英語聽力：自然百科 動物大遷徙

Every fall, this herd of pronghorn antelope treks more than 100 miles from Grand Teton National Park to their winter habitat in the Upper Green River Valley of Wyoming. This phenomenal migration, one of the longest in North America, has been taking place for the past 6,000 years.

As the snows develop and forage becomes unavailable, they migrate and move back, 100 to 150 miles back to the sagebrush basins of the Upper Green River basin where they can access forage through the winter.

Relying on information collected from VHS radio collars, researcher Hall Sawyer mapped this particular pronghorn migration route in 1998, with a half a million animals roaming the range. Wyoming is home to almost as many Pronghorn as people.

Within a hundred-mile radius of Casper, Wyoming, which is located right in the center of state, there’s more pronghorn than anywhere else in the world. Wyoming sustains most of the Pronghorn in North America.

But part of the pronghorn habitat is becoming increasingly fragmented by human development. And some worry that crucial corridors are being severed. Two men have an ambitious plan to find out which obstacles the pronghorn have to navigate. Conservationist and adventurer Rick Ridgeway and photographer Joe Riis have set out to document "This Year's Migration".

Joe proposed to walk with those pronghorn on foot, nobody had ever done that before on foot, and more importantly document the migration in photographs, so that we could increase theawareness about the migration and the threats to the Pronghorn.

Joe Riis is one of National Geographic young explorers. He is a biologist and a wildlife photographer.

We walked, er, the distinct path that the pronghorn migrate in the fall and in the spring. We wanted to view the landscape through the eyes of the pronghorn and see the obstacles that the pronghorn see, see the beauty of the landscape as well as that human obstacles.

The duo quickly establishes that pronghorn are encountering some modern inconveniences along their ancient pathway.

In one day, we estimated a herd of about 800 animals moved, past us near Trappers Point, Wyoming. It's a natural bottleneck that has been reduced by a subdivision. We watched the pronghorn dodge cars and people along highway 191 and 352.

When they hit a migratory bottleneck, animals are confined to a narrow passageway. The squeeze can be caused by vegetation or human development. In the case of Trappers Point. It's both.

Every year, between 1,500 and 2,000 pronghorn cross through this bottleneck twice a year, once in the spring and once in the fall. Vehicle wildlife collisions are a problem all across the state, but they become particularly problematic during these migratory time periods when there is a lot of animals moving across the roads.

The most common barriers the animals face are roads and fences.

Fences pose a problem to the pronghorn because they are not evolved to jump over fences, they move underneath them, and if fences are woven wire, they can't move under, a lot of times they simply can't pass through.

In order to capture intimate photographs of the migration, Joe uses remote cameras.

Pronghorn Antelope can see 10 times better than we can. So if you move at all, they see you and they will stop and go the other way. You have to use remote camera systems that don't make any movement and take the pictures by themselves.

He positions the cameras in specific areas along the migration path often hiking miles to find the perfect spot. Joe’s ultimate goal is to get shots of the herd in motion, to capture the essence of this unique long distance journey.

I...I photograph this migration to inspire people to care about it. It's an incredible migration and people need/ to know about it. It’s happening right in the middle of the United States, it’s the second longest migration in the Western Hampshire.

Joe also documents the migration from the air. With the help of pilot Chris Boyer of LightHawk, an organization that provides free flights for conservationist. Aloft Joe can clearly see potential roadblocks.

During one of my aerial photography flights, I documented human development winthin the corridor. As more and more barriers are installed, that threat to this migration is increased.

The team discovers that oil and gas fields are also threatening the corridor. There’re already more than 8,000 wells on public lands, with plans to double that in the next decade.

These remarkable aerial photographs documented the changes since the 1980s.

In the last 100 years, the migration route has been threatened, by human development, by suburbs, and by ranchettes, and now by oil and gas development in the southern part of their range.

The team's extensive documentation of pronghorn corridor may help officials target where they need to act.

There're ways to protect wildlife corridors that are still compatible with human occupation in an area, and for example, a rancher could install fences that allow Pronghorn antelope, for example, to go under them without getting hung up on the barbed wire.

The team also hopes their efforts will spark an interest in protecting the animals.

The pronghorn migration corridor is 125 miles long and only a mile wide, it’s not gonna take that much to protect it.

If the human impact on this migration were to continue, then it's possible that the migration route might be cut off and if that were to happen there would be no mere pronghorn antelope in the Grand Tetons, and that would be tragic for everyone.

With commitment and planning, the pronghorn could continue their great annual migration just as they've done since the last Ice Age.

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