Latitudinal gradients in species diversity provides high niche opportunities for range-expanding phytophagous insect

Background/Question/Methods

When species undergo poleward range expansions in response to anthropogenic change, they likely experience less diverse communities in new locations. If low diversity communities provide weak biotic interactions, such as reduced competition or predation, range-expanding species may experience “high niche opportunities.” Here, we uncover if oak gall wasp communities experience a latitudinal diversity gradient (LDG) and if weaker local interactions at the poles create high niche opportunities for a poleward range-expanding community member (Neuroterus saltatorius). To this end, we performed systematic surveys of oak gall wasps on a dominant oak, Quercus garryana, throughout most of its range, from northern California to Vancouver Island, BC. In six regions, on 540 trees at 18 sites, we identified 23 morphotypes in three guilds (foliar detachable, foliar integral, and stem galls). We performed regressions to uncover latitudinal patterns in richness. We performed partial correlations at the local scale to reveal patterns in oak gall wasp associations and interactions on individual host plants - the scale they are most likely to compete.

Results/Conclusions

We found a negative relationship between oak gall wasp richness and latitude, suggesting the community follows a LDG as predicted. We also found a negative relationship between oak gall wasp abundance and latitude, but only without the range-expanding species. N. saltatorius had a positive relationship with latitude, as it is outbreaking in its expanded range. N. saltatorius associated on host plants most closely with species in similar gall guilds (foliar detachable galls). As species richness in this guild most strongly decreased with latitude, interactions with N. saltatorius were lost in the expanded range at northern latitudes. Negative relationships between N. saltatorius abundance and abundance of other detachable foliar galls co-occurring on trees suggest potential antagonistic interactions. Taken together, ecological release of the range-expanding species may result from high niche opportunities, 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.