WASHINGTON -- Rural America now accounts for just 16 percent of the nation's population, the lowest ever.The latest 2010 Census numbers hint at an emerging America where, by midcentury, city boundaries become indistinct and rural areas grow less relevant. Many communities could shrink to virtual ghost towns as they shutter businesses and close schools, demographers say.More metro areas are booming into sprawling megalopolises. Barring fresh investment that could bring jobs, however, large swaths of the Great Plains and Appalachia, along with parts of Arkansas, Mississippi and rural areas of North Texas, could face significant population declines.These places posted some of the biggest losses over the past decade as young adults left and the people who stayed got older, moving past childbearing years."This place ain't dead yet, but it's got about half a foot in the grave," said Bob Frees, 61, of Moundsville, W.Va., which now has a population of just over 9,000. "The big-money jobs are all gone. We used to have the big mills and the rolling plants and stuff like that, and you could walk out of high school when you were 16 or 17 and get a $15-an-hour job."Demographers put it a bit more formally."Some of the most isolated rural areas face a major uphill battle, with a broad area of the country emptying out," said Mark Mather, associate vice president of the Population Reference Bureau, a research group in Washington, D.C. "Many rural areas can't attract workers because there aren't any jobs, and businesses won't relocate there because there aren't enough qualified workers. So they are caught in a downward spiral."Many rural areas, the Great Plains in particular, have been losing population since the 1930s, with few signs of the trend slowing in coming decades, according to census figures.The share of people in rural areas over the past decade fell to 16 percent, passing the previous low of 20 percent in 2000. The rural share is expected to drop further as the U.S. population balloons from 309 million to 400 million by midcentury, leading people to crowd cities and suburbs and fill in the open spaces around them.In 1910, the population share of rural America was 72 percent. Such areas remained home to most Americans until 1950, amid post-World War II economic expansion and the baby boom.Among the struggling rural areas are vast stretches of West Virginia in Appalachia.Other rural U.S. counties seeing big declines include Issaquena, Jefferson and Sharkey in Mississippi; Sheridan and Towner in North Dakota; Kiowa in Kansas; Cimarron in Oklahoma; Tensas Parish in Louisiana; Monroe in Arkansas; and Cottle, King and Culberson in Texas.All had percentage losses of 20 percent or more over the past decade.The numbers are based partly on an analysis by the Population Reference Bureau. "Rural" is generally defined as nonmetro areas with fewer than 50,000 people.The Census Bureau will soon begin to define new "combined statistical areas" -- often referred to by demographers as megapolitan areas or megalopolises -- based on growth and overlapping commuter traffic.Some analysts point to a merger of areas between Austin and San Antonio, between Tampa and Orlando, Fla., and possibly between Phoenix and Tucson, with the Washington, D.C.-Baltimore region extending southward to Richmond, Va.