Predicting heat death in species more reliable with new mathematical model

September 07, 2020

An international research with the involvement of the Universitat Autònoma de Barcelona (UAB), published in Science, has developed a new dynamic mathematical model which represents a change in paradigm in predicting the probability of heat-related mortality in small species. The study, which has validated field experiments conducted with the Drosophila subobscura fly, concludes that the current standard model subestimates species' vulnerability to climate warming and extreme temperatures.

When faced with global warming, finding tools which can predict as reliably as possible the probability of mortality from death of an organism and its probability of survival can pose a challenge. Particularly when it comes to small species, in which temperatures can fluctuate notably within days and across seasons.

The model proposed in this study by researchers Mauro Santos (UAB), Enrico L. Rezende and Francisco Bozinovic (Pontificia Universidad Católica de Chile) and András Szilágyi (Eötvös Loránd University, Hungary) is based on the premise that the accumulative effect of any thermal stress varies with temperature and over time. According to the researchers, the model represents a change in paradigm in the prediction of mortality related to extreme temperatures from a static model based on critical limits to another more solid and realistic dynamic model.

The standard approaches currently used are based on the concept of "thermal safety margin", which is the difference between a species' maximum tolerance to heat (called Ctmax), and the warm air temperatures it often experiences in the field.

According to researchers, this thermal safety margin can often be surprisingly high in mid-latitude species (from 23.5ºC to approximately 66.5ºC in both hemispheres), which suggests that many ectotherms (cold-blooded animals) could easily withstand summer air temperatures up to 20°C above the current average. However, this approach has strong limitations because the harmful effects of heat stress accumulate over time, and focusing only on the end-point (CTmax) completely ignores the survival function or the proportion of individuals that will reach this point.

"With the model we have developed we can show that a single survival function or probability of death, which depends on temperatures and the amount of time until the organism dies of thermal stress, perfectly describes the laboratory results obtained in 11 Drosophila fly species", explains Mauro Santos.

In addition, when the model was applied to field data using hourly information of air temperatures from mid-winter to the end of summer in a mid-latitude Chilean location (Santiago), a population crash was predicted in the species Drosophila subobscura towards the middle or end of summer. And that is precisely what occurred during 8 consecutive years, although the maximum air temperature had always remained well below the CTmax of these flies.

"The population crashes will accelerate with global warming. The message is that current CTmax-based inferences underestimate species' vulnerability to climate warming and extreme temperatures, which will be more and more frequent. What is still lacking is a mechanistic understanding of heat-related mortality", the UAB researcher concludes.

The model proposed is not limited to the Drosophila fly, given that it can in principle be applied to other cold-blooded animals, whose survival can be measured in laboratory and in thermal microhabitats which can be estimated with precision on the field.
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Universitat Autonoma de Barcelona

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