Some like it hot, some like it cold: Thermal tolerance in Australian alpine grasshoppers

Alpine species are distributed across steep environmental gradients and turnover of closely related species along these clines is common. Thermal tolerance is frequently inferred as a proximal driver of both high- and low-elevation range limits across a wide range of species, as temperature decreases rapidly with elevation. Furthermore, locally adaptive genetic variation within species can be maintained where populations exist along an environmental gradient. Three species of alpine-endemic grasshopper (Kosciuscola genus) occupy overlapping elevation zones in the mountains of southeastern Australia. We explored role of thermal tolerance in shaping distribution patterns of the Kosciuscola in the NSW alpine region, and the potential for local adaptation in thermal limits. All species showed remarkable thermal tolerance ranges of over 50°C, reflecting the highly variable and unpredictable climate of the Australian alpine region. There were marked differences in cold tolerance between species but little variation in upper thermal limits. Species occupied thermal environments close to their cold-tolerance limit, suggesting a role for thermal adaptation in shaping patterns of species turnover. Within species, however, high-elevation populations showed greater heat tolerance than low-elevation populations. Contrasting patterns of between- and within-species thermal tolerance variation point to different selective forces acting at the two ecological scales.