COS 265-5 - Assessing intra- to interspecific variation in thermal tolerance in a species complex of seed-dispersing ants

Abstract: Ants in the genus Aphaenogaster are the main seed-dispersers of herbaceous plants in eastern North American deciduous forests. Yet, we are only starting to understand functional variation among several recognized species of these ecologically important ants. For instance, two named species (A. rudis and A. picea) are polyphyletic and morphologically challenging to distinguish, and may not be two true species, or may be hybridizing. To better partition ecologically significant variation, we uncover potential continuous variation in traits and behaviors among populations in this species complex. Here, we assess population-level (in addition to named species-level) variation in thermal tolerances of A. rudis and A. picea, an important trait that affects the timing of seed dispersal. We tested heat tolerance or critical thermal maximum (CTmax) and cold tolerance or critical thermal minimum (CTmin) of 217 Aphaenogaster colonies from 35 sites (i.e., populations) across latitudinal (eight states from West Virginia to Vermont) and elevational (30–500m) gradients. In addition to binning populations into putative species using traditional diagnostic traits, we performed 2D morphometrics on ant thoraxes to capture intermediate phenotypes in this species complex. Best-fit models (using Akaike Information Criterion) included variation in thermal tolerance between named species, along variation in 2D morphometrics, and across environmental gradients. 2D morphometrics revealed continuous variation in the species complex, with more A. rudis-like colonies, more A. picea-like colonies, and intermediate ants. Thorax shape or 2D morphometrics was the best-fit model explaining variation in Ctmin, showing more A. rudis-like colonies having higher Ctmin. Latitude was the best-fit model that explained variation in thermal range (Ctmax-Ctmin). A significant positive relationship showed colonies at higher latitudes had greater thermal ranges. Future work includes incorporating microhabitat and BIOCLIM variables in models to determine whether habitat influences thermal tolerance variation. This study provides insights into how ants with different thermal tolerances will shift their ranges or foraging timing in response to thermal challenges, and how this shift may influence which plant species in communities are dispersed. By accurately partitioning variation, we can better understand how functional differences in seed-dispersing ants can scale up to affect mutualist communities in eastern deciduous forests.