Now a coalition of environmental groups think they have a way to make the long trek easier for wolves and grizzlies. They are pushing an ambitious and controversial project to construct an animal-friendly corridor that would run 1,800 miles from Yellowstone to Canada’s Yukon Territory. Dubbed Y2Y, the project would replant forests, build animals-only highway overpasses and curtail cattle grazing - all in order to create a ““bright green thread’’ where animals could safely pass. Pushed by groups like the Wilderness Society, the proposal is calculated to tug at North Americans’ nostalgia for the frontier. Critics aren’t moved. Says Chuck Cushman of the American Land Rights Association in Battle Ground, Wash., ““There is no end to the greed of the environmental groups. They’ve got to save all these animals, and they’ve got to have my house to do it.''

Y2Y and similar projects around the world are prompted by the controversial science of conservation biology. Its adherents argue that wildlife populations need to get around and swap genes with distant populations to avoid extinction. ““All you need is one horny male once every 10 years for genetic exchange,’’ says Steve Gatewood of the Wildlands Project. The poster animal for Y2Y is the grizzly, which numbers between 200 and 400 in Yellowstone. Biologists contend that the 2.2 million-acre park is at best a small ““postage stamp’’ island for the bear population. Bears need big chunks of space to get enough food to survive. And because Yellowstone’s grizzlies are isolated and reproduce at a very slow rate, they are not genetically diverse. ““We really need to give the bears more space,’’ says Barrie Gilbert, bear biologist at Utah State University.

Not all scientists agree with that assessment. It hasn’t been proven, they say, that species are threatened with extinction simply because they live in isolation. What’s more, carving out an 1,800-mile animal track is about as simple as making a bear talk. (No one is even guessing how much it would cost.) Y2Y’s proponents talk about various degrees of protection for animals making the trek. In some areas, typically wilderness, most human use would be forbidden. For example, British Columbia is considering imposing that restriction on 2.5 million acres of wild land in the northern Rockies. (In an additional 8 million acres it would allow some development.) In the least protected areas in the Y2Y plan, humans and animals would be allowed to coexist, though this has never been easy when the animals in question are wolves and grizzlies. It is also difficult to imagine getting large animals to use highway overpasses, but supporters of the plan say they are hopeful that such overpasses at Banff National Park will work. Scientists hope to use satellite maps of animal movements to draw their travel routes.

But as any vacationer knows, it’s easy to get lost. ““Bears can’t read maps,’’ says Kirk Moore, a 31-year-old rancher in Choteau, Mont. His home is already in a bear-protected area, which means he already sees increasing numbers of grizzlies traipsing around. He and other ranchers worry that under Y2Y migrating bears will endanger their livestock, if not themselves. ““It’s kind of like in the big city if you know you have a murderer around your neighborhood. Are you comfortable?’’ Moore asks. The biggest battle will be over land; property owners fear government- mandated ““land grabs.’’ Y2Y supporters prefer to focus on the potential jobs that would be generated by the project, such as replanting forests.

Is all this pie in the sky? Y2Y is one of several such projects, including a seven-country plan in Central America. In October supporters hold their first international conference, near Glacier National Park in Montana. But it will be a long time before lawmakers focus on the idea. Yellowstone-to-Yukon dreamers face an even more difficult trek than Pluie the wolf did.