Detroit stands as the ultimate expression of industrial depopulation. The Motor City offers traffic-free streets, burned-out skyscrapers, open-prairie neighborhoods, nesting pheasants, an ornate-trashed former railroad station, vast closed factories, and signs urging "Fists, Not Guns." A third of its 139 square miles lie vacant. In the 2010 census it lost a national-record-setting quarter of the people it had at the millennium: a huge dip not just to its people, but to anxious potential private- and public-sector investors.

Is Detroit an epic outlier, a spectacular aberration or is it a fractured finger pointing at a horrific future for other large shrinking cities? Cleveland lost 17 percent of its population in the census, Birmingham 13 percent, Buffalo 11 percent, and the special case of post-Katrina New Orleans 29 percent. The losses in such places and smaller ones like Braddock, Penn.; Cairo, Ill.; or Flint, Mich., go well beyond population. In every recent decade, houses, businesses, jobs, schools, entire neighborhoods -- and hope -- keep getting removed.

The subtractions have occurred without plan, intention or control of any sort and so pose daunting challenges. In contrast, population growth or stability is much more manageable and politically palatable. Subtraction is haphazard, volatile, unexpected, risky. No American city plan, zoning law or environmental regulation anticipates it. In principle, a city can buy a deserted house, store or factory and return it to use. Yet which use? If the city cannot find or decide on one, how long should the property stay idle before the city razes it? How prevalent must abandonment become before it demands systematic neighborhood or citywide solutions instead of lot-by-lot ones?

Subtracted cities can rely on no standard approaches. Such places have struggled for at least two generations, since the peak of the postwar consumer boom. Thousands of neighborhoods in hundreds of cities have lost their grip on the American dream. As a nation, we have little idea how to respond. The frustratingly slow national economic recovery only makes conditions worse by suggesting that they may become permanent.

Subtracted cities rarely begin even fitful action until perhaps half the population has left. Thus generations can pass between first big loss and substantial action. Usually the local leadership must change before the city's hopes for growth subside to allow the new leadership to work with or around loss instead of directly against it. By then, the tax base, public services, budget troubles, labor forces, morale and spirit have predictably become dismal. To reverse the momentum of the long-established downward spiral requires extraordinary effort.

Fatalism is no option: Subtracted cities must try to reclaim control of their destinies. They could start by training residents to value, salvage, restore and market unused sites and the material found there. They might supplement school drug-free zones with subtraction-action ones by reacting quickly when nearby empty properties show neglect. Children who see debris-filled plots and boarded-up buildings learn not to expect much from life. Just planting a few trees often makes a deserted lot look cared for.

The cities should encourage gardens on abundant unoccupied lots. Plants can sometimes go directly into the ground. Or people can truck in soil and build raised beds. Community gardens upgrade the food supply, offer a positive neighborhood project, teach business and social skills, and may create new enterprises based on common resources. Trucks, backhoes and other small earth-moving equipment often get shared for home maintenance. Above all, community gardens rouse people for tangible local efforts rather than just against City Hall.

Subtracted cities can scream walkability and public transit. They still have downtowns, main streets and public works that should support entertainment, retail and the arts. Clearing vacant lots and structures on the downtown's edge can create parks, outdoor amphitheaters and sports facilities. When these measures work, nearby homes, perhaps with solid architecture, become more valuable.

Subtracted cities should not commit the sacrilege of obliterating their past and the contributions of their people. They should retain and reuse old structures. Factories and mills can become restaurants, apartments and business incubators, and rail lines hiking trails. The answer to desertion should always be to use any chance to get beyond it without necessarily concealing it. But the first step begins with facing the on-the-ground subtraction without pretending or praying that it will go away when it hasn't for generations.

Deborah E. Popper teaches geography at the College of Staten Island/City University of New York and Princeton University. Frank J. Popper, who teaches land-use planning at Rutgers University and Princeton University, contributed to this article. They wrote this article for the American Geographical Society Writers Circle.