Despite these extremes, many plants and animals such as wildflowers, pronghorn antelope, mule deer, prairie dogs, and coyotes make their homes in the prairie grasslands. In addition, small, isolated wetlands dot the dry prairies, providing much-needed water and aquatic habitat for birds. In the Northern Great Plains, these wetlands formed as the glaciers receded and left round, sunken areas behind them. Rain and groundwater fill these depressions during certain times of year, creating scattered wetland habitat known as "prairie potholes." The Prairie Pothole Region in the Northern Great Plains contains 5-8 million small wetlands and some of the most important freshwater resources in North America. Bullrushes, sedges, and cattails grow on the edges of these potholes because they prefer standing water, and these plants in turn provide food and shelter for other species, such as birds. More than half of the migratory waterfowl in North America depend on prairie potholes for their survival.