The landscape of the American Southwest has been shaped by naturally occurring fires for thousands of years. Fire is a major natural driver in some ecosystems, such as the grasslands of the Great Plains. Fire influences ecosystem processes and patterns, such as nutrient cycling and plant community composition and structure. Without frequent small and occasional large fires to create a mosaic of plant communities in different stages, biodiversity declines and leaf litter and deadfall accumulate faster than they can return nutrients to the soil through decay.