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Jason Witherspoon/Design Pics/Newscom

Scientists at the University of Alberta, Edmonton, found that caribou populations have declined by nearly 60 percent in the past three decades.

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Throughout history, people have repeatedly contemplated teeming herds of animals – or schools of fish, as the case may be – and thought, “Nothing we humans can do will ever put a dent in this bounty.”

They’ve repeatedly discovered the op­­posite: Humans can – and have – put a big dent in what they thought was nature’s limitless abundance.

That’s the story with cod off the coast of New England. The once seemingly innumerable fish have yet to recover from overfishing in the late 20th century. And that’s the case with the American bison of the Great Plains – by some estimates once the most numerous large mammal on earth. Then came the slaughter of the 19th century – an estimated 40 million bison gone.

Now, a study in the journal Global Change Biology sees trouble for a species historically considered so numerous – and so distant from human activity – that most assumed it was beyond human ability to affect it.

Caribou and reindeer, a single grazing species with regional variations, inhabit the northern reaches of Eurasia and North America, from Scandinavia to Siberia and from Alaska to Greenland. Yearly, their vast herds make the single greatest migration of any ungulate in the Northern Hemisphere.

But in what they characterize as the first worldwide caribou census, scientists at the University of Alberta, Edmonton, find that populations have declined by nearly 60 percent in the past three decades.

Fluctuations in caribou numbers are normal, as they are for any animal that inhabits a harsh and highly variable environment. But these declines differ from usual population expansions and contractions in one crucial aspect, says Liv Vors, lead author on the study: They’re occurring simultaneously around the world.

“[The numbers] all seem to be going down at the same time,” she says. “Usually, some would go down, and some would go up, and it would be more mixed.”

The causes of the declines differ according to habitat. In the high Arctic, the authors fault global warming. Arctic temperatures have risen twice as fast as the global average, and spring now arrives earlier. Caribou migrations now arrive late, missing young plants when they’re most nutritious.

Hotter summers also bring more insects. The larger swarms harass animals with greater intensity, interrupting their feeding. And more winter precipitation arrives as freezing rain rather than snow. Rain turns to ice on the ground, sealing in the animals’ winter food, lichen.

In the vast Canadian forests, the authors suspect industrial development – roads, logging, and oil and gas exploration – has a greater impact on caribou than changing climate. Caribou prefer old-growth, coniferous forests. They shun roads and other human development.

But other animals, such as moose and deer, thrive in disturbed forest. They also reproduce faster than caribou, so disturbed forests inadvertently favor deer and moose over caribou. With more prey available, wolves also increase. Soon, caribou have less preferred habitat, they face more wolves, and they reproduce more slowly than the new arrivals.

We can’t necessarily control climate, says Ms. Vors, but we can control where roads and mines are located. We ought to place caribou calving grounds off limits, she adds. “Giving calves the best chance possible is one way to keep populations as healthy as possible. That’s one thing we can control.”

( More stories )