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 or predation, range-expanding species may experience high niche opportunities.2. Here, we investigated if oak gall wasp communities follow a latitudinal diversity gradient (LDG) and if lower diversity communities provide weaker interactions at the poles for a range-expanding community member, Neuroterus saltatorius.3. We performed systematic surveys of 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 oak gall wasp morphotypes in three guilds (leaf detachable, leaf integral, and stem galls). We performed regressions between oak gall wasp diversity, latitude, and other abiotic (e.g. temperature) and habitat (e.g. oak patch size) factors to reveal if gall wasp communities followed an LDG. To uncover patterns in local interactions, we first performed partial correlations of gall wasp morphotype occurrences on trees within regions).We then performed regressions between abundances of co-occurring gall wasps on trees to reveal if interactions are putatively competitive or antagonistic. 4. Q. garryana-gall wasp communities followed an LDG, with lower diversity at higher latitudes, particularly with a loss of detachable leaf gall morphotypes. Detachable leaf gall wasps, including the range-expanding species, co-occurred most on trees, with weak co-occurrences on trees in the northern expanded region. Abundances of N. saltatorius 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 associations at the poles that might facilitate ecological release in a range-expanding community member. 5. Given the ubiquity of LDGs in nature, poleward range-expanding species are likely moving into low diversity communities. Yet, understanding if latitudinal diversity pattern provides weak biotic interactions for range-expanding species is not well explored. Our large-scale study documenting diversity in a related community of phytophagous insects that co-occur on a host plant reveals that

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).…”