Scientists have warned for years that a warming climate will threaten many of the world's species. But for one diminutive alpine creature, the threat has already arrived.

The American pika is disappearing from much of its mountain habitat across the western United States, with rising temperatures a driving factor, a new study says. The findings, said lead author Erik A. Beever, a research ecologist with the United States Geological Survey, also point to a much larger problem.

"This is kind of an indication of what's happening in our mountain ecosystems," Beever said, pointing out that the rivers that feed the West's cities have their origins in the pika's habitat. The findings, published last week in the Journal of Mammalogy, paint a troubling picture.

Beever and 14 colleagues surveyed more than 900 locations across three Western regions—in northern California, the Great Basin and southern Utah—where pikas had been known to live. Their searches, carried out in 2014 and 2015, found that the creature had vanished from locations across each region. In California, pikas had disappeared from 38 percent of the sites. In the Great Basin, which lies between the Rockies and the Sierra Nevada mountains, 44 percent of locations were pika-free. They were unable to find a single one in Zion National Park, in southern Utah, where the animals had been recorded as recently as 2011.

Beever and other researchers have been warning about and chronicling the pika's decline for more than a decade, but the new work broadened the research across a greater extent of the animal's range, which stretches from New Mexico to southern Canada and is limited almost exclusively to rocky outcrops high in the mountains. The hamster-like creatures, which are a distant relative of the rabbit, weigh less than eight ounces, but they're talkative and easy to spot, making them prime subjects for study. Their remote habitat is also relatively undisturbed by human development.

The pika has been labeled one of the cutest creatures alive. It rubs its cheeks on rocks to mark its territory. It is not, however, well suited for a warming world.

"It has this characteristic of essentially being a big fur ball, which is a really great strategy if you live on the top of a snowy cold mountain and want to stay active in those temperatures," said Mark C. Urban, who studies the effects of climate change on animals at the University of Connecticut. Urban, who was not involved in the pika research, compared its condition to wearing a fur coat on a hot summer day. "Humans can take off that fur coat, but the American pika can't."

Their mountain habitats are like islands in the sky, surrounded by warm, often uncrossable valleys, so they have trouble shifting to new territory. And as climate change pushes warm temperatures farther up mountain slopes, these "sky islands" are becoming smaller and scarcer.

Scientists had long theorized that creatures in isolated ecosystems would be among the most vulnerable to climate change, Urban said, and that they'd decline at a faster rate than other species. The new study solidifies the theory, he said.

"The pika serves as an early warning for other species," Urban said.

Despite these threats, the Obama administration in 2010 rejected a bid to add the American pika to the endangered species list, though the animal is up for consideration again.

It's not all bad news for the pika. Beever and colleagues have found that they are thriving in the Teton mountains of Wyoming, in the colder, northern part of their range. They also found that the decline in California likely was not due solely to climate change, but probably a combination of other factors such as disease or fire.

The scientists warn that the loss of pikas could have a profound impact on their mountain habitats. Despite their small stature, the animals play an outsized role in the ecosystem, spreading seeds and redistributing nutrients. And, Beever said, in the areas with the greatest body of research, the data indicate a near-certain decline.

"At our sites in the Great Basin, we're really not seeing any of those patches they're lost from being recolonized," he said. "It's kind of a one-way trip."