Tricky to help animals ‘beat the heat’

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Predicting how species, from fish to insects, will respond to climate change and temperature extremes is not easy, says climate scientist Susanna Clusella-Trullas.

It is this lack of understanding that hinders efforts to predict their vulnerability, says the associate professor in physiological ecology at the Centre for Invasion Biology at Stellenbosch University.

Ecologists and physiologists based at the centre study the diverse ways that ectothermic — or cold-blooded — species such as reptiles and insects respond to environmental change.

Defining what taxonomic groups are most vulnerable is “very tricky”, because of the myriad factors that need to be taken into account, including the capacity of a species to adapt, its resilience and the type of region and climate in which it lives.

“Given ongoing climate change and future predictions, we need to forecast the vulnerability of species to climate change in an effective way, often at very large scales. Scientists have been trying to devise simple and efficient and cost-effective ways to estimate the vulnerability of organisms to climate change,” says Clusella-Trullas.

One popular approach is using thermal vulnerability indices, which compare the sensitivity of a species to its exposure to warming, but this has several limitations, she says. 

In a new paper published in the journal Ecology and Evolution, she and colleagues from Stellenbosch University and the University of Melbourne outline proposals on how to improve the current, widely adopted thermal vulnerability index. 

“The paper focuses on highlighting what questions can and cannot be answered with this approach and offers ways to improve the formulas to incorporate more information, both in terms of species responses and exposure, which is the temperatures species experience in the field,” they say.

According to Ary Hoffmann, of the school of biosciences at the University of Melbourne: “It is very hard to devise a test of vulnerability in a laboratory test tube that accurately reflects what happens in nature where animals can adapt to a stress. Yet we often make conclusions about vulnerability based on such assessments.” 

There are too few studies in South Africa and sub-Saharan Africa to guide management efforts to help animals beat the heat, says Clusella-Trullas. “We have a poor understanding of the frequency and severity of extreme heatwaves and there isn’t enough monitoring of animal populations in the field and in the long term to be able to detect mortalities or fitness losses of animal populations.

“However, we are aware that impacts on organisms are occurring. We just need to look for them and have the personnel and funding to do so. The risks of stress and mortality due to exposure to extreme events [warm and cold] are high, even for humans.”

Species, she says, can use multiple ways to respond to warming and temperature extremes, including through behaviour — thermoregulation and migration — and adaptive responses, including short-term changes of traits such as colour and performance. 

This provides flexibility to respond to climatic changes and evolutionary adaptation, she says. But if these strategies are not sufficient or are not taking place for some species, the risks of population extinction increases. 

“Species with limited distribution ranges, low ability to disperse, or that live in environments very close to their tolerance limits should be especially vulnerable to climate change.” 

The effects of temperature change are expected to be highest in ectotherms such as reptiles, amphibians and insects, she says. “Marine ectotherms will likely be highly vulnerable given the potential compounding effects of warming, deoxygenation and acidification.”

“As the climate changes further, we are witnessing dramatic changes — for example, drier landscapes or landscapes disturbed by exotic plants — and … the compounding effects of landscape changes and restrictions of food and shade availability.” 

The latest report of the Intergovernmental Panel on Climate Change (IPCC) concludes that human-induced greenhouse gas emissions have led to an increased frequency and intensity of some weather and climate extremes, in particular for temperature extremes.

Evidence has strengthened for several types of extremes since the previous IPCC report, in particular for precipitation, droughts, tropical cyclones and compound extremes, including fire-related weather, Clusella-Trullas says.

“This means these sorts of thermal vulnerability indices are going to be increasingly relied upon to forecast impacts of extreme events.”