Succession is a scientific term describing the long-term progression of biological communities that occurs in a given area. Ecological succession breaks down into three fundamental phases: primary and secondary succession, and a climax state. The study of ecological succession generally focuses on the plants present on a particular site. But animal populations also shift over time in response to the changing habitat.

Primary succession occurs when organisms colonize an area devoid of life, usually after a catastrophic natural event that leaves the land barren. Often the first organisms to take hold are algae, fungi and simple plants such as lichens and mosses. Over time a thin layer of soil builds up so that more advanced plants, such as grasses and ferns, can take root. Along with the successful colonization of plants come animals such as insects, birds and small invertebrates. One example of primary succession is the pioneer communities that begin to inhabit a newly created lava bed, where life cannot exist until the rock surface cools to a moderate temperature.

Most ecological change occurs as secondary succession. In fact, most biological communities are in a continual state of secondary succession. This term describes the process in which an established community is replaced by a different set of plants and animals. Secondary succession is gradual, always moving toward the climax community. Most ecosystems, however, experience disturbances -- either natural events such as wildfires or flooding, or man-caused events such as logging -- that set back the progress of succession.

An ecosystem undergoes many intermediate stages of succession. These changes form a continuum between the two endpoints, with the actual stages being merely a fixed glance at the never-ending progression of plants and animals. The emergence of the climax state of succession may occur more quickly in some ecosystems, and likely never occur in other biomes that experience routine disturbances. Examples of quickly forming climax communities are the short-grass and long-grass prairies of the Great Plains of the United States.

Climax communities are relatively stable and can vary widely in a given region, especially when the landscape consists of high mountains and low valleys. In such cases, the final biological matrix of plants and animals can cover vast tracts of land or be limited to a very small pocket within the landscape. Overall, a climax community is very dependent on rainfall, soil, altitude and temperature. California, for instance, includes many different and distinct ecosystems. One of the most unique places is the redwood forest, which can be found only in the fog banks along the coastal waterways of the northern part of the state.