Lisa Bjerregaard Jørgensen, Hans Malte and Johannes Overgaard
Insects typically have very limited ability for physiological thermoregulation and body temperature therefore follows ambient temperature closely. Accordingly, environmental temperature is one of the most important factors that dictate insect distributions. Detailed information on species heat tolerance can therefore help us to understand where species can live under current climatic conditions, and potentially also inform us of how specific species will deal with climate change. It is therefore necessary to have methods that measure heat tolerance in a simple and repeatable way that also describe important ecological components of species heat tolerance.
In the present study we examined the relation between the intensity of temperature stress and time to heat knockdown in 11 fruit fly species (Drosophila). We found for all species that higher temperature led to faster knockdown, but also that the species were characterised by large interspecific differences in temperature tolerance. We did not find any trade-off between tolerance to intense acute heat stress and moderate chronic heat stress in the 11 species tested, meaning that warm-adapted species can tolerate both moderate and intense heat stress for a longer period of time than species not adapted to warm environments.
Heat tolerance measures in insects have historically been tested using either static assays (where temperature is constant and the time to knockdown is recorded) or with dynamic assays (where temperature gradually increases until the animal falls over at a knockdown temperature). In this study we construct a mathematical model based on data from our static assays that can accurately predict the knockdown temperature of a dynamic assay. Effectively, this means that the two types of heat tolerance measures can be reconciled as they give comparable information on heat tolerance of the studied species.
Importantly, we show that both static and dynamic measures of heat tolerance can be used as predictors of species distribution as these measures of heat tolerance are strongly correlated to the environmental temperature and aridity of the species origin.