Latitudinal gradients in the diversity of oak gall wasps (Hymenoptera: Cynipidae) provides open niche opportunities for range-expanding species

Species are shifting their ranges poleward in response to climate change. Species will likely not shift in concert and interacting species may be left behind or novel associations formed in new locations. Patterns in diversity often follow latitudinal diversity gradients (LDG) and species’ undergoing poleward range expansions may experience less diverse communities in expanded ranges. As a result, range-expanding species may experience open niche space in their expanded range, resulting in “ecological release” (i.e., higher performance than native range). Here, we study a recent poleward range-expansion by Neuroterus saltatorius (Hymenoptera: Cynipidae), an oak gall wasp, that occurs on Quercus garryana from northern California to Washington State. This species expanded its range to Vancouver Island, BC where it is outbreaking. We performed surveys of Q.garryana-cynipids over 3 periods at 18 sites (540 trees) from California to Vancouver Island, BC. We found 24 cynipid morphotypes, with a negative relationship between cynipid richness and latitude. We found a negative relationship between cynipid abundance and latitude, but only without N. saltatorius. N. saltatorius had a positive relationship with latitude, outbreaking in the expanded range. N. saltatorius associated (on trees) most closely with similar gall types (foliar detachable galls), and its associations were lost in northern latitudes. Ecological release may be a result of open niche space, resulting from release from direct or indirect competition with foliar detachable galls that are lacking in northern latitudes. This work offers valuable insight into how commonly found patterns in LDG affect the dynamics of poleward range-expansions.