Ecological succession
Ecological succession is the process of how species compositions change in an ecological community over time.
The two main categories of ecological succession are primary succession and secondary succession. Primary succession occurs after the initial colonization of a newly created habitat with no living organisms. Secondary succession occurs after a disturbance such as fire, habitat destruction, or a natural disaster destroys a pre-existing community.
Both consistent patterns and variability are observed in ecological succession. Theories of ecological succession identify different factors that help explain why plant communities change the way they do.
Succession was among the first theories advanced in ecology. Ecological succession was first documented in the Indiana Dunes of Northwest Indiana by Henry Chandler Cowles during the late 19th century and remains a main ecological topic of study. Over time, the understanding of succession has changed to include a more complex cyclical model that argues organisms do not have fixed roles or relationships. Ecologists and conservationists have since used the theory of succession to aid in developing ecological restoration strategies.
Stages of succession
Mechanisms
In 1916, Frederic Clements published a descriptive theory of succession and advanced it as a general ecological concept.The process of succession occurs over multiple stages.- Nudation refers to a bare area that has little to no life forms or organic matter. In secondary succession, nudation occurs after a disturbance.
- Migration refers to arrival of propagules.
- Ecesis involves establishment and initial growth of vegetation.
- Competition occurs as vegetation becomes well established, grows, and spreads, various species begin to compete for space, light and nutrients.
- Reaction refers to the phase where autogenic changes such as the buildup of humus affect the habitat, and one plant community replaces another.
- Stabilization occurs when a supposedly stable climax community forms.
Seral community
Climax community
According to classical ecological theory, succession stops when the sere has arrived at an equilibrium or steady state with the physical and biotic environment. Barring major disturbances, it will persist indefinitely. This end point of succession is called climax.The final or stable community in a sere is the climax community or climatic vegetation. It is self-perpetuating and in equilibrium with the physical habitat. There is no net annual accumulation of organic matter in a climax community. The annual production and use of energy is balanced in such a community.
Characteristics
- The vegetation is tolerant of environmental conditions.
- It has a wide diversity of species, a well-drained spatial structure, and complex food chains.
- The climax ecosystem is balanced. There is equilibrium between gross primary production and total respiration, between energy used from sunlight and energy released by decomposition, between uptake of nutrients from the soil and the return of nutrient by litter fall to the soil.
- Individuals in the climax stage are replaced by others of the same kind. Thus the species composition maintains equilibrium.
- It is an index of the climate of the area. The life or growth forms indicate the climatic type.
Types of climax
; Edaphic Climax: When there are more than one climax communities in the region, modified by local conditions of the substrate such as soil moisture, soil nutrients, topography, slope exposure, fire, and animal activity, it is called edaphic climax. Succession ends in an edaphic climax where topography, soil, water, fire, or other disturbances are such that a climatic climax cannot develop.
; Catastrophic Climax: Climax vegetation vulnerable to a catastrophic event such as a wildfire. For example, in California, chaparral vegetation is the final vegetation. The wildfire removes the mature vegetation and decomposers. A rapid development of herbaceous vegetation follows until the shrub dominance is re-established. This is known as catastrophic climax.
; Disclimax: When a stable community, which is not the climatic or edaphic climax for the given site, is maintained by man or his domestic animals, it is designated as Disclimax or anthropogenic subclimax. For example, overgrazing by stock may produce a desert community of bushes and cacti where the local climate actually would allow grassland to maintain itself.
; Subclimax: The prolonged stage in succession just preceding the climatic climax is subclimax.
; Preclimax and Postclimax: In certain areas different climax communities develop under similar climatic conditions. If the community has life forms lower than those in the expected climatic climax, it is called preclimax; a community that has life forms higher than those in the expected climatic climax is postclimax. Preclimax strips develop in less moist and hotter areas, whereas Postclimax strands develop in more moist and cooler areas than that of surrounding climate.
Theories
There are three schools of interpretations explaining the climax concept:- Monoclimax or Climatic Climax Theory was advanced by Clements and recognizes only one climax whose characteristics are determined solely by climate. The processes of succession and modification of environment overcome the effects of differences in topography, parent material of the soil, and other factors. The whole area would be covered with uniform plant community. Communities other than the climax are related to it, and are recognized as subclimax, postclimax and disclimax.
- Polyclimax Theory was advanced by Tansley. It proposes that the climax vegetation of a region consists of more than one vegetation climaxes controlled by soil moisture, soil nutrients, topography, slope exposure, fire, and animal activity.
- Climax Pattern Theory was proposed by Whittaker. The climax pattern theory recognizes a variety of climaxes governed by responses of species populations to biotic and abiotic conditions. According to this theory the total environment of the ecosystem determines the composition, species structure, and balance of a climax community. The environment includes the species' responses to moisture, temperature, and nutrients, their biotic relationships, availability of flora and fauna to colonize the area, chance dispersal of seeds and animals, soils, climate, and disturbance such as fire and wind. The nature of climax vegetation will change as the environment changes. The climax community represents a pattern of populations that corresponds to and changes with the pattern of environment. The central and most widespread community is the climatic climax.
Factors
Diversity of possible trajectories
Ecological succession was formerly seen as an orderly progression through distinct stages, where several plant communities would replace each other in a fixed order and eventually reach a stable end point known as the climax. The climax community was sometimes referred to as the 'potential vegetation' of a site, and thought to be primarily determined by the local climate. This idea has been largely abandoned by modern ecologists in favor of nonequilibrium ideas of ecosystems dynamics. Most natural ecosystems experience disturbance at a rate that makes a "climax" community unattainable. Climate change often occurs at a rate and frequency sufficient to prevent arrival at a climax state.The trajectory of successional change can be influenced by initial site conditions, by the type of disturbance that triggers succession, by the interactions of the species present, and by more random factors such as availability of colonists or seeds or weather conditions at the time of disturbance. Some aspects of succession are broadly predictable; others may proceed more unpredictably than in the classical view of ecological succession. Coupled with the stochastic nature of disturbance events and other long-term changes, such dynamics make it doubtful whether the 'climax' concept ever applies or is particularly useful in considering actual vegetation.