Limited tolerance by insects to high temperatures across tropical elevational gradients and the implications of global warming for extinction
Significance Tolerance to high temperatures will determine the survival of animal species under projected global warming. Surprisingly little research has been conducted to elucidate how this trait changes in organisms living at different elevations of similar latitudes, especially in the tropics. DNA barcodes demonstrate that insect species previously thought to have broad elevational distributions and phenotypically plastic thermal tolerances actually comprise cryptic species complexes. These cryptic species occupy discrete elevational ranges, and their thermal tolerances seem to be locally adapted to temperatures in their life zones. The combination of high species endemism and local adaptation to temperature regimes may increase the extinction risk of high elevation insects in a warming world. The critical thermal maximum (CTmax) the temperature at which motor control is lost in animals, has the potential to determine if species will tolerate global warming. For insects, tolerance to high temperatures decreases with latitude, suggesting that similar patterns may exist along elevational gradients as well. This study explored how CTmax varies among species and populations of a group of diverse tropical insect herbivores, the rolled leaf beetles, across both broad and narrow elevational gradients. Data from 6,948 field observations and 8,700 museum specimens were used to map the elevational distributions of rolled leaf beetles on two mountains in Costa Rica. CTmax was determined for 1,252 individual beetles representing all populations across the gradients. Initial morphological identifications suggested a total of 26 species with populations at different elevations displaying contrasting upper thermal limits. However, compared with morphological identifications, DNA barcodes (cytochrome oxidase I) revealed significant cryptic species diversity. DNA barcodes identified 42 species and haplotypes across 11 species complexes. These 42 species displayed much narrower elevational distributions and values of CTmax than the 26 morphologically defined species. In general, species found at middle elevations and on mountaintops are less tolerant to high temperatures than species restricted to lowland habitats. Species with broad elevational distributions display high CTmax throughout their ranges. We found no significant phylogenetic signal in CTmax, geography, or elevational range. The narrow variance in CTmax values for most rolled leaf beetles, especially high elevation species, suggests that the risk of extinction of insects may be substantial under some projected rates of global warming.
Proceedings of the National Academy of Sciences