One characteristic of all communities and ecosystems is that their structures constantly change in response to changing environmental conditions. The gradual change in species composition of a given area is called ecological succession. During succession some species colonize an area and their populations become more numerous, whereas populations of other species decline and even disappear. Ecologists recognize two types of ecological succession: primary and secondary. Primary succession involves the gradual establishment of biotic communities on nearly lifeless ground. In contrast, secondary succession, the more common type of succession, involves the reestablishment of biotic communities in an area where a biotic community was once present.

Primary succession begins with a lifeless area where there is no soil in a terrestrial ecosystem or no bottom sediment in an aquatic ecosystem. Examples include bare rock exposed by a retreating glacier or severe soil erosion, newly cooled lava, or a pond or lake created when bare rock gets covered with water. Before a community of producers, consumers, and decomposers can become established on land, there must be soil: a complex mixture of rock particles, decaying organic matter, air, water, and living organisms that, depending mostly on the climate, takes natural processes several hundred to several thousand years to produce. Soil formation begins when spores of hardy pioneer species attach themselves to inhospitable patches of bare rock. Examples are wind-transported lichens and mosses, which can withstand the conditions typical of such habitats-temperature extremes and the lack of moisture and soil nutrients. These species can extract nutrients from dust in rain or snow and from bare rock. They start the soil formation process on patches of bare rock by trapping windblown soil particles and tiny pieces of detritus (loose material that is worn away from rocks, like stone fragments and silt), producing tiny bits of organic matter and secreting mild acids that slowly fragment and break down the rock. This chemical breakdown is hastened by physical breakdown such as the fragmentation of rock when water freezes in cracks and expands.

As patches of soil build up and spread, eventually the community of lichens and mosses is replaced by a community of small grasses and herbs (ferns in tropical areas), whose seeds germinate after being blown in by the wind or carried to the area in the droppings of birds or on the coats of mammals. These early successional plant species grow close to the ground, can establish large populations quickly under harsh conditions, and have short lives. Some of their roots penetrate the rock and help break it up into more soil particles, and the decay of these plants adds more nutrients to the soil

After hundreds of years the soil may be deep and fertile enough to store the moisture and nutrients required to support the growth of less hardy, midsuccessional plant species of herbs, grasses, and low shrubs. These, in turn, are usually eventually replaced by trees that need lots of sunlight and are adapted to the area's climate and soil. As these tree species grow and create shade, they are replaced by late-successional plant species (mostly trees) that can tolerate shade. Unless fire, flooding, severe erosion, tree cutting, climate change, or other natural or human processes disturb the area, what was once bare rock eventually becomes a complex forest community. The specific composition of pioneer, early successional. midsuccessional. and late-successional communities and the rates of primary succession vary from one site to another. Generally, however, primary succession occurs fastest in humid tropical areas and slowest in dry polar areas

In contrast to primary succession, secondary succession begins with an area where the natural community of organisms has been disturbed, removed, or destroyed but the soil or bottom sediment remains. Terrestrial candidates for secondary succession include abandoned farmlands and burned or cut forests. Aquatic candidates include streams recovering from heavy pollution and land that has been dammed or flooded. Because some soil or sediment is present, new vegetation usually can begin to germinate within a few weeks. This is aided by seeds already present in soils and dispersal