Critical thermal maxima in neotropical ants at colony, population, and community levels

Global warming is exposing many organisms to severe thermal conditions and is having impacts at multiple levels of biological organisation, from individuals to species and beyond. Biotic and abiotic factors can influence organismal thermal tolerance, shaping responses to climate change. In eusocial ants, thermal tolerance can be measured at the colony level (among workers within colonies), the population level (among colonies within species), and the community level (among species). We analysed critical thermal maxima (CT_max) across these three levels for ants in a semiarid region of northeastern Brazil. We examined the individual and combined effects of phylogeny, body size (BS), and nesting microhabitat on community-level CT_max and the individual effects of BS on population- and colony-level CT_max. We sampled 1864 workers from 99 ant colonies across 47 species, for which we characterised CT_max, nesting microhabitat, BS, and phylogenetic history. Among species, CT_max ranged from 39.3 to 49.7°C, and community-level differences were best explained by phylogeny and BS. For more than half of the species, CT_max differed significantly among colonies in a way that was not explained by BS. Notably, there was almost as much variability in CT_max within colonies as within the entire community. Monomorphic and polymorphic species exhibited similar levels of CT_max variability within colonies, a pattern not always explained by BS. This vital intra- and inter-colony variability in thermal tolerance is likely allows tropical ant species to better cope with climate change. Our results underscore why ecological research must examine multiple levels of biological organisation.