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Hereabouts was where E had spent most of her summer that year, mating at least once and fattening herself on rodents for the homeward migration, another winter in the den, and pregnancy. It was productive habitat but also risky, Jørgensen said, what with all the agricultural machinery that could dice a snake like zucchini, all the farm-road traffic that could flatten her like an alligator belt. The changes that had come to this landscape did not favor long-distance rattlesnake migration. At that moment, as though to embody those changes within one human memory, a man named Aldo Pederzolli pulled up on his four-wheeler.

Pederzolli was the farmer on whose land we stood and who had genially welcomed Jørgensen's study. He wore a black polo shirt, rubber boots, and a cap that read "Cee-Gee Earthmoving." He was a fit-looking man of 80, with squinty brown eyes, a high voice, a sun-ripened Canadian smile. Introduced to me, hearing the reason for my visit, he said, "Oh, I just love rattlers." This wasn't irony. Got enough good snakes, he added, and you don't need to worry about gophers. Back when he was young, Pederzolli recalled, he would see nice fat old rattlers, that big around, when he seeded a fallow field. Don't see such big ones anymore. There was a den near the river, he said wistfully, and they'd migrate six miles up to a nice patch of open prairie full of gophers. Not anymore.

Although it's only a hypothesis, Dennis Jørgensen suspects that natural selection—in this case, the death of the venturesome—may be turning his migrating rattlesnakes into a population of stay-at-homes.

Biological diversity entails more than a gross tally of species. Diversity of ecosystems, behaviors, and processes are important too, contributing richness and beauty, robustness and flexibility and interconnectedness to the living communities on Earth. To lose the long-distance migrations performed by some species would be a grievous diminution. Joel Berger has made this point in the journal Conservation Biology and elsewhere, with reference to migrating species around the world and one creature close to home: the pronghorn (Antilocapra americana), North America's only endemic species of ungulate.

Loose talk sometimes mistakes the pronghorn for an antelope, but in fact it belongs to a family all its own. Its extreme speed (fastest land mammal of the New World), more than necessary to evade any living North American predator, probably reflects adaptation for escaping the now extinct American cheetah of the Pleistocene. Besides traveling fast, though, the pronghorn also travels far. One population migrates hundreds of miles across the Great Plains from north-central Montana into southern Saskatchewan and Alberta. Another population follows a narrow, tenuous route from its summer range in Grand Teton National Park, across a divide at the headwaters of the Gros Ventre River, and down onto the plains south of Pinedale, Wyoming, in the Green River Basin. There the pronghorn mingle with thousands of others arrived from other parts of Wyoming, where they try to distance themselves from the natural gas wellheads and drilling teams, and wait out the frozen months, feeding mainly on sagebrush blown clear of snow.

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