Succession is the transformation or creation of a biological community as new species are introduced and modify the environment. A community does not remain in a fixed state throughout time. Even a small change can upset any balance that does exist, and this can then alter the whole structure of the community. Under normal circumstances, a community will start off with a simple structure and gradually become more complex with time. Primary succession is the progressive colonization of a previously barren area. Secondary succession is the recolonization of an area that has been disturbed by fire, hurricanes, field clearing, tree felling, or some other process that removes most plant and animal species. Intermediate successional communities are known as seral stages or seres.

One example of primary succession is a newly emerged volcanic island. Initially, the area is barren. The first organisms to inhabit the area are lichens--one of the only living organisms capable of surviving on bare rock. While participating in their natural life cycles, lichens break the volcanic rock down, allowing dust particles and humus to collect in crevices. This provides a habitat for mosses, ferns, and eventually grasses. With more available humus, the soil becomes more enriched and larger plants move into the area. With more plants come animal species. Eventually, the vegetation is dominated by one or two tree species. At this time the overall diversity in the system decreases, creating a climax community. The climax community is more or less stable and there is no new influx of species into the community. If there is a change in the environment it may alter the climax community, returning the whole area back to an earlier point in the successional series.

Succession alters with time, but it is possible to observe the stages of succession as changes occur within an environment. A classical example is the transition from an open sandy beach to sand dunes, and finally to the climax woodland. Within woodland, change is observed on a smaller scale when a tree falls. The gap in the forest canopy created by the fallen tree creates opportunity for less dominant woodland species that thrive on the extra sunlight. This type of succession is called gap succession.