How Climate Change May Shrink Species

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Soay sheep. Credit: Frank Lukasseck / Corbis

The people of Soay Island, off the west coast of Scotland, have notice something strange. Over the years, their sheep have begun to shrink, as I wrote in 2009:

Why? In short, because of climate change. Generally, the sheep’s life cycle goes like this: they fatten up on grass during the fertile, sunny summer; then the harsh winter comes, the grass disappears and the smallest, scrawniest sheep die off, while their bigger cousins survive. That’s how you end up with big sheep, which — according to Darwin’s laws of natural selection — will pass on their big genes to the next generation.(See pictures of sheep and other animals.)

But over the past 25 years, the average Soay Island wild sheep has decreased in size, according to a report in the July 2 issue of Science by a team of researchers led by Tim Coulson of Imperial College London. Thanks largely to global warming, the winters on Soay Island are becoming shorter and milder. That makes food more abundant and allows some of the smaller, more vulnerable and younger sheep to survive. Then they go on to have offspring that tend to be small themselves — and have a better chance of survival because of the increasingly mild winters. “The environmental and evolutionary processes are intertwined,” says Coulson “There’s still natural selection, but it’s not leaving as big a signature as it used to. There’s still a disadvantage to being small, but not as much.”

That was one case. But now a team of scientists writing in the October Nature Climate Change believe that sheep won’t be the only animals to shrink because of warming temperatures. Jennifer Sheridan of the University of Alabama and David Bickford of the National University of Singapore reviewed existing studies and argue that species have shrunk in past periods of warming, and that they are shrinking now even as temperatures warm faster than they have in millions of years.

Why would warming temperatures mean smaller organisms? Sheridan and Bickford posit that plants could become smaller and weaker as temperatures increase—and crucially, water supplies fall in dry areas of the world. Smaller plants mean that the animals that depend on green stuff for their diets will need to eat more plants to get the same amount of calories, which could affect their growth. The carnivores that feed on those herbivores will need to survive on smaller meals, causing them to shrink as well.

It might sound far-fetched, but in experiments fruits tend to shrink by 3-17% for every degree Celsius increase in warming. Similar levels of warming can reduce the body size of marine invertebrates by 0.5%-4%, fish by 6-22% and salamanders by 14%. As Sheridan and Bickford write:

Collectively, comparative and experimental studies indicate that a broad array of taxa are likely to get smaller with continued global warming, and that the rate and degree of shrinkage will vary widely.

This doesn’t mean that we’ll wake up tomorrow with dwarf cats and dogs because temperatures have gone up a bit—in all likelihood, we won’t notice the change. And in one way, the shrinking is how organisms might adapt to climate change, becoming smaller to deal with a restricted diet, rather than simply dying off.

But if different species shrink at different rates, it cause a serious ecological imbalance, as Bickford told the New York Times:

This isn’t a ubiquitous or general law of ecology, but when some animals are affected and others aren’t, that’s when an imbalance will happen, and that’s what we’re most concerned about.

In case you weren’t worried enough about climate change, now warming could make nature literally disappear beneath your feet.

Bryan Walsh is a senior writer at TIME. Find him on Twitter at @bryanrwalsh. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME