low food supplies all served as checks keeping pre-industrial cities much smaller than their modern counterparts. Ancient Athens and Rome, scholars estimate, had peak populations of 80,000 and 500,000. The onset of the Industrial Revolution in Europe in the late 19th century removed many of these checks. As food supplies increased and stabilized and industrialized centers arose, cities began to support larger populations, sparking the start of rural flight on a massive scale. The United Kingdom went from having 20% of the population living in urban areas in 1800 to more than 70% by 1925. While the late 19th century and early 20th century saw much of rural flight focused in Western Europe and the United States, as industrialization spread throughout the world during the 20th century, rural flight and urbanization followed quickly behind. In the early twenty-first century, rural flight was especially distinctive phenomenon in China and sub-Saharan Africa. Resource repletion and ecological factors: case study of the Dust Bowl in 1930s North America The shift from mixed subsistence farming to commodity crops and livestock began in the late 19th century. New capital market systems and the railroad network began the trend towards larger farms that employed fewer people per acre. These larger farms used more efficient technologies such as steel plows, mechanical reapers, and higher-yield seed stock, which reduced human input per unit of production. The other issue on the Great Plains was that people were using inappropriate farming techniques for the soil and weather conditions. Most homesteaders had family farms generally considered too small to survive (under 320 acres), and European-American subsistence farming could not continue as it was then practiced. During the Dust Bowl and the Great Depression of the 1930s, large numbers of people fled rural areas of the Great Plains and the Midwest due to depressed commodity prices and high debt loads exacerbated by several years of drought and large dust storms. Rural flight from the Great Plains has been depicted in literature, such as John Steinbeck's novel The Grapes of Wrath (1939), in which a family from the Great Plains migrates to California during the Dust Bowl period of the 1930s. Since World War II Post-World War II rural flight has been caused primarily by the spread of industrialized agriculture. Small, labor-intensive family farms have grown into, or have been replaced by, heavily mechanized and specialized industrial farms. While a small family farm