Succession is the gradual replacement of one community of organisms by another.
Succession begins with the pioneer community, which colonizes any unoccupied habitat such as bare ground, open water or rock. Each community is replaced in turn by another community of plants and animals. This new community is successful at replacing the older community because the older community has modified the habitat. Some of the many changes include more shade, greater amounts of humus in the soil, and more moisture available to the plants. New plants and animals find the changed habitat to their liking.