Powell, Department of Biological Sciences, Binghamton University (SUNY), Binghamton, NY

Background/Question/Methods
Species are expanding their ranges poleward as a consequence of global climate change. As interacting species are not likely to move in concert, range-expanding species may experience “ecological release” from natural enemies and competitors in their expanded range. 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 competitors are not present in the expanded range. We study a recent poleward expansion by oak gall wasp Neuroterus saltatorius (Hymenoptera: Cynipidae), that occurs on Quercus garryana, from mainland western North America to Vancouver Island, BC. In its expanded range, it is outbreaking and threatening oak savanna ecosystems. We examine how interactions with enemies (parasitoids) and competitors (co-occurring cynipids on Q. garryana) change over the range of Q. garryana and in the native and expanded range of N. saltatorius, and if changes relate to ecological release. We systematically sampled cynipids on 540 trees at 18 study sites from northern California to BC, and are identifying emergent parasitoid wasps to morphospecies using taxonomic keys.
Results/Conclusions
We found 24 cynipid morphospecies that co-occur with N. saltatorius on Q. garrayna throughout its range, with a subset of cynipids in the expanded range. We found that N. saltatorius had a different composition of parasitoid enemies in its expanded range compared to its native range that resulted from a change in generalist parasitoids attacking the range-expanding host, but not a loss of specialists. We also identified parasitoids from several co-occurring competitor cynipids, finding no overlap in parasitoids with Andricus quercuscalifornicus, suggesting that it is not a competitor and does not contribute to the release of N. saltatorius. However, A. kingi and A. coortus, share enemies in the native range but do not occur in the expanded range, making them competitors that likely contribute to release. So far, we are finding that the range-expanding species is likely experiencing release from antagonistic interactions via a loss of apparent competition. We will also report results of parasitoids species attacking other cynipid competitors. Our study demonstrates the effect global climate change has on host-parasitoid communities via asynchronous range shifts and decoupling interactions, a phenomenon that is increasingly occurring in insect-parasitoid communities.