The concept of ecological succession is fundamental to ecological studies. The concept explains how the composition of species (the community) in an area gradually change over time, along with changes in physical characteristics of the environment.
The term first originated in Sweden by a scientist called Hult. However, it became a topic of study only in the early 1900’s with scientists like Fredrick Clements and Henry Cowles interested in understanding community dynamics. The standard definition of ecological succession is-
The occurrence of relatively definite sequence of communities over a period of time in the same area.
Characteristics of ecological succession
Ecological succession has certain interesting ecological features-
- It is directional and orderly. Therefore, it is possible to predict the possible community compositions in the future.
- It occurs due to modification of the environment by the community itself. For example, fungus first colonizes bare rock. This fungus community begins to derive its nutrients from that rock, thereby dissolving it over time. It eventually leads to the formation of a thin layer of soil. This allows other species of plants to colonize the area, and they replace the fungus community.
- Species diversity and biomass continue to increase through each succession stage.
- Net annual yield continues to decrease through each succession stage.
- It culminates in a stabilized ecosystem: single dominant species, maximum possible species diversity, high biomass and low annual yield.
Fun fact: The low annual yield is because the community is in equilibrium with itself and with the environment. Therefore, the energy produced by the ecosystem is consumed within itself. Little is left as surplus.
The stages of ecological succession
The stages of ecological succession can be summarized in 5 steps:
- Competition and Coaction
This is the development of an area without any form of life. This can be a bare area with no life before (like a freshly exhumed rock; leading to primary succession) or it can be an area which had had life before, but was destroyed by external factors (like natural disasters or human activities; leading to secondary succession).
Once this bare area is formed, it is attacked by primitive forms of life that can survive in bare minimum environmental conditions. This can be plant life or animal life (like fungus, bacteria, etc.). They come from different areas. The process is subdivided into three steps-
The organisms migrate from other areas by means of wind, water, animals, etc. Organisms that migrate to begin an ecological succession on bare land are called pioneer species.
This is otherwise called “establishment”. Once pioneer species have reached the area, they set up shop. They adjust with the prevailing conditions and find their ecological niches.
Once the pioneer species is established, it reproduces like crazy. Many, many offsprings are formed and they begin to aggregate and form small colonies in this area.
All this while, the environmental factors like soil, water, temperature, humidity, etc are being influenced by the species living there. As this modification continues, new species begin to arrive and they form a community…
Competition and Coaction
Once a community is formed, the various organisms in the community begin to fight for common resources of raw material and shelter. The competition is both among different species and also between individuals of the same species. Think of it this way: if you are given a tasty piece of sausage to eat when your dog and your siblings are around, what would happen?
Organisms begin to affect each other’s way of life, both directly and indirectly. Primitive food chains with two or three trophic levels begin to form. This process is called coaction.
This is the time when the most influential changes in the environment takes place due to action of the community living in the area. Due to this modification, the area eventually becomes unsuitable for the existing community. For example, fungus cannot live on well developed soil. Therefore, the community is replaced by the next community that can live there stably.
Each community that takes its place in the area is called a seral community or seral stage.
Finally, a time comes where the community existing in the place reaches equilibrium with the area and the conditions there. It remains in the area for much longer than other communities before it. This community, is not replaced and is called climax community.
Where does ecological succession occur?
The process of succession is found in every place on Earth that supports life. They occur in water bodies (called hydrarch succession), they occur in dry regions like deserts (called xerarch succession), and they occur on sand (called psammosere succession). There is a general trend in successional processes to reach the “middle path”; they go from extreme to moderate conditions.
However, in nature, communities rarely get a chance to reach the climax stage. This is because there are always environmental disturbances that reverts the succession back to previous stages. If, say, a flood occurred during a hydrarch succession, then the life forms that worked so hard to leave the water and enter the land habitat would be submerged back into the water. The community would go back many seral stages.
In fact, it is also possible that certain organisms in the community can have a negative influence on the entire community and the succession process becomes reversed. Pathogens and parasites can have this effect, especially if the succession is still in its early stages. This is called retrogressive succession.
Therefore, the process keeps going back and forth, always changing. This entire concept of succession has been successfully used to explain what is called community dynamics in an ecosystem. It has many other applications as well; in landscape studies, in restoration studies, etc.