Is climate change making animals smaller?

When Sifrhippus, an early precursor to the horse, began roaming the forests of what is now northwestern Wyoming, it was about the size of a Border terrier. At the time, some fifty-six million years ago, the planet was undergoing an intense period of warming known as the Paleocene-Eocene Thermal Maximum. For reasons that are still unknown, carbon flooded the atmosphere and acidified the oceans, triggering a wave of extinctions in the deep sea and causing global temperatures to rise by at least nine degrees. Sifrhippus apparently responded to these changes by losing thirty per cent of its body size in about a hundred and thirty thousand years, dwindling to the proportions of a Persian cat.

Shrinkage in the face of environmental stress, according to a paper out today in the journal Nature Climate Change, may be a kind of survival strategy for modern animals, too. The paper describes a study by a group of British and Italian scientists of animal life around a series of volcanic seeps off the coast of Sicily. It is believed that these seeps have been active for thousands of years, pouring carbon dioxide into the sea and acidifying the water. In the area around the seeps, many common species are absent entirely. Among those that persist, some appear to have adapted to the high-CO2, low-pH conditions by shrinking.

Specifically, the researchers found, representatives of two species of sea snail—Cyclope neritea and Nassarius corniculus—that live by the seeps are significantly smaller than their counterparts elsewhere in the Mediterranean. “It shows us that not everything dies, which is a sort of a glass-half-full way of looking at it,” one of the scientists involved in the study, Jason Hall-Spencer, a marine biologist at Plymouth University, told me. “And it shows that some things have got tricks up their sleeves that they can use to get by.”

Near the seeps, sea snails and other mollusks must be careful how they exert themselves. The dissolved CO2,, in effect, eats up carbonate ions from the water, making it harder for the animals to build and maintain their shells. When Hall-Spencer and his colleagues measured the energy use of some of the snails that they gathered off of Sicily, they found, not surprisingly, that the smaller ones used less (although, on a gram-for-gram basis, their metabolic needs were higher). Their smaller size allowed the creatures to compensate, at least partially, for the greater demands placed on them by the water chemistry, just as Sifrhippus is thought to have compensated for its hotter surroundings.

The group’s findings, Hall-Spencer told me, tie in well with the fossil record. Paleontologists have noted that creatures that survived mass-extinction events tended to be diminutive, a phenomenon that has become known as the Lilliput effect. But the findings are also relevant to today. CO2 emissions from tailpipes, factories, and power plants are making all of the world’s oceans more acidic. As the scientists put it in their paper, the environmental conditions at the seeps “forewarn of changes we can expect in marine ecosystems as CO2 emissions continue to rise unchecked.” Some of those changes are already being observed. Over the past few decades, for instance, as the North Sea has warmed, the maximum size of haddock and whiting has declined. “The increase in temperature of the North Sea is actually quite subtle—approximately two degrees Celsius—yet this appears to be having a detectable impact on growth rates of fish,” Alan Baudron, a research fellow at the University of Aberdeen, said in a press release.

Whether this is true of creatures in other parts of the world—whether we are witnessing the beginnings of the Lilliput effect—is not known at this point. But we do know that past instances of intense warming and acidification have been associated with some of the worst crises in the history of life, including the Permian-Triassic extinction, roughly two hundred and fifty million years ago, which killed off some ninety per cent of all species on Earth. Another recent paper, which appeared this month in Science, argues that the acidification at the end of the Permian period was the result of a release of CO2 that, on an annual basis, was comparable to global emissions today. No one is entirely sure where all that carbon came from—the paper’s authors suggest that a huge and prolonged volcanic eruption burned through deposits of limestone—but, in total, it was probably a great deal larger than humans will manage, since the release extended over thousands of years. Still, as we grow accustomed to a hotter world, we should be prepared for one that is getting smaller, too.

Elizabeth Kolbert, The New Yorker, 20 April 2015. Article.

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