Ecological succession is a term developed by botanists to describe the change in structure of a community of different species, or ecosystem. The concept of ecological succession arose from a desire to understand how large and complex ecosystems like forests can exist in places known to be recently formed, such as volcanic islands.
An important characteristic of all communities is that their composition and structure constantly change in response to the changing environmental conditions. This change is orderly and sequential, parallel with the changes in the physical environment. These changes lead finally to a community that is in near equilibrium with the environment and that is called a climax community. The gradual and fairly predictable change in the species composition of a given area is called ecological succession. During succession some species colonise an area and their populations become more numerous, whereas populations of other species decline and even disappear.
The entire sequence of communities that successively change in a given area are called sere(s). The individual transitional communities are termed seral stages or seral communities. In the successive seral stages there is a change in the diversity of species of organisms, increase in the number of species and organisms as well as an increase in the total biomass.
The present day communities in the world have come to be because of succession that has occurred over millions of years since life started on earth.
The different types of ecological succession exists during different phases of an ecosystem, and depend on how developed that ecosystem is. In the concept of ecological succession, ecosystems advance until they reach a climax community. In the climax community, all of the resources are efficiently used and the total mass of vegetation maxes out. Many forests that have not been disturbed in many years are examples of a climax community.
Plant succession is two types
(i) Primary succession
It begins in area which is originally bare i.e. without any plants. It could be just water, bare rock, or sand. Thus, the area does not have any existence of vegetation previously. The process of succession begins with the colonization and establishment of pioneer species in/on the bare area.
(ii) Secondary succession
This kind of succession begins in area where the vegetation was present previously but it got destroyed or disturbed. It occurs in areas of forest fires, deforestation, flood, erosions or abandoned agricultural lands.
Based on the causes of succession, it has been categorized as
Allogenic succession – Succession is caused by change in environmental conditions. It changes the composition of the plant community.
Autogenic succession – Succession occurs due to changes in soil structure, composition and activities of plants. The changes in the plant community support growth of few species and causes adverse effects on growth of other.
Succession can be also be categorized as
Progressive succession – It is a type of succession in which sequential development of community occurs. A simple community with few species and low productivity develops into a complex community that contains more species and greater productivity.
Retrogressive succession – It is a kind of succession where the community becomes simplistic and contains fewer species and less biomass over time.
Processes in Succession
Generally the process of plant succession consists of nine steps. The steps include nudation, migration, germination, ecesis, colonization and aggregation, competition, invasion, reaction and stabilization.
An area in which no living form is present is referred as nude area. The process of establishment of a species or primary succession occurs in this area. In this stage, one pioneer species establishes itself. It can be a seed or bacteria or an insect. Pioneer species thrive well in the new habitat. These species are ‘r-selected’ species that are fast growing and well-dispersed. Early succession is therefore dominated by so called ‘r-selected’ species.
The seeds, spores and other structures involved in the propagation process establish in the nude area.
The process involves the germination of seeds or spores on the onset of favorable conditions.
Successful germination of propagules and their establishment in a bare area is known as ecesis. The pioneer species establishes itself in an ecosystem and is able to sustain its basic needs – such as food, water and safe habitat.
- Colonization and Aggregation
In this stage of succession, the individuals of the species increase in number as the result of reproduction. The life form begins to enter into a pattern that allows for a cycle of life to continue in the ecosystem. There is little migration outside of the ecosystem. As succession continues, more species enter the community and begin to alter the environment. These are called ‘k-selected’ species. Life forms are breeding and growing.
- Competition and Co-action
Species compete (inter and intra-specific competition) with each other for resources. The better suited species begin to succeed over the other species. These are superseded by new species. The species with better vigor dominates the area while weaker species get eliminated.
Species from other areas invade come and try to establish spaces left by the elimination of plants due to competition.
The new plants invade the area and affect the growth earlier existing ones. As a result of reaction, becomes unsuitable for the existing community. The replaced by other community.
Finally, a stage is reached in which plant species gets successfully established in an area and gets stabilized with the passage of time. Community is dominated by stable and small number of called climax or the state of equilibrium. The climax community maintains itself in equilibrium with the climate of the area. The climax community has larger size of individuals, complex organization with efficient use of energy and nutrients.
Examples of Ecological Succession
Acadia National Park
Acadia National Park, in Maine, suffered a large wildfire in 1947 of over 10,000 acres. Being nearly 20% of the parks size, many were concerned that the park would be destroyed forever. Restoration efforts were left to nature however, and many think that the choice to not intervene paid off. While the first years were ugly, and only small plants could colonize the burnt soil, many years has led to a great amount of diversity in the tree species. While the trees before the fire were mostly evergreen trees, deciduous forests now dominate the landscape. This example shows how quickly secondary ecological succession can change a landscape.
Coral Reef Ecological Succession
While ecological succession is a term coined by botanist, it also applies heavily to animal population that go through a disruption. Take, for instance, a coral reef. The coral reef as an ecosystem did not just pop into existence, but like many plant communities had to be formed over time through ecological succession. The primary ecological succession in a coral reef is the colonizing of rocks by small coral polyps. These polyps will grow and divide many times to create coral colonies. The shapes and shelter of the coral colonies eventually attract small fish and crustaceans that live in an around the coral. Smaller fish are food for larger fish, and eventually a fully functioning coral reef exists. The principles of ecological succession, while developed in context to plants, exists in all established ecosystems.
Other examples of observable secondary succession include:
- The growth of hardwood trees (including ash, poplar and oak) within the red pine planting area. The consequence of this the increased shading and subsequent mortality of the sun loving red pines by the shade tolerant hardwood seedlings. The shaded forest floor conditions generated by the pines prohibit the growth of sun seedlings and allow the growth of the hardwoods. The consequence of the growth of the hardwoods is the decline and senescence of the pine forest. 2. The raspberry thickets growing in the sun lit forest sections beneath the gaps in the canopy generated by wind require sunlight to grow and thrive. Beneath the dense shade canopy particularly of the red pines but also beneath the dense stands of oaks, there is not sufficient sunlight for the raspberry’s survival. The place in which there has been a tree fall the raspberry canes, dense thickets proliferated. The macro-ecosystem change within the red pine stand and all along the more open sections of the trail has been noted. Within these raspberry thickets, dense growths of hardwood seedlings were found. The raspberry plants are generate seedlings and also protect tree seedlings from being destroyed by young trees. By providing a shaded haven for growth of raspberry plants, future tree canopy was protected.