英語(yǔ)聽力：自然百科 瀕臨滅絕的鼠兔

Autumn in the Rocky Mountains, a time of change. Aspens turn golden and the bull elk, in the midst of their mating season, work hard to catch the eye and ears of the ladies. Up in the highest peaks, another call rings out. It's the pint-sized prince of the Alpine tundra, a relative of the rabbit, known as the pika. Pikas are among the world's highest dwelling animals, often choosing the most frigid mountain tops to call home. Pikas, once common in mountain ranges around the world, are vanishing, even in protected places, like Rocky Mountain National Park. To investigate these disappearances, national geographic grantee Rob Guralnick and his pika research team are making the trek to the tundra. Throughout the summer months, the biologists will routinely canvass sites where pika had been spotted in the past, in order to determine how many are left.

We are losing a very characteristic Alpine denizen, something that a lot of people recognize and see when they got there, that's sort of now going missing on mountaintops. So the question becomes, you know, what's going on in the Rockies where that the rarest stronghold is in this, in this very rich area for pika habitat.

Today, the pikas are elusive. But, fortunately, they leave their telltale signs of their presence behind.

It's a great, a great connection of scat here.

Yeah, a little urine’s, urine station is white. You can see the white.

But each of these little peppercorn-like things, it's, it's a fresh pika. It's a pika scat, some fresh, some not.

Rob's team also finds plenty of miniature haystack, grass and flowers the animals lay out to dry.

Unlike most of the Alpine species, they don't shut down in the winter. Marmots, other mammalshibernate. Pikas don't hibernate. That's part of what they are doing when they hay is collect that vegetation for the winter so that they sustain themselves.

Back in the lab, the team analyzes the pika data. The first result suggests that the pika is headed the way of many other small Alpine mammals. In a wider study, Rob mapped the temperature and historical distribution of many small Alpine mammals. He found that most of the species used to live at low elevations across larger areas. Something is driving the mammals higher up the mountains. Rob looks to the climate for answers. As carbon dioxide and other greenhouse gas emissions increase, temperatures around the global are climbing, even in the Rockies.

They have to really have a good little internal heater going, and a high metabolism so that they can generate enough heat to stay warm in the winter. Then in turn, because they're generating so much heat, that's not something you can necessarily shut off in the summer. So, most of the time, Alpine stays pretty cool. In the summer, but if you get really hot streak in the Alpine, the pikas are not gonna be able to thermoregulate or cool themselves down very easily, 'cause their metabolism is high.

When it gets too hot, the little pikas overheat and die. As the climate warms, some species have been moving to higher elevations to seek cooler weather, but the pikas already inhabit the highest point. There's nowhere left for them to go.

As well as being a loss of a great charismatic, wonderful taxon, it'd be that bellweather of further changes that are likely to be occurring not just the pikas, but to all organisms that live in the Alpine. And of course that's a real concern. We start losing ecosystems and the services they provide to humans.

At least this year, Many pikas have been able to beat the heat. And next spring, the call of the pika will ring out once again across the Alpine. Sponsored by National Geographic Mission Programs, taking science and exploration into the new millennium.