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Emilene Ostlind | Jun 13, 2011 05:00 AM

When the first winter storms buried northeast Montana last November, the thousands of pronghorn antelope that spent the summer around the state's border with Alberta and Saskatchewan started making their way south. Normally, they move into the north side of the state's Milk River valley and find enough sagebrush sticking out of the snow to forage through the winter, but this year, the snow was too deep, so they kept pushing on.

"Winter came a month earlier and stayed a month longer. We had historical snow at 9 feet deep," says Montana Fish, Wildlife and Parks biologist Kelvin Johnson who, in late November and December, watched groups of 200 to 400 antelope walking south in single file. They crossed Highway 2 along the Milk River and followed Rock Creek and Larb Creek, eventually reaching the north shore of frozen and snow-covered Fort Peck Reservoir on the Missouri River, in northeastern Montana. Johnson saw, "herds of great size going across the lake."Antelope in Missouri River Michael Forsberg

Pronghorn antelope are experts at survival. The only ungulate endemic to North America (meaning they evolved here and nowhere else), they're also the only remaining species in their family. They've lived on this continent for 20 million years, about 19.99 million years longer than humans. They shared the plains with American cheetahs and enormous dire wolves, now extinct, plus millions of bison that used to surge over the land. They watched the Teton Range rise up out of the Snake River plain and they weathered ice ages.

One strategy that's helped them prosper for so long is migration, the ability to seasonally cross huge distances of country to seek better climate and habitat. Some of the antelope this winter traversed 250 miles of Montana before they found adequate range south of the reservoir, near the town of Jordan and along Highway 200. When Johnson went there in February, he noticed something strange: the antelope's legs were dark gray rather than the usual white. He found they were scabby and bloodstained with the hair worn off from traveling through brush and crusty snow.

Making this difficult journey south of the worst snow paid off at first. The antelope fared well for the rest of the winter and were in good health when spring started to soften Montana, while those that stayed north of the Missouri River died by the thousands from freezing, starvation and collisions with trains when they went up on the tracks to escape deep snow. However, for those that did migrate, their main trial was still ahead, and like the trains, it was something for which evolution did not prepare them.

When the snow pulled back and the antelope started to make their way north to the Missouri River, it was no longer the white plain they had crossed months before. Instead, they faced a mile-wide reservoir of open water. Just a couple weeks earlier, wildlife biologist Ashley Beyer was flying along the south edge of the reservoir to survey great blue herons when she noticed pronghorn pacing the shore. She started counting them, and along just half the length of the reservoir, she observed 2,710 individuals.

Dennis Jorgensen, program officer with World Wildlife Fund's Northern Great Plains program, spent time over a couple of weeks with photographer Michael Forsberg watching the pronghorn trying to cross the reservoir. As he tells it:

The pronghorn would come down to the river in morning. They were moving along the shore looking for a place to cross. Sometimes they would go all day without attempting to cross. Sometimes one individual would plunge in with others following. We saw groups of 15 to 20 pronghorn set out, often groups of does. The animal leading the way was so driven to get to the other side. Most would get a third to half way across, and then peel off and head back to the south shore. When an animal did continue to north side, she could barely stand. The most I ever saw make it was a group where eight bucks started and six made it across.

This is not the first time antelope have faced extreme weather. The last record floods on the Milk River happened just 50 years ago, a mere blink in antelope evolutionary history. But recent landscape changes such as dams, fences and subdivisions threaten migration as a survival strategy for the pronghorn. Add to that, the record 391 parts per million of carbon dioxide in the atmosphere and dire predictions for more extreme weather to come. In an angry Washington Post op-ed responding to flooding throughout the Missouri River watershed plus recent wildfires and tornadoes, climate change activist Bill McKibben wrote, "you might find your thoughts wandering to, oh, global warming, and to the fact that climatologists have been predicting for years that as we flood the atmosphere with carbon we will also start both drying and flooding the planet, since warm air holds more water vapor than cold air." (I recommend listening to a reading of the letter here.)

It's this dual threat of human-built obstacles like reservoirs and fences blocking migration on the ground and a shifting climate pushing wildlife to cover more distance in their search for suitable habitat that puts migrating pronghorn in danger. They survived the die-off of prehistoric species and a brush with extinction from overhunting a century ago. Now the landscape these animals evolved with for millions of years is deteriorating. If we can't protect the climate and habitat enough to enable this long-term survivor to continue its ancient journeys, I have to wonder if we're doing enough to protect the land for our own fledgling species.

Photo © 2011 Michael Forsberg / michaelforsberg.com.

Post updated 10:28am 6/13/11.

Emilene Ostlind is HCN's editorial fellow.

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