As the world warms in upcoming decades less than half the current number of Costa Rican leatherback turtles will succeed in their first, vital, journey from sandy nest to sea. That’s according to a team of US researchers who have closely monitored how regular climate fluctuations affect egg and hatchling survival. That’s allowed them to show a clear relationship that they can use to predict the turtles’ future prospects, explained James Spotila from Drexel University in Philadelphia, Pennsylvania. “With the projected warming that’s going to happen in this century, these eggs and hatchlings are going to have a serious problem,” Spotila told Simple Climate. “We’ll have to do some kind of mitigation to keep these animals alive.”
James, who is also chairman of the Leatherback Trust, has been studying nesting turtles, considered “critically endangered”, at Las Baulas Park in Costa Rica for 22 years. Over that time, he and his fellow scientists had noticed more hatchlings in some years than others, and wanted to know the cause. The close watch they keep on the turtles gave them the first clues that climate played a role. “We noted that as the season would progress, and got hotter and drier, you had a reduction in hatching success of the eggs,” James said.
To find a detailed link, the scientists focused on one important nesting area in the Las Baulas Park – Playa Grande – over 6 seasons, from 2004-2010. Over that time they tracked temperature and rainfall measurements recorded at a nearby airport. But the hardest part – much of which was done by James’ Drexel colleague Pilar Santidrian Tomillo – came after the nests hatched.
“Two or three days later you excavate the nest where you saw hatchlings come out – or if they didn’t come out when they were expected, you excavate a week later,” James explained. “You dig down a metre to get into the nest, and then you take out every egg and examine it. If it hatched, you have eggshells, if it didn’t hatch, then you have a dead egg and you examine it to see what stage it died. You might also find hatchlings that didn’t get out the nest. It’s a very painstaking, detailed process, but that long, tough, effort over many years turned out to be pretty valuable.”
Leatherback eggs spend two months in the nest before hatching, so the researchers focused on the effects of air temperature during that time, while for rainfall they considered different timespans. In a paper published in research journal PLoS One on Wednesday, they found that temperature was responsible for about one fifth of the variation in hatching rates. Rainfall was responsible for the remaining four fifths. But the situation for whether, once they’d hatched, turtles emerged from the nests was slightly different. Air temperature explained three fifths and rainfall two fifths of the variation in emergence. The variations in both rates were also strongly linked to variation in the natural cycle known as the El Niño Southern Oscillation, which drives differences in climate from year to year.
Turning to turtles’ aid
The biologists could then use the figures on how weather affects egg and hatchling survival to predict what would happen to them in the future. They used data from 17 models provided to the Intergovernmental Panel on Climate Change on what Playa Grande’s weather would be like through to 2100. 13 projected a decrease in rainfall, and all models projected an increase in air temperature. Today, around half of all leatherback eggs hatch, and then four out of five of the hatchlings emerge from the nest. James, Pilar and their colleagues’ research suggests that the hatching rate will likely fall below one in five by the end of the century, and that one in three of those hatchlings will escape the nest.
Leatherbacks already face threats that include egg poaching and human fishing practices. Rising temperatures are also biasing the sex balance of Playa Grande’s leatherback turtles so that around nine out of ten hatchlings are female. James and his colleagues are now looking at what these different pressures mean for the population overall, but noted that leatherbacks will likely need human help to avoid extinction. “We have to move nests some place more appropriate, or perhaps shade the nests to keep them cooler in the hot years,” James said. “We have to determine what the best strategy is.”
The scientist also emphasised that the fate of creatures like leatherback turtles is one of many reasons that the world should get serious on climate change. “It’s one more piece in the puzzle,” he said. “It really points to the fact that we’re playing Russian Roulette with climate here, except in this case we have five out of six chambers with a bullet in it.”