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

Abstract

1. When species undergo poleward range expansions in response to anthropogenic change, they likely encounter less diverse communities in new locations. If low diversity communities provide weak biotic interactions, such as reduced competition for resources or predation, range-expanding species may experience high niche opportunities. 2. Here, we uncover if oak gall wasp communities experience a latitudinal diversity gradient (LDG) and weaker interactions at the poles that might create high niche opportunities for a range-expanding community member. 3. 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, British Columbia. On 540 trees at 18 sites, we identified 23 species in three guilds (leaf detachable, leaf integral, and stem galls). We performed regressions between oak gall wasp diversity, latitude, and other abiotic and habitat factors to reveal if cynipid communities follow an LDG. To uncover patterns in local interactions, we performed partial correlations on oak gall wasps co-occurring on trees within regions. Finally, we performed regressions between abundances of co-occurring gall wasps on trees to reveal potential interactions. 4. Q. garryana-gall wasp communities followed an LDG, with lower diversity at higher latitudes, particularly with a loss of detachable leaf gall species. Detachable leaf gall wasps, including the range-expanding species, co-occurred most on trees and had weak interactions in the northern region. Abundances of the range-expander and detachable and integral leaf galls co-occurring on trees were negatively related, suggesting antagonistic interactions. Overall, we found that LDGs create communities with weaker antagonistic interactions at the poles that might facilitate ecological release in a range-expanding community member. 5. Here, we uncover how regional and local scale patterns and processes create high niche opportunities for a range-expanding community member. This research provides insight into how biogeographical patterns in communities and species interactions influence the outcome of range expansions. Given the ubiquity of LDGs, these outcomes might be expected in other phytophagous insect communities.

Host-enemy interactions provide limited biotic resistance for a range-expanding species via reduced apparent competition

“…Poleward range-expanding species may experience weaker biotic interactions when they move into low diversity communities at the poles (21,23) except see (85). We found fewer host and parasitoid species in the poles and expanded range.…”

“…The agamic generation of Nsal occurs at higher abundance on Q. garryana in its expanded range, with higher frequency of infested trees. Some trees are infested in the native range of Nsal, but at low frequency (18,23,65). Particularly high abundances can cause foliar scorching with negative effects on oaks and oak associated species (18,48,49,55).…”

“…In 2017, we chose 10 study sites that were patches of oaks. We chose four sites in Nsal's native range that were the largest oak patches with Nsal closest to its expanded range (18,23) and six sites in the expanded range that are some of the largest intact Q. garryana sites in the expanded range (Fig. 1).…”

“…Yet, there are growing examples of short-distance expanders (i.e., intra-continental expanders) experiencing ecological release (3,(17)(18)(19). Short-distance expanders may experience release not just from coevolved or coadapted species, but also from coadapted populations (20)(21)(22), and especially when moving poleward may encounter a reduction in community diversity in northern locations (9,18,21,23). Moreover, instances of short-distance expansions are particularly tractable for testing hypotheses about the consequences of altered biotic interactions because the similarity of ecosystems and underlying species interaction networks over short distances allows for more direct comparisons of the biotic interactions affecting the focal species.…”

“…Host gall morphology and phenology influence the assemblages of enemies attacking host species (42,51,53). Oak gall wasp-enemy communities are tractable multi-trophic communities that are excellent systems for uncovering direct and indirect trophic interactions and how interactions are altered under anthropogenic change (18,23,38,(50)(51)(52)54,55).…”