Neotropical ants are at greater risk from global warming in savanna than in adjacent forest

Abstract

Determining how the thermal tolerances of species are related to climatic conditions at multiple spatial scales can improve our understanding of species distributions and their vulnerability to climate change. We compare the warming tolerances—a metric of warming vulnerability—of arboreal and ground-dwelling ants from savanna and adjacent semideciduous forest in Brazil's Cerrado. Warming tolerance was estimated using the difference between an ant's upper thermal limit and a thermal index of its habitat and stratum. We also evaluated if differences in the upper and lower thermal limits of Cerrado ant assemblages conform to the thermal adaptation (TAH) and the niche asymmetry (NAH) hypotheses. We found that the mean critical thermal maximum (CT_max) and range (CT_range) were higher for ants in savanna than in forest, a pattern that is consistent with the TAH as savanna had higher maximum and more variable air temperatures. However, arboreal ants had lower CT_min than those on the ground despite the similarities in minimum temperatures between the two strata. CT_max was lower for ground than for arboreal ants even though in the savanna (but not in the forest) average maximum air temperatures on the ground were 2°C higher than in trees. Further, the greater heat tolerance of savanna ants was less than the ~7°C difference at the ground stratum in mean maximum temperatures between savanna and forest. A moderate phylogenetic signal was found for CT_max, CT_min, and CT_range. However, accounting for phylogeny did not change our results. Our key finding is that vulnerability to global warming cannot be adequately predicted based on heat tolerance alone—species having a similar CT_max can have very different vulnerability to global warming because of differences in exposure to direct insolation of their preferred habitat or stratum. In our study system, savanna ground ants are more vulnerable to global warming compared to ants living on the forest floor or to arboreal ants more generally. This may have important implications for conservation of the Brazilian savanna ant fauna since most Cerrado species, including several endemics, nest and forage on the ground.