Ecological succession is the slow and gradual process by which one type of community gradually replaces another due to changes in environmental conditions until a stable community, known as the climax community, is established.
It involves a series of changes in community structure and species composition. Typically, these changes correspond with shifts in the physical conditions of the environment.
For instance, when wetlands are drained, they gradually transform into drier areas like grasslands. The aquatic or semi-aquatic organisms begin to die out, and species better suited to drier conditions take over. Over time, these grasslands can evolve into shrublands and eventually woodlands as new species invade.
Ecological succession is a gradual and continuous process that does not cease until a stable community is formed. This ultimate stable community is referred to as the climax community. The stages of vegetation or communities formed during ecological succession are known as seres or seral stages.
Different Types of Ecological Succession Based on Different Criteria
- Primary Succession: This occurs when an area in any basic environment (terrestrial, freshwater, or marine) is colonized by organisms for the first time. Primary succession begins on a sterile area, such as newly exposed rock or a sand dune, where the initial conditions are not favorable for life.
- Secondary Succession: This happens in areas where the previous plant community has been cleared by some event (such as burning, grazing, tree felling, or a sudden change in climate). Secondary succession typically progresses faster than primary succession due to better nutrient availability and other favorable conditions from the previous plant cover.
- Autogenic Succession: In most cases, once ecological succession begins, the community itself modifies the environment through its interactions, leading to its own replacement by new communities. This process is known as autogenic succession.
- Allogenic Succession: In some instances, the replacement of one community by another is mainly due to external forces rather than the community’s impact on the environment. Allogenic succession can occur in highly disturbed or eroded areas or in ponds where external nutrients and pollutants modify the environment and the resident communities.
- Autotrophic Succession: This type of succession is marked by the early and continued dominance of autotrophic organisms, such as green plants. It starts in a predominantly inorganic environment, with a sustained energy flow that gradually increases the organic matter content.
- Heterotrophic Succession: This ecological succession is characterized by the early dominance of heterotrophic organisms, such as bacteria, actinomycetes, fungi, and animals. It begins in an organic matter-rich medium, such as small river or stream areas heavily polluted with sewage or small pools receiving large amounts of leaf litter.
- Induced Succession: Human activities like overgrazing, frequent scraping, shifting cultivation, or industrial pollution can degrade ecosystems. Agricultural practices often lead to the retrogression of a stable ecosystem to a younger state due to deliberate human actions.
- Retrogressive Succession: This is a return to a simpler, less dense, or impoverished community from an advanced or climax community. Usually caused by external forces, such as severe grazing by animals from surrounding villages or excessive removal of wood and leaf litter, leading to the degradation of natural forests into shrubs, savannas, or desert-like stands.
- Cyclic Succession: This type of succession occurs locally within a larger community. Cyclic succession involves the repeated occurrence of certain stages of ecological succession whenever open conditions are created within a large community.
Depending primarily on the environment where ecological succession begins, it can be categorized into the following types;
- Hydrosere: This occurs in regions with abundant water, such as ponds, lakes, streams, swamps, and bogs.
- Mesosere: This takes place in areas with adequate moisture conditions.
- Xerosere: This happens in environments with minimal moisture, such as deserts and rocky areas.
- Lithosere: Ecological succession that starts on rocks.
- Psammosere: Ecological succession that begins on sand.
- Halosere: Ecological succession that occurs in saline water or saline soil.
Causes of Ecological Succession
- The primary cause of ecological succession is environmental changes. Thus, changes in succession typically reflect changes in the environment.
- Sometimes, changes in vegetation or communities occur due to the effects of the vegetation itself, known as autogenic succession.
- When ecological succession is driven by external environmental factors, it is called allogenic succession.
- Autogenic succession can result from changes in organic matter, nutrients, or soil pH caused by the plants. For example, larger trees may provide shade that inhibits the growth of light-dependent smaller plants, leading to their replacement by species that require less light. These conditions are created by the vegetation, thus resulting in autogenic succession.
- Allogenic succession is caused by external factors such as animal influences, climatic changes (e.g., rainfall, droughts, extreme temperatures), or alterations in soil conditions due to erosion, landslides, or silt accumulation. These factors lead to large-scale ecological succession.
General Process of Ecological Succession
Nudation
Nudation is the formation of a bare area without any life forms. Exposure of a new surface can result from various causes, such as landslides, erosion, deposition, or other catastrophic events. These causes of nudation are categorized into three main types:
- Topographic: Soil erosion by gravity, water, or wind can lead to the disappearance of the existing community. Other topographic causes include sand deposition, landslides, volcanic activity, and similar factors.
- Climatic: Glaciers, dry periods, hailstorms, frost, and fires can also destroy communities.
- Biotic: Human activities are a significant biotic factor, leading to the destruction of forests and grasslands for industry, agriculture, and housing. Other factors include disease epidemics caused by fungi, viruses, and other pathogens that can wipe out entire populations.
Invasion
This is the successful establishment of a species in a bare area. The species arrive at this new site from another area. Invasion involves three steps:
- Migration (Dispersal): Seeds, spores, or other propagules of the species reach the bare area, usually carried by air, water, etc.
- Ecesis (Establishment): After reaching the new area, the species adjust to the prevailing conditions, establishing themselves successfully. In plants, seeds or propagules germinate, seedlings grow, and adults start to reproduce.
- Aggregation: Colonization by successive offspring and new migrants increases the population, a process called aggregation. Pioneers are the first plants or autotrophic organisms to colonize and aggregate. Pioneer communities are typically dynamic, have low nutrient requirements, and can utilize minerals in complex forms. They are small and make fewer demands on the environment.
Competition and Coaction
With the aggregation of many individuals of the species in a limited space, competition (both interspecific and intraspecific) for space and nutrition develops. The interaction between individuals of a species, called coaction, affects each other’s survival. Species that fail to compete with others are eventually eliminated. Reproductive capacity and wide ecological amplitude help species withstand competition.
Reaction
This stage involves the modification of the environment by living organisms. Significant changes occur in soil, water, light conditions, temperature, etc., due to the influence of the organisms. As a result, the environment becomes unsuitable for the existing community, which is eventually replaced by another community (seral community).
The entire sequence of communities that replaces one another in a given area is called a sere, with different communities constituting the sere known as seral communities, seral stages, or developmental stages.
Stabilization (Climax)
The final stage in the process is when the terminal community becomes established for an extended period and maintains equilibrium with the area’s climate. This final community is not replaced and is known as the climax community, with the stage referred to as the climax stage.
Concept (Theories) of Climax
The concept of climax has long been a topic of controversy and discussion. Following are three main theoretical approaches to understanding climax:
Mono-Climax Theory
Developed largely by Frederick Clements, this theory posits that there is only one climax community in a region, determined solely by the climate, regardless of the initial variety of environmental conditions. All seral communities in a region, given enough time, would converge to this single climax.
The landscape would be dominated by a uniform plant and animal community. Other communities are seen as stages in ecological successional development related to the climax, such as subclimax, disclimax, preclimax, and postclimax:
- Subclimax: A stage just before the climatic climax community.
- Disclimax: A type of vegetation maintained by recurrent disturbances, mainly biotic, preventing the establishment of the climatic climax community.
- Preclimax: Vegetation of lower life forms than the adjacent community, resulting from different soil conditions.
- Postclimax: A strip of higher life forms within a climatic climax, such as a forest along a stream in a grassland community.
Numerous other terms like coclimax, superclimax, quasiclimax, anticlimax, and pseudoclimax were later introduced by post-Clements ecologists to describe specific situations. This theory is supported by Cowles, Ranganathan, and Puri but strongly opposed by Daubenmire (1968).
Poly-Climax Theory
This theory was developed by Tansley, it argues that the climax vegetation of a region consists of a mosaic of vegetational climaxes controlled by factors such as soil moisture, soil nutrients, topography, slope exposure, fire, and animal activity. Each stable community is considered a climax and described with a prefix, such as edaphic climax, topographic climax, biotic climax (zootic, grazing, or anthropogenic), and fire climax.
For instance, grassland communities in central India, Sri Lanka, and parts of California are considered biotic climaxes due to the influence of fire, grazing, and other biotic factors, as noted by Misra, Pandeya, and Holmes.
Climax Pattern Theory
This theory was put forward by Whittaker, MacIntosh, and Sellack, it suggests that the composition, species structure, and balance of a climax community are determined by the total environment of the ecosystem, not just by climate. Factors include species characteristics, biotic interrelationships, availability of flora and fauna, chance dispersal of seeds and animals, and soil and climate conditions.
The climax vegetation pattern changes with environmental changes, forming a gradient of communities, or ecoclines. The most widespread community in this pattern, the prevailing or climatic climax, most clearly reflects the area’s climate.
Information Theory
This theory was proposed by Leith, Odum, and Golley, this theory considers ecological succession and climax in terms of ecosystem development.
In autotrophic succession (ecosystem development), species diversity increases with an increase in organic matter and biomass supported by available energy. In a climax community, the available energy and biomass, or information content, increase.
In contrast, heterotrophic succession involves a gradual energy depletion, as respiration rates exceed production rates.
In an ecosystem, both autotrophic and heterotrophic successions operate coordinately. Autotrophs derive mineral elements from the soil and atmosphere, while heterotrophs return nutrients through the decomposition of dead organic matter.
