Green turtle swimming over corals

Many species of turtle, such as this green sea turtle (Chelonia mydas) are endangered. Photo by Wikimedia Commons user Brocken Inaglory, licensed under CC-BY-SA 3.0.

Turtles can tolerate warmer temperatures, given time

New research shows turtles and tortoises can tolerate warmer conditions than they experience today - but their ability to cope with climate change will likely depend on how quickly temperatures rise.

As habitat degradation and collection from the wild put nearly half of all turtle and tortoise species at risk of extinction, the additional impact of rising global temperatures is a significant concern.

A team led by the Museum and the University of Bristol set out to test the ability of turtles, tortoises and terrapins - collectively known as testudines - to respond to long-term global warming.

Looking at the Late Cretaceous Period, approximately 72 to 66 million years ago, the team investigated how testudines survived this time of warmer global temperatures.

They found that many groups flourished in these conditions, suggesting testudines could potentially cope with a rise in global temperatures today.

The team also discovered, however, that the amount of rainfall was an important factor in the distribution of freshwater turtles, including the softshell turtle family (Trionychidae). Since rainfall patterns are also affected by climate change, the rising temperatures may have more of an impact on these groups.

Florida softshell turtle

Rainfall levels were particularly important for species in the family Trionychidae, such as this Florida softshell turtle (Apalone ferox)

 

The researchers also point out that climate change today is occurring far more quickly than it did in the past, which could make adaptation more difficult.

'These groups have historically withstood warmer climates, so in isolation, higher future temperatures may not be harmful for them,' says Amy Waterson, lead author of the paper and a joint Museum and University of Bristol PhD student.

'But the rate of modern temperature change is critical. The ability of turtles to adapt or acclimatise quickly enough will be key to their resilience under future environmental change.'

Aldabran giant tortoise

Terrestrial testudine species, such as this Aldabra giant tortoise (Aldabrachelys gigantea), may be able to cope with higher temperatures, if given time to adapt

 

Adapting to change

Around 40% of modern turtle and tortoise species are recognised by the International Union for Conservation of Nature (IUCN) as vulnerable, endangered or critically endangered. Much of this loss is due to habitat destruction and trade in the species, but climate change is also an increasing threat to testudine numbers.

Knowing that many of these species have been around for millions of years, the researchers looked to a period in the past to predict their reaction to this changing climate. They chose the Maastrichtian Age (part of the Late Cretaceous Period) because of its warmer climate and good fossil record.

Using modern climate data, the team first constructed a computer model of current testudine distribution, and checked its accuracy against reported observations of these species. This gave them an idea of the environmental conditions that testudines currently thrive in.

The researchers then used the same model with climate data from the Maastrichtian, to predict where the testudines may have lived, based on their modern climatic preferences.

By comparing this prediction to the fossil record, the team were able to determine if testudines were indeed confined to these areas, or if they were able to thrive in other, warmer regions.

They found that land-dwelling tortoise species were able to survive in much warmer conditions in the past, suggesting they may be able to adapt to future changes. 

A young spotted turtle

Freshwater turtles, such as this spotted turtle (Clemmys guttata), may be especially vulnerable if habitat destruction confines them to particular habitats © Ryan M Bolton/Shutterstock.com

 

Conservation challenges

However, their ability to adapt depends on the rate of change, warns Professor Daniela Schmidt, a palaeobiology researcher at the University of Bristol.

'The largest difference between the warm Cretaceous and today is that this earlier warming happened over tens of thousands of years - giving these animals a chance to adapt to these conditions - not a century.'

In addition, freshwater turtles were found in habitats with similar temperatures to their modern environments, suggesting a limit to their ability to adapt to warmer climates.

The team found these turtles were primarily affected by the amount of rainfall, as the species expanded their habitats in wet areas. As climate change alters worldwide rainfall levels, these turtles could be vulnerable to population decline if they find themselves confined to certain areas.

According to Museum palaeontologist Professor Paul Barrett, 'If turtles find their current habitats unsuitable, other conservation threats - such as manmade habitat degradation and barriers to movement - might be as important in determining the fates of turtles in a warming world as the warming itself.'