READ: Communities
7. Ecological Succession
Ecological Succession
Communities are not usually static. The numbers and types of species that live in them generally change through time. This is called ecological succession. Important cases of succession are primary and secondary succession.
Primary Succession
Primary succession occurs in an area that has never before been colonized. Generally, the area is nothing but bare rock. This type of environment may come about when
- lava flows from a volcano and hardens into rock.
- a glacier retreats and leaves behind bare rock.
- a landslide uncovers an area of bare rock.
The first species to colonize a disturbed area such as this are called pioneer species (see Figure below) They change the environment and pave the way for other species to come into the area. Pioneer species are likely to include bacteria and lichens that can live on bare rock. Along with wind and water, they help weather the rock and form soil. Once soil begins to form, plants can move in. At first, the plants include grasses and other species that can grow in thin, poor soil. As more plants grow and die, organic matter is added to the soil. This improves the soil and helps it hold water. The improved soil allows shrubs and trees to move into the area.
Primary Succession. On an island near New Zealand, bare rocks from a volcanic eruption are slowly being colonized by pioneer species.
Secondary Succession
Secondary succession occurs in a formerly inhabited area that was disturbed. The disturbance could be a fire, flood, or human action such as farming. This type of succession is faster because the soil is already in place. In this case, the pioneer species are plants such as grasses, birch trees, and fireweed. Organic matter from the pioneer species improves the soil. This lets other plants move into the area. An example of this type of succession is shown in Figure below.
Secondary Succession. This formerly plowed field in Poland is slowly changing back to forest.
Climax Communities
Many early ecologists thought that a community always goes through the same series of stages during succession. They also assumed that succession always ends with a final stable stage. They called this stage the climax community. Today, most ecologists no longer hold these views. They believe that continued change is normal in most ecosystems. They think that most communities are disturbed too often to become climax communities.
CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/