Ever vigilant in promoting the advantages of big cities, the U.S. Conference of Mayors recently released a highly publicized report showing that metropolitan areas are the engines of the American economy.

The report goes to great lengths to show that U.S. metro economies are large in worldwide terms, pointing out not only that New York and Los Angeles have economies as big as South Korea and Australia, but even that Little Rock's economy rivals that of Kazakhstan. Most important, the mayors' report finds that the 319 U.S. metropolitan areas account for the vast majority of America's economic activity-- about 85 percent of the gross domestic product.

The mayors' point is that metropolitan regions are now the foundation of the American economy. This is not exactly news, of course. But there's a crowing tone to the report, as if the mayors don't want us to miss the fact that the rural areas of our nation (which are 80 percent of the country's land area) are eating their dust. So it's worth pondering what this really means for economic development in the 21st century.

We may live in a "metropolitan world" now. But that wasn't always the case, and even big cities have often relied on a rural hinterland-- usually to the mutual benefit of both city and countryside. As history professor William Cronon vividly showed in his classic book, "Nature's Metropolis: Chicago and the Great West," it is impossible to separate the 19th-century city from the rural areas with which it was intertwined. When you admire a weathered farmhouse in rural Nebraska today, remember that the wood probably came from trees in Wisconsin and Michigan that floated down Lake Michigan more than a century ago and then sat in woodpiles in Chicago before being shipped by rail out West. The rural West would not have existed without Chicago--but the reverse is also true.

Today, it's harder to make such an explicit connection between city and countryside. It's often said that the modern metropolis can function without the traditional access to raw materials--but that's an overstatement. Even regions driven by the "new economy," such as Silicon Valley, need huge quantities of raw material: wood and cement and food and silicon chips and clothes and gadgets. But they rarely get these commodities from adjoining rural areas anymore. Silicon Valley's rural hinterland isn't just California's Central Valley. It's Asia and South America and any place in the developing world able to grow, sell and ship things to the West Coast of the United States. To explain this, we're reduced to abstractions, such as the "ecological footprint," which seeks to translate the raw materials required to sustain a metro area into an acreage statistic, even though the acres are scattered across the globe.

A few rural areas in America still do a good job of exploiting their symbiotic relationship with big cities. By and large, however, it's a beauty contest. The pretty places can draw tourists and their money, especially if they are located close to thriving metro areas, while urbanites can glean "quality of life" benefits from nearby rural areas, such as when fresh fruits and vegetables are grown in the outskirts of the metropolis.

Meanwhile, the mundane locations--many of which profited in the past from resource extraction and agriculture--are being left behind. Resource extraction has become an iffy business in the United States, although that could turn around somewhat if the Bush administration pursues its energy and resource commodity policies aggressively. And farmers must compete in an increasingly competitive global marketplace.

So the Conference of Mayors' report on metropolitan economies is not all good news. It underscores the problems created by the rapidly changing relationship between prosperous cities and their traditional hinterlands in the United States.

Of course, the mayors' conference has been working on that one, too, at least to some extent. A couple of years ago, the mayors teamed up with the American Farmland Trust to promote a combination of rural farmland preservation and urban brownfields redevelopment. That's a good idea, but it addresses only one aspect of the city-countryside relationship--land.

Rural areas need more than just undeveloped land to thrive. They require a partnership with metropolitan regions in figuring out what role rural areas in America will play in the future. If lumber from Wisconsin no longer winds up housing farmers on the Nebraska plains, then what will be the new model of how city and countryside will prosper together?

Too often, economic developers in prosperous urban areas are so entranced by the miracle of metropolitan economic growth that they don't seem to care about their rural neighbors. But if we can get our grapes from South America and our clothes from Asia, does that mean that someday we won't need places like South Dakota? I don't think so.