Luther Propst, executive director of the Sonoran Institute, a nonprofit conservation group that focuses on planning issues, sees it as one of the major threats facing the West. "Part of the challenge is that this exurban growth is being driven by some very powerful and fundamental demographic and economic forces," he explains.

The impending retirement of baby boomers adds to the urgency of the problem. "Even if you have just a regular working career in San Francisco, you are in the position to walk away with hundreds of thousands of dollars in profit from the sale of your home," says Propst. "Under the tax code, you are obligated to turn (that profit) back into housing, so you buy five to 10 to 20 acres, and build a house."

Tom Cova, a geographer at the University of Utah, has been watching this phenomenon in bucolic Grants Pass, Ore., where he grew up. Grants Pass began losing its mining, logging and paper-mill jobs 30 years ago, and they’ve been replaced by low-paying service-oriented jobs. Yet home prices and the population have been increasing rapidly. There is, he points out, a fundamental disconnect between traditional sources of wealth and this new rural living.

To get a grasp on just how swiftly the open spaces could disappear, look at a Web site created by the Center of the American West at the University of Colorado-Boulder. Maps show development patterns in the West from 1960 to 2000, then extrapolate to 2040. Cities and suburbs expand rapidly in the coming decades, but the biggest change is in exurban development. In Colorado, for example, the maps show 10-acre developments crawling across the base of the Rocky Mountains and sweeping up toward the Continental Divide.

A December 2004 Brookings Institution report contains sobering statistics. It predicts that in the West, about 45 percent of the houses being used in 2030 will have been built since 2000. Most of this growth, says the report, will occur at the urban edge in natural or agriculture areas. Arizona, Nevada and Utah will lead the nation, with Idaho, Colorado, New Mexico, Oregon and Washington also in the top 10.

If the new exurbs offer expansive living, they come at a considerable cost — and not only to the people who actually live there.

Richard Knight, a professor of wildlife conservation at Colorado State University, says exurban development takes a toll on biodiversity. Human-adapted species such as magpies, robins and raccoons thrive, while native species such as the yellow warbler are pushed out (HCN, 10/28/02: Shadow creatures). Sometimes the removal is deliberate. "What is the level of tolerance in ranchette development for prairie rattlesnakes?" asks Knight. In other instances, it’s a byproduct of human-subsidized predators, such as cats, dogs and what Knight calls "metallic carnivores" — cars and trucks (HCN, 2/7/05: Caught in the Headlights). "Fire is an absolutely essential ecological process for rangelands," he says — and the same is true for forests. "But there is zero tolerance for fire in exurban development."

Exurbanites’ clean air also comes at a cost. Exurban commuters contribute more than their fair share to air pollution. Consider Conifer. According to the U.S. Census Bureau, in 2000, the average commute time from that area was 38 minutes, compared to 24-26 minutes from the closer-in suburbs and within Denver itself. While the air quality in many major cities has improved in recent decades, new studies show that people living close to freeways face heightened risks of cancer, asthma and other illnesses. Children are particularly at risk. There are also implications for global warming: A commuter who drives 50 miles to work — 25,000 miles a year — pumps out enough carbon dioxide, a key greenhouse gas, to fill a Goodyear blimp.

The extensive transportation infrastructure has other impacts. Exurbia requires more miles of road per person than do the suburbs or cities. Water runs off those roads, even the gravel ones that dominate the exurban infrastructure, washing soil into streams and other waterways, harming fish and other wildlife. It also makes it more expensive to treat water for drinking, says Michelle DeLaria, the stormwater water-quality coordinator in Jefferson County.

Individual water wells and septic fields also pose problems. The foothills west of Denver, including Evergreen and Conifer, have 25,000 individual septic fields. When those septic fields are too close to water wells, it can mean trouble. Nitrate levels in groundwater in several hotspots along the Highway 285 corridor have doubled and tripled, says Russ Clayshulte, a consulting water-quality specialist in the Evergreen-Conifer area. Elsewhere in Colorado, he notes, nitrate in groundwater has caused the death of infants. Also of concern is the migration of viruses from septic fields into water supplies. New studies by the Environmental Protection Agency show that viruses can migrate hundreds of feet from their point of origin in septic fields, and remain in the soil for as long as two years.

The most dramatic problems with exurbia involve fire. This became wildly apparent in 2002, when television cameras showed thousands of people fleeing their exurban homes in Colorado, Arizona and Oregon in front of billowing clouds of smoke. The 137,000-acre Hayman Fire destroyed 132 homes in the region south of Conifer.

Firefighters have died trying to control wildfires, but the University of Utah’s Cova fears the worst is yet to come. "Sooner or later, we will have a fire going up a canyon in the opposite direction of the exit," trapping residents inside the fire, he says. "It just hasn’t happened yet."

The U.S. Forest Service has never assessed the cost of fighting and preventing fires to benefit exurban homeowners, but it is surely astronomical. Extinguishing the Hayman fire alone cost $42.2 million. While some expense is inevitable, anecdotal evidence suggests trying to prevent flames from reaching homes along the "wildland-urban interface" inflates the cost. Fires like the Hayman spur more calls for expensive thinning — also for the benefit of the exurbanites (HCN, 4/14/03: Forest thinning slows fires, increases concerns).

Brian Muller, a geographer at the University of Colorado, says that insurance companies in much of the Intermountain West have only recently increased premiums or deductibles for people living in this dangerous interface. "The basic point, from our perspective, is that until recently, homeowners did not really absorb the cost of the risk," he says. Society bears other costs, too. In the exurbs, everything is more expensive: School buses have to go farther to pick up kids; telephone and utility lines stretch to reach scattered homes; law enforcement, ambulances, building inspectors and other county officials have to travel considerable distances. The greatest single cost, however, comes with the roads, which are usually built and maintained at taxpayer expense. In Jefferson County, where Conifer is located, the average number of households per mile of road in the mountain exurban areas is 18. In the cities, it’s 59 households per mile. Partly in response to exurban commuters, the state government since 1991 has been incrementally four-laning Highway 285, at a cost to taxpayers so far of $105 million.

County government has long seen new development as a source of tax revenue, but in some places, it costs more to service exurban development than the county nets in taxes. A 2002 study by economists Roger Coupal and Andy Seidl of the Colorado State University Cooperative Extension Service found that county and school district governments in Colorado spent $1.65 for every $1 in revenue received from dispersed 35-acre ranchettes. In some places, including La Plata and Jefferson counties, they found $5 in expense for every $1 in revenue. In other words, the city mice heavily subsidize the scurrying country mice.

Supreme Court Justice Oliver Wendell Holmes Jr. famously said that your right to swing your fist ends where the other man’s nose begins. Exurban living is smacking noses left and right. The task of planners, activists, and government officials is to control this bully as the West’s population continues to mushroom.

"You can put a lot of people in the landscape, but with less impact if you do it in certain ways," says William Travis, a geographer at the University of Colorado-Boulder. "We will have to pay a lot of attention to the patterns rather than the raw numbers."

The crucial discussion must occur in the county courthouses and state legislatures, where land-use rules and laws are written. However, in most states in the West, governments are traditionally reluctant to curb private-property rights. The right to subdivide farms and ranches into smaller ranchettes or hobby farms goes without question. In Colorado, as long as the lots are 35 acres or larger, you need no permission to subdivide. Many county governments routinely bless even smaller divisions, such as the 5- 10- and 20-acre parcels favored by the exurban hobbyist.

That mentality is starting to bend, however.