For millennia, Colorado's Yampa River Valley has followed the rhythms of wildlife mating and migration, the habits of elk and grouse and bear. The arrival of ranching in the 1880s altered the pattern a little, but radical change didn't occur until the last half of the 20th century. That's when the big ranches began to be broken up into small ranchettes and vacation-home lots, the kind of low-density exurban sprawl responsible for habitat fragmentation across the West.

Desperate to preserve Routt County's character, in the mid-1990s its commissioners fought to pass Land Preservation Subdivision ordinances, or LPS. It was an early form of conservation development, an increasingly popular land-planning tool that develops part of a property to fund the preservation of the rest.

Conservation development is usually regulated at the county level. Ordinances encourage developers to cluster houses on a portion of land and leave 40 to 80 percent of it as open space, and often give a "density bonus" for such clustering, allowing up to 70 percent more housing units per project.

Such developments typically sell well and command premium prices. They feel in touch with an agricultural past, where people can live within walking distance of hiking trails and fishing ponds. And they've found favor across the West: The passage of such ordinances took off in the '90s and has more than doubled in the last decade.

They seem to offer a way for mountain communities to have it all. A 2011 study estimated that conservation development has preserved nearly 10 million acres across the U.S. since the 1960s. But questions about its effectiveness remain: Is that open space really helping to maintain biodiversity?

"The key to integrating nature and urban growth is scale," says Armando Carbonell, chair of the Department of Planning and Urban Form at the Lincoln Institute of Land Policy in Cambridge, Mass. Local land planners and developers, he says, need to understand both the ecosystem context and the ecological consequences of their actions.

Sarah Reed, a conservation biologist with Colorado State University and the Wildlife Conservation Society, co-founded the Center for Conservation Development at CSU in fall 2010 to assess county development choices and their ecological consequences.

In 2010, Reed and her coworkers examined land-planning ordinances in all 414 counties of the 11 Western states. While over a third of the counties had regulations that promoted some form of conservation development, many did so in ways unlikely to preserve critical wildlife habitat or other natural values. Few promoted land stewardship, or ensured that open space parcels were contiguous within or among developments.

One of the biggest issues, Reed concluded, is the quality and type of data used to create the conservation design. Her preliminary results show that only 13 percent of the West's conservation development ordinances mandate a study of the property's ecological attributes. "There's no reason to believe that (the land that) got protected is any better than what got developed," Reed says. In contrast, she points to Routt County, which specifically requires developers to identify and avoid "Critical Habitat of Threatened and/or Endangered species, including nesting, roosting, mating, birthing and feeding areas."

Then there's the question of who manages the conserved land once the houses are built. Reed found that few ordinances require any sort of post-development oversight: That's left up to homeowner associations. Some make weed control, wildfire reduction, habitat restoration and riparian management a priority and set up funding; others don't. And there are other flaws; Wyoming and Colorado, in particular, are notorious for allowing reserved land to be reopened for development after 65 and 40 years, respectively.

Another problem is lot size. In 2011, Reed examined 372 conservation developments in Colorado and found that the average total size of a single project was 501 acres, with varying amounts set aside as open space, mostly in small scattered parcels. A 2006 study of developments near Boulder with open-space parcels ranging in size from 200 to 500 acres found that they were no different in terms of wildlife variety than traditional exurban sprawl.