Arboreality drives heat tolerance while elevation drives cold tolerance in tropical rainforest ants

Determining how species thermal limits correlate with climate is important
for understanding biogeographic patterns and assessing vulnerability to
climate change. Such analyses need to consider thermal gradients at
multiple spatial scales. Here we relate thermal traits of rainforest ants
to microclimate conditions from ground to canopy (microgeographic scale)
along an elevation gradient (mesogeographic scale) and calculate warming
tolerance in the Australian Wet Tropics Bioregion. We test the thermal
adaptation and thermal niche asymmetry hypotheses to explain interspecific
patterns of thermal tolerance at these two spatial scales. We tested
CTmin, CTmax, and calculated CTrange using ramping assays for 74
colonies of 40 ant species collected from terrestrial and
arboreal habitats at lowland and upland elevation sites and recorded
microclimatic conditions for one year. Within sites, arboreal ants were
exposed to hotter microclimates and on average had a 4.2°C (95% CI: 2.7 –
5.6°C) higher CTmax, and 5.3°C (95% CI: 3.5 – 7°C) broader CTrange than
ground-dwelling ants. This pattern was consistent across the elevation
gradient, whether it be the hotter lowlands or the cooler uplands. Across
elevation, upland ants had significantly lower CTmin than lowland ants,
whereas the change in CTmax was less pronounced, and CTrange did not
change over elevation. Differential exposure to microclimates, due to
localised niche preferences, drives divergence in CTmax while
environmental temperatures along the elevation gradient drive divergence
in CTmin. Our results suggest that both processes of thermal adaptation
and thermal niche asymmetry are at play depending on the spatial scale of
observation, and we discuss potential mechanisms underlying these
patterns. Despite the broad thermal tolerance range of arboreal rainforest
ants, lowland arboreal ants had the lowest warming tolerance and may be
most vulnerable to climate change.