You might assume that being 鈥渃old blooded鈥� would offer some protection against global warming. In fact, ectothermic species in hot countries could be hit as hard as the rest of us, a new model suggests.
Depending on external warmth to keep energy levels up means you are more likely to overheat as temperatures keep climbing, according to a study by of the University of Melbourne, Australia, and colleagues.
鈥淓ven if they manage to find shade in the tropics, they are going to be heat stressed, and possibly too hot to reproduce and, ultimately, to survive,鈥� says biologist of the University of Washington in Seattle, who was not involved in the study.
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Virtual creatures
Kearney鈥檚 team created a computer model that married two techniques. One predicts an animal鈥檚 body temperature from its immediate environment, and from characteristics such as body shape and colour. The other creates maps of air temperature, wind speed, cloud cover and shade availability, using geographic information systems.
Uniquely, the model also factored in the ability of reptiles to respond to changing temperatures by moving into the shade, underground, or into the Sun.
The model was designed to answer specific questions about how species need to adapt to changing conditions 鈥� for example, how decreases in shade caused by bush fires will increase the risk of heat stress in a particular insect species. But the team also used it to get a general sense of how ectotherms will be affected by global warming.
To do this, they fed into their model the characteristics of a 鈥渢ypical鈥� ectotherm 鈥� say a lizard, or a grasshopper 鈥� foraging at body temperatures between 20 and 40 掳C, and moving fastest at an optimal body temperature of 33 掳C.
Some like it hot
Under current conditions, the model showed that on a summer鈥檚 day the body temperature of the typical ectotherm exceeds 40 掳C in the shade for at least an hour 鈥� near lethal conditions for most Australian lizards.
But with a 3 掳C increase in average temperatures 鈥� the mid-range estimate for the end of the 21st century according to data from the Intergovernmental Panel on Climate Change 鈥� typical ectotherms living in most of the tropics and deserts of Australia experience those temperatures.
To stay cool enough to survive, the ectotherms in the model are forced to remain in their burrows for longer. But in real life that could limit their ability to find enough food to survive, says Kearney.
However, the model suggests that climate change would benefit ectotherms living in temperate Australia. 鈥淭hey were winners. They increased their active season by an extra month,鈥� says Kearney. In temperate regions, Ectotherms typically go into torpor 鈥� a semi-dormant state 鈥� during the winter, when temperatures are low.
鈥楽evere impact鈥�
Less detailed climate data is available for the whole globe compared to Australia. Nonetheless, when the Kearney team ran global simulations, the pattern was repeated with the area over which a typical ectotherm would experience heat stress increasing from 1.6 to 18% of land surface.
鈥淭hey already have enough problems with loss of habitat. With global warming too, the human impact on tropical species is going to be severe,鈥� says Huey.
Last year, a study of 38 species of insects concluded that their reproduction rates will fall as the tropics heat up. Some biologists have suggested relocating animals that are likely to be affected by climate change to more suitable regions of the world.
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