Strangers in a strange land: Globally unusual thermal tolerance in Collembola from the Cape Floristic Region

Specific trait–environment interactions have led to globally unusual botanical radiations in the Cape Floristic Region (CFR) of South Africa. Yet it is unclear whether such interactions have similarly resulted in the diversification of the fauna independently of associations with plants. We explore this question for the Collembola genus Seira, which includes 89 species in the CFR, and appears to have diversified substantially into warm, dry fynbos shrublands—habitats atypical of those usually occupied by Collembola. Specifically, we determine whether thermal tolerance is higher for members of the genus in these shrublands compared with congeners in cooler, moister Afrotemperate Forests in the region, and with Collembola globally. Fynbos-inhabiting Seira species have remarkable critical thermal maxima (CT_max) of on average 43°C, 5–6°C higher than congeners occupying the Southern Afrotemperate Forests, and Collembola globally, even when taking CT_max variation associated with regional climate into consideration. Thermal tolerance trait variation among the Seira species from different habitats is likely the consequence of variation in habitat temperature: fynbos shrublands are on average ~17°C warmer than Afrotemperate Forests. Moreover, periodic fires in the fynbos have a large effect on microhabitat temperatures, increasing them substantially (by ~7.5°C) in the year after fire, which may be especially relevant in selecting for higher thermal tolerance in species from this habitat. These results suggest that the remarkable richness of Seira in the CFR may in part be due to the evolution of very high CT_max values which have enabled them to take advantage of a hot, dry habitat that is unusual for Collembola. A free Plain Language Summary can be found within the Supporting Information of this article.