Thinking Anew About a Migratory Barrier: Roads The mountains in and around Glacier National Park teem with bears. A recently concluded five-year census found 765 grizzlies in northwestern Montana, more than three times the number of bears as when it was listed as a threatened species in 1975. To the south lies a swath of federally protected wilderness much larger than Yellowstone, where the habitat is good, and there are no known grizzlies. They were wiped out 50 years ago to protect sheep. One of the main reasons they have not returned is Interstate 90. To arrive from the north, a bear would have to climb over a nearly three-foot high concrete Jersey barrier, cross two lanes of road, braving 75- to 80-mile-an hour traffic, climb a higher Jersey barrier, cross two more lanes of traffic and climb yet another barrier. Some experts believe that habitat fragmentation, the slicing and dicing of large landscapes into small pieces with roads, homes and other development, is the biggest of all environmental problems. “By far,” said Dr. Michael Soulé, a retired biologist and founder of the Society for Conservation Biology. “It’s bigger than climate change. While the serious effects from climate change are 30 years away, there’s nothing left to save then if we don’t deal with fragmentation. And the spearhead of fragmentation are roads.” Fragmentation cuts off wildlife from critical habitat, including food, security or others of their species for reproduction and genetic diversity. Eventually they disappear. There are some four million miles of roads affecting 20 percent of the country, and in the last 10 years the new field of road ecology has emerged to study the many impacts of roads, and how to mitigate the damage....