Before Europeans settled in the USA, there were no urban areas. There were only forests, grasslands, mountains, deserts, and rivers, all full of wildlife. Native Americans killed animals for food and clothes, but they killed only the number of animals that they needed. As the population grew and people moved west, more and more animals were killed. The number of large bison,(also called buffalo) on the Great Plains in Central USA fell from millions to only a few hundred by the end of the19th century.