Every community of organisms is subjected to a changing and fluctuating environment, and in turn, is itself changing in composition. There are diurnal, monthly, and annual cycles, long term trends and short term disturbances. The rates of flow of energy, nutrients, and water change. Plants and animals die and are replaced by new generations. A community of organisms can appear relatively stable while at the same time containing a mosaic of patchiness caused by fires, blow-downs, floods, or other disturbing events. The long term constancy and recovery of a community structure is an indication of its relative stability.
When a community of plants and animals is severely disturbed by fire, insect infestation or lumbering, these will occur great changes in the composition of the community and ecological succession will result. Ecological succession is an orderly process of community development that involves changes in species structure and community processes with time an ends in a stabilized ecosystem. For example, a virgin forest may be cut down for lumbering, or destroyed by fire, it then becomes an open field of grasses and herbs, followed by shrubs and young trees of certain species, followed by other tree species. After hundreds of years the mature forest may once again occupy the site.
At each stage of this preceding succession, the micro-climate of the site was modified by the plants themselves. At first it was an open exposed sunlit site, the soil was warm and dry, as grasses and fern covered the surface the soil became cooler and more protected against moisture loss. After the establishment of trees, the soil was even more shaded, cooler and more moisture retentive. The wind blowing over the site, was a completely different during each stage of the succession, the ecosystem had a different character during each progressive succession.
Succession does not always lead to climax communities nor is succession only limited to plants. It occurs among all organisms, plants and animals.
It is important to realize the dynamics that occur within ecosystems and the patchiness of communities. There is always disturbance in nature, fire, wind, thaw, drought, flood, earthquake and the result is a certain pervasive patchiness to any community. Whenever there is disturbance by humans there will be opportunity for succession.