Thermal diversity of North American ant communities: Cold tolerance but not heat tolerance tracks ecosystem temperature

Abstract

AbstractAimIn ectotherms, gradients of environmental temperature can regulate metabolism, development and ultimately fitness. The thermal adaptation hypothesis assumes that thermoregulation is costly and predicts that more thermally variable environments favour organisms with wider thermal ranges and thermal limits (i.e., critical thermal minima and maxima, CTmin and CTmax) which track environmental temperatures. We test the thermal adaptation hypothesis at two biological levels of organization, the community and species level.LocationContinental USA.Time periodMay–August 2016 and May–August 2017.Major taxa studiedAnts (Hymenoptera:Formicidae).MethodsWe used ramping assays to measure CTmax and CTmin for 132 species of North American ants across 31 communities spanning 15.7° of latitude.ResultsAnts were cold tolerant in cooler environments particularly at the community level where CTmin was positively correlated with the maximum monthly temperature (CTmin = 0.24Tmax − 0.4; R2 = .39, p < .001). In contrast, most ant communities included some highly thermophilic species, with the result that CTmax did not covary with environmental temperature means or extremes. Consequently, we found no evidence that thermally variable environments supported ant communities with broader thermal ranges. We found a strong phylogenetic signal in CTmax but not CTmin. Species level responses paralleled community data, where maximum monthly temperatures positively correlated with species CTmin but not CTmax, which was significantly lower in subterranean species.Main conclusionsOur results suggest a large fraction of continental trait diversity in CTmax and CTmin can be found in a given ant community, with species with high CTmax widely distributed regardless of environmental temperature. Species level analyses found the importance of local microclimate and seasonality in explaining thermal tolerances. Frequent invariance in CTmax of insects at a large scale might be caused by (a) local adaptations to a site's microclimates and (b) species acclimation potential, both of which cannot be accounted for with mean annual temperatures.