Primary and secondary succession occurs in ecosystems, thereby creating new ecosystems or changing existing ones. Primary succession happens when a major catastrophic disturbance occurs—typically affecting rocks, cliffs and sand dunes. Secondary succession results from the natural aging process of the elements or when humans or animals have made a disruption.
Primary succession is very slow. It starts where no previous soil existed and can take several hundred years to produce fertile soil naturally. Bacteria and lichens, e.g., fungi and algae, colonize bare rock. The algae photosynthesize and the fungi absorb nutrients from the bare rocks and begin to hold water. As time goes on, the fungi and algae break down the rock matter. Water freezes and then thaws out in the cracks and cervices of the rock. Lichens eventually die and accumulate in the cracks. Moss begins to grow and die in the cracks and starts to create fertile soil. The fertile soil is made from the broken rocks, decayed organisms, water and air.
Secondary succession, which is more common, occurs where an ecosystem previously existed. For example, after a natural disaster—such as a forest fire—plant life and soil is wiped out or damaged. A fire burns and destroys plant life in a previous ecosystem. Once the land has been wiped out, organisms begin to colonize any area that may still be habitable. These organisms are called the pioneer species because they take the lead in sprouting new plant life. In time, plants, flowers, new trees and shrubs start to grow. The climax community enters the finalizing stages and becomes stable. It changes in small ways, but continues undisturbed unless another natural disaster occurs. When humans or animals contribute to secondary succession, they deplete the ecosystem. Organisms begin to colonize in the same manner as when a natural disaster occurs, eventually leading to a climax community.
Many times animals depend on fires for secondary succession to develop because they depend on the newly sprouted vegetation to feed on. For an example of primary succession, look at the sidewalks in your neighborhood. Another example of secondary succession is currently taking place at the site of the 1980 eruption of Mount Saint Helens in Washington State.