COS 146-1 - The nightshift lowdown: ants take the heat out of tropical rainforest, but will it be enough in a warming world?

Background/Question/Methods

Tropical species are thought to be especially vulnerable to climate change because they are not adapted to high temperature variation and don’t cope well with temperature extremes. Rainforest species may be able to buffer climate change in situ by avoiding unfavourable temperatures, but this has not been thoroughly investigated in both spatial and temporal dimensions. At the local site scale, tropical rainforest species are potentially active over two thermal dimensions, the vertical gradient from ground to canopy (spatial) and the transition from day to night (temporal). Together these spatial and temporal gradients create a multidimensional thermal landscape within which rainforest biota can operate. Here, we investigate the spatiotemporal activity and thermal physiology of ants at contrasting elevation sites in the Australian Wet Tropics. We consider: 1) are current and projected microscale temperatures close to the upper thermal limits of ants, and 2) do ants have the potential to buffer temperatures based on their activity in space and time? At lowland and upland elevation sites, we quantify localised thermal exposure, test upper thermal limits (using ramping assays for 71 colonies of 40 ant species from ground and arboreal habitats) and characterise ant activity patterns in vertical space and time.

Results/Conclusions

Within sites, arboreal ants were exposed to hotter microclimates and, on average, had upper thermal limits 4.2°C (95% CI: 2.7 – 5.6°C) higher than ground-dwelling ants. This pattern was consistent across the elevation gradient, whether it be the hotter lowlands or the cooler uplands. Despite their tolerance to hot temperatures, lowland arboreal ants had upper thermal limits that were very close to current and projected temperature extremes. However, we found that a high proportion of species did have the capacity to be active when (at night) and where (on or near the ground) it is cooler. Such species had scope to buffer temperatures that are close to their upper thermal limits. Our results indicate that rainforest ants might have more potential for in situ behavioural buffering than previously considered, thereby reducing the impacts of global warming. Ant species that are canopy specialists with obligate daytime activity do not have this flexibility and are therefore most at risk. This is likely to apply more broadly for lowland ectotherms that are diurnal canopy specialists, which may represent a large proportion of tropical rainforest biodiversity.