The nightshift lowdown: can ants buffer climate change through shifts in vertical and temporal activity?

Abstract

Temperature fluctuations across space and time create a multidimensional thermal landscape within which organisms are exposed to local climates while conducting their daily activities. Tropical species are considered to be particularly sensitive to climate change, with narrow thermal safety margins - the buffer between operative and lethal temperatures. In tropical rainforests, however, species can hypothetically mediate thermal exposure via activity over two local thermal dimensions: vertical (ground-canopy) and temporal (day-night). Such spatiotemporal flexibility could protect species from elevated temperatures and improve thermal safety margins, but this mechanism has not been previously investigated. We test this hypothesis using rainforest ants at a warm lowland and cool upland site (100, 1200 m a.s.l.) in the Australian Wet Tropics. At lowland and upland sites, we quantified microclimate, foraging activity, community composition, and thermal ecology of ants across vertical and temporal dimensions. To assess spatiotemporal flexibility as a climate change mitigation strategy, we calculated thermal safety margins (TSM) as the difference between a species upper thermal limit (CTmax) and mean activity temperature (Te). For each species in each of their spatiotemporal niches (ground-arboreal-day-night) we test whether shifting activity to cooler niches increases TSM using the hottest niches (arboreal and/or daytime) as a baseline. At both lowland and upland sites, ant species were highly stratified vertically, but the large majority (77 - 87.5%) were active both day and night, indicating widespread temporal generalisation. Shifting activity to cooler parts of the thermal landscape substantially improved TSMs: in the lowlands, species with arboreal diurnal foraging increased their TSM by an average of 4.4 {degrees}C ({+/-} 1.7 SE) by shifting to the ground and 6.7 {degrees}C ({+/-} 1.63 SE) by shifting to nocturnal foraging. Improvements were more modest in the uplands: arboreal diurnal foragers increased TSM by 2.1 {degrees}C ({+/-} 2.07 SE) and 2 {degrees}C ({+/-} 0.28 SE) for ground and nocturnal shifts respectively. We therefore demonstrate that foraging niche flexibility is an important climate-change mitigation trait and is most beneficial in the lowlands. Lowland diurnal canopy specialists, however, are most at risk. This represents a large proportion of tropical rainforest biodiversity, supporting previous hypotheses of lowland biotic attrition under climate change.