Fire ecology is concerned with the processes linking the natural incidence of fire in an ecosystem and the ecological effects of this fire. Many ecosystems, particularly prairie, savanna, chaparral and coniferous forests, have evolved with fire as a necessary contributor to habitat vitality and renewal. Many plant species in naturally fire-affected environments require fire to germinate, establish, or to reproduce. Wildfire suppression not only eliminates these species, but also the animals that depend upon them. Finally, fire suppression can lead to the build-up of flammable debris and the creation of less frequent but much larger and more destructive wildfires.Campaigns in the United States have historically molded public opinion to believe that wildfires are always harmful to nature. This view is based on the outdated belief that ecosystems progress toward an equilibrium and that any disturbance, such as fire, disrupts the harmony of nature. More recent ecological research has shown, however, that fire is an integral component in the function and biodiversity of many natural habitats, and that the organisms within these communities have adapted to withstand, and even to exploit, natural wildfire. More generally, fire is now regarded as a 'natural disturbance', similar to flooding, wind-storms, and landslides, that has driven the evolution of species and controls the characteristics of ecosystems. The map below right shows how each ecosystem type in the United States has a characteristic frequency of fire, ranging from once every 10 years to once every 500 years. Natural disturbances can be described by key factors such as frequency, intensity and area. The map also shows intensity, since some fires are understory fires (light burns that affect mostly understory plants) while others are stand replacement fires (intense fires that tend to kill the adult trees as well.)Fire suppression, in combination with other human-caused environmental changes, has resulted in unforeseen consequences for natural ecosystems. Some uncharacteristically large wildfires in the United States have been caused as a consequence of years of fire suppression and the continuing expansion of people into fire-adapted ecosystems. Land managers are faced with tough questions regarding where to restore a natural fire regime.