Succession
In the maintenance of ecosystem stability, you have learned about the important factors, sustainable practices, pollution, and restoration. However, sometimes ecosystem destruction (such as in natural wildfires) is beneficial for the restart of an ecosystem. During this restart, the ecosystem has to be rebuilt over time via ecological succession.
Ecological succession refers to the process of sequential change over time in an ecosystem. Usually, it comes about in response to a disturbance and is the result of the interactions (dynamic) between biotic and abiotic conditions. Earlier communities modify the environment, making it more favourable for the future species in the communities that will come next. In time succession may result in stable, mature or climax communities. Succession that occurs where there is no soil is called primary succession.
Primary succession
Primary succession occurs when new land is formed or exposed for the first time.
This could be the result of cooling lava creating new rock, or when a glacier retreats, exposing rocks that have no soil on them. It can also happen if a previous community has somehow been extinguished.
During primary succession there are multiple changes in the composition of the communities that develop over time. Each stage in succession is referred to as a sere or a seral community. The early seres have a simple structure and a lower species diversity than the later stages. They will host the smaller plants capable only of low primary production, limited nutrient cycling and very simple food webs. Over time the developing community modifies the abiotic environment. The process is as follows:
- The earliest pioneer species are microorganisms, such as cyanobacteria. They can survive the substrate free environment thanks to their autotrophic ability, using only sunlight to make food.
- Lichens begin the process of forming soil by breaking down substrate, and adding organic matter to it when they die and decay. Their presence, life and death therefore create a favorable environment for vascular plant growth.
- Then, the community evolves to house intermediate species. These are typically larger and denser plants such as shrubs and pine trees. There is now increased primary productivity, and this leads to stable nutrient recycling and more complex food webs than in the former seres.
- Eventually, the climax community is formed with the arrival of shade tolerant trees such as oak and hickory. Climax communities in a state of stable equilibrium and are deemed to be the final or end stage of succession.
Ecosystems then undergo one of two pathways: cyclical succession or arrested succession.
Cyclical succession
Some ecosystems are subject to cyclical change. This is because some communities are subject to constant changes and so experience cyclical succession. The removal or appearance of species that happens due to natural cycles such as fires or changes in lifestyle stages in animals, causes repeated succession to a climax community.
Cyclical succession can be triggered by events that reoccur, such as the death of old trees, or by wildfires that change the composition of a community, and this can happen in repeated cycles. It can vary in its length depending on the community in question.
For example, the chaparral ecosystem on the California coast is subject to natural wildfires that occur every 30-150 years. Each time this happens a succession of different plant species replace one another, in the same order.