Over a 20-year period, the 100 largest Urbanized Areas examined in this study sprawled out over an additional 14,545 square miles. That was more than 9 million acres of natural habitats, farmland and other rural space that were covered over by the asphalt, buildings and sub-divisions of suburbia. And that was just for the half of Americans who live in those 100 cities. A major controversy in the efforts to halt the rural land loss is whether land-use and consumption decisions are the primary engines of urban sprawl, or whether it is the nation’s continuing population boom providing most of the power driving the expansion. A careful analysis of U.S. Census Bureau data found that the two sprawl factors share equally in the blame: per-capita sprawl and population growth. Immigration is the largest cause of population growth.