Are biologists asking the wrong questions on climate change?

Could species be vulnerable to the effects of climate change at temperatures lower than biologists currently think?

That’s the question a team of scientists is asking in a recent opinion in the journal Trends in Ecology and Evolution.

Often, species’ survival in an area is considered in terms of the temperature at which they collapse, stop moving or die, the researchers say. But many species may become sterile at lower temperatures than this “critical thermal limit,” they suggest, putting them at risk of extinction at lower temperatures than models indicate.

“There is really good evidence that gametes in particular sperm are vulnerable to high temperatures,” said University of Liverpool evolutionary biologist Tom Price, lead author of the paper. “It’s the big explanation for why mammals have external testes. It’s to keep their sperm cool.”

Mammals are pretty good at thermoregulation, Price said, so they may be less affected. But what about cold-blooded species or aquatic species? What about bees, or wildlife like the flightless cormorant (Phalacrocorax harrisi) with limited ranges that can’t move to higher elevations or latitudes as temperatures warm?

“The big question is, is this going to cause populations do die out?” Price said. “And we just don’t know that yet. It could be that the big factors are going to be changes in disease, changes in food, changes in dehydration. But the two fundamental things an animal has to do is, an animal has to survive and an animal has to breed. We’re finding that the breeding point is a bit more vulnerable than people have appreciated.”

In the paper, Price and his colleagues urge scientists to look at not just the critical thermal limit, but the “thermal fertility limit” to predict what species may be at risk under climate change. “Given the importance of fertility for population persistence, understanding how climate change affects TFLs is vital for the assessment of future biodiversity impacts,” they wrote.

“If climate change does severely impact the fertility of wild populations, this may be most clearly seen in failures to breed after extremely hot summers,” Price said. “For example, biologists and conservationists might observe high proportions of unfertilized eggs in birds, reptiles or amphibians, or changes in age structure in vulnerable species, where adults survive the high temperatures, but younger animals are not replaced. We’d be extremely interested if anyone sees anything like this in nature.”

Highlighting data from a variety of plants and animals that suggests that organisms lose fertility at lower temperatures than their critical thermal limit, the researchers suggest that further research could help biologists work out if this is an issue conservationists should worry about, identify which organisms may be vulnerable and help craft conservation programs to address it.

“People who are working in all these different systems but are asking different questions need to work together,” Price said. “There are universal lessons we can learn from this that we can apply more broadly.”

Header Image: The flightless cormorant has a limited range that makes it unable to move to higher elevations or latitudes as temperatures warm. ©Charles J. Sharp