Primary and secondary successions are two phases of ecological regrowth. Primary succession refers to the growth of plants and trees in an area where no life exists, which secondary succession is regrowth in an area where some life already exists. The type of plants that begin to populate an area often determine the eventual ecology of that area.
When life begins in an area where it has not been before, it's called primary succession. An area may be devoid of life because of a catastrophic event such as glacial movement, a landslide, or a flood. A geological event such as a newly-formed volcanic island may also provide an opportunity for primary succession.
Secondary succession occurs when an area retains some life, but it has been denuded in some way. A farmer’s field or a housing development may occupy land where forests once stood. Secondary succession ensures that the forest will eventually return if fields lie fallow or a housing development is not kept clear of invading plants and trees.
One difference between primary and secondary successions is the condition of the soil when each process begins. In primary succession, an area scraped raw by a glacier may be bare rock or covered in gravel. Over time, these rocks break down into smaller and smaller pieces, eventually creating soil. The wind brings in both soil and organic material that decomposes to add vital nutrients. In secondary succession, soil is already present, though its composition is largely dependent on the types of plants already inhabiting the area.
These two phases also differ in the types of plants that first appear in their respective processes. In primary succession, only plants that can live without soil are able to inhabit the area. These first plants, usually some form of lichen, are called pioneer species. In secondary succession, soil and some plants are present, making it easier for new plants to take hold.
Time is another difference. Primary succession takes much longer than secondary succession, often centuries or millennia. Secondary succession can be relatively quick. For example, after a forest fire, it may take only a few decades for the forest to return to its prior state.
The progress of primary and secondary successions does not always follow through to a large stand of huge trees. Some end-stage communities, known as climax communities, are small shrubs, grasses, or succulents. Climate and soil conditions may restrict an area to grassland or cactus.