Changing faster than the climate
These threatened species are unexpectedly surviving, even harnessing, global warming
Kathryn Doyle • March 21, 2012
A letter to Nature in 2004 predicted that 15-37 percent of species will be “committed to extinction” by 2050 due to climate-warming scenarios.
This is not their story.
Global temperature increase, caused by humans, is in fact happening. And don’t take the polar bear’s word for it; ask the animals that are quickly adjusting, despite the dire outlook for their ecosystems.
Let’s talk about these exceptions to the rule. Some species and some populations are doing surprisingly well, against the odds. At least one has actually benefitted. They don’t indicate that climate change isn’t as bad as we thought, but they do demonstrate the flexibility and strength of the world around us.
In a new paper in Proceedings of the Royal Society B, researchers made the surprising discovery that sea turtles are surmounting climate change, at least so far, with their mating patterns. Sea turtles, in this case the endangered green sea turtle, have long been an important indicator of environmental changes. They are already threatened by climate change in a number of ways, including rising sea levels on Brazilian beaches that reduce their nesting habitats. But they have been surprisingly resilient in one important way.
Turtles belong to a group of animals, including other reptiles and some birds, with temperature-dependent sex determination, or TSD. The temperature of the nest where eggs are laid controls which turtles come out male and which come out female. Cooler temperatures, between 22 and 27 degrees Celsius, yield males, and warmer temperatures, around 30 degrees Celsius, result in females. When a turtle digs her nesting site in a sandy beach, the eggs at the bottom of the pile end up incubating at a cooler temperature – if you’ve ever dug a moat around a sand castle you may have noticed that the top layer of sand is warmed by the sun, and deeper down the sand is wet and cool. That’s where male sea turtles come from.
Climate change due to global warming has caused a gradual increase in temperature at these nesting sites, and now most sea turtle populations hatch around 90 percent female. A 2010 study of loggerhead turtles in Greece authored by Graeme Hays of Swansea University in the UK anticipated the potential negative effects of having too many females: “This ecological enigma has provoked concern that climate change may induce the production of single sex generations and hence lead to population extirpation.”
“All species of sea turtles exhibit TSD, many are already endangered, and most already produce sex ratios skewed to the sex produced at warmer temperatures (females).” If the population skews toward the female too much, then there won’t be enough males to produce more turtles, and future generations are susceptible to inbreeding and genetic drift, when genes potentially necessary to evolution are lost by random chance. This is especially true in severely shrinking populations. Lead researcher Lucy Wright explains via email, “In a finite population genetic drift could cause the loss of a particular variation of a gene, for example for color – meaning that by chance one color could be lost from the population due simply to failure of that gene variant being passed to the next generation.” With less genetic diversity, the chance of survival in a changing environment dwindles. This surely spells doom for these sensitive marine animals.
Not just yet.
Both studies found that although turtles hatch almost universally female babies, something changes between hatchlings and adults to fix the problem, breeding-wise. Though the hatchling sex ratio is lopsided, the “operational sex ratio” ends up nearly 50:50. There’s roughly one mating male for every mating female. The first study found that although there are a small number of breeding loggerhead turtle males, they visit breeding sites 2.6 times more frequently than females, which may explain the evened-out operational sex ratio. But if this is the case, then one father turtle would have many clutches of eggs with many different females, and genetic diversity would decrease.
The new study shows by genetic analysis, in one population of green turtles in Cyprus, there are at least 1.4 breeding males for every breeding female. No father had more than one clutch of eggs, so genetic diversity is still high and inbreeding isn’t yet a problem. How are they doing it? No one knows, really, but somehow these turtles are beating the system. Probably, males mate more often than females and also travel between breeding sites over long distances, whereas females stay generally grouped together. Females may be able to store sperm to use in breeding later batches of eggs, the way some freshwater turtles do. “We do not suggest that marine turtle mating patterns have changed in response to climate change, but that their mating patterns may allow them to cope with future increasing temperatures,” says Wright.
Albatross, on the other hand, are not only dodging the climate change bullet, they are actually thriving because of it. Once again, this is an exception to the rule, as many species are seriously threatened by global warming. But other long-voyaging seabirds could learn from their albatross cousins in the Southern Ocean.
Warming trends have caused a shift in global wind patterns. A paper in Science by Henri Weimerskirch of the Centre d’Etudes Biologiques de Chizé, explains “climate change-induced alterations in oceanic wind regimes and strength have already occurred and are predicted to increase.” Westerly winds in the Antarctic Ocean are now faster, and moving toward the South Pole. Wandering albatrosses do just as their name suggests: they wander from nesting grounds, like those on the Crozet Islands, an archipelago at the very bottom of the Indian Ocean, and sail on prolonged food gathering trips Northward over the ocean up to 2,175 miles away. Breeding pairs take turns; the male sits on the nest while the female forages, and vice versa.
Now that the winds are stronger and more southerly, the albatross’s range is smaller. Foraging trips have shifted south, closer to the nesting sites. Faster winds mean the birds cover the same distance in a shorter time. The nesting bird has less alone time, and so egg failure rates are lower because the incubator is less likely to leave the nest out of hunger. Plus, the birds have actually gained 1 kilogram in mass, without changing in overall dimension, over the last 20 years. This is probably a combination of fasting less when sitting on the egg, and the presence of stronger winds that can carry a heavier bird.
And there are still other benefits. In the past, wandering albatross adults have been incidentally caught in on hooks of tuna fishery longlines, dragged underwater and drowned, which decrease bird populations. The new study from the Centre d’Etudes Biologiques de Chize suggests, “If the range of breeding females moves southward due to environmental changes, they will be less likely to overlap with tuna longliners, whose effort has not shifted southward during the past 50 years,” says the paper in Science.
These hopeful stories are far outnumbered by those touting the negative effects of climate change happening every day: polar bears turning to cannibalism, coral reefs teetering on the brink of massive bleaching. But we shouldn’t stifle the turtle eggs or the albatross winds, just because they don’t fit the trend. Maybe we can learn from these species. Their adaptations are biding us time, giving us a window to fix the problem, before it’s too late. Someday there might be only female turtles; someday the winds might be so strong that none of the albatross can fly. But for now, they abide.
A “few species” are thriving? Look up “carboniferous age”.
Is it possible that nesting behavior for sea turtles may be changing as well, with nesting spreading to beaches beyond the traditional beaches, moving to cooler regions?