­­­Uncovering intra- and interspecific variation in thermal tolerance in species complex (Aphaenogaster sp.) of seed-dispersing ants along environmental gradients

Background/Question/Methods

Climate change is an imminent threat to the persistence of biodiversity. By altering timing of functionally important species interactions, climate change causes cascading effects in ecosystems. One way to predict organisms’ responses to climate change is to measure thermal tolerance traits, such as critical thermal limits defined as when organisms can no longer function. Here, we measure variation in thermal tolerance in a species complex (Aphaenogaster sp.) of the main seed-dispersing ants in North American eastern deciduous forests. This complex consists of several named species, including polyphyletic A. rudis and A. picea, which are not well delineated morphologically. Given that putative species are unresolved, with intermediate phenotypes, we are uncovering population-level (in addition to species-level) variation in thermal tolerance along environmental gradients to predict how climate change affects seed dispersal by Aphaenogaster sp. We tested heat tolerance or critical thermal maximum (CTmax) and cold tolerance or critical thermal minimum (CTmin) of 179 Aphaenogaster colonies from 25 sites (i.e., populations) across latitudinal (from West Virginia to Vermont) and elevational gradients (30–500m). In addition to binning populations into putative species using traditional diagnostic traits, we perform 2D morphometrics on ant thoraxes to capture intra- to interspecific variation in this species complex.

Results/Conclusions

In our preliminary analysis, binning colonies into putative species, we found variation in thermal tolerance between species and across environmental gradients. In particular, as expected based on previous studies, putative A. picea had lower CTmin than A. rudis. CTmax was also different between species and decreased with latitude (with no interaction). Finally, thermal range (CTmax- CTmin) increased with latitude. We also found significant unexplained variation among ant populations. To uncover intra- to interspecific population variation in this complex, we are performing 2D morphometrics on ant thoraxes that have diagnostic (yet overlapping) characteristics. We predict that variation in thermal tolerances will be better explained by continuous variation in complex thorax shape than binning colonies into putative species. Future work includes additional colony collections to fill in gaps across gradients and species. We will also incorporate microhabitat and BIOCLIM variables to uncover if habitat variation helps explain variation in thermal tolerances. This study contributes to informing future trajectories of functionally important mutualists. If species (or populations) of seed-dispersing ants with different thermal tolerances shift their ranges in response to climate change, it may negatively impact plant partners reliant on seed dispersal by ants.