Ecological succession is the replacement of one biological community with another community until eventually, an equilibrium is achieved. When an area that was uninhabited, as in a new volcanic island, eventually becomes a thriving habitat, succession has occurred. This type of succession is called primary succession. The pioneer organisms come first--lichens and moss which help biologically weather the rocks to form soil. Next, grasses can grow, adding more humus to the soil and enriching it. Eventually, shrubs can take hold, followed by trees. Each change in the plant community allows different animals with specific adaptations to the new community, to become established. Eventually, a climax community is achieved and will remain in equilibrium indefinitely, unless a natural(flood, fire) or man-made catastrophe occurs. If that happens, secondary succession will then take place. If a forest burns down, in the future the same area will once again see the return of grasses and weeds. Over a long period of time, the forest will eventually grow back again, thus achieving equilibrium.