Ecological succession
Ecological succession is the process of change in the species that make up an ecological community over time.
The process of succession occurs either after the initial
Succession was among the first theories advanced in
History
Precursors of the idea of ecological succession go back to the beginning of the 19th century. As early as 1742 French naturalist Buffon noted that poplars precede oaks and beeches in the natural evolution of a forest. Buffon was later forced by the theological committee at the University of Paris to recant many of his ideas because they contradicted the biblical narrative of Creation.[6]
Swiss geologist
H. C. Cowles
Gleason and Clements
From about 1900 to 1960, however, understanding of succession was dominated by the theories of
Two quotes illustrate the contrasting views of Clements and Gleason. Clements wrote in 1916:
The developmental study of vegetation necessarily rests upon the assumption that the unit or climax formation is an organic entity. As an organism the formation arises, grows, matures, and dies. Furthermore, each climax formation is able to reproduce itself, repeating with essential fidelity the stages of its development.
— Frederic Clements[14]
while Gleason, in his 1926 paper, said:
An association is not an organism, scarcely even a vegetational unit, but merely a coincidence.
— Henry Gleason[15]
Gleason's ideas were, in fact, more consistent with Cowles' original thinking about succession. About Clements' distinction between primary succession and secondary succession, Cowles wrote (1911):
This classification seems not to be of fundamental value, since it separates such closely related phenomena as those of erosion and deposition, and it places together such unlike things as human agencies and the subsidence of land.
— Henry Cowles[16]
Eugene Odum
In 1969, Eugene Odum published The Strategy of Ecosystem Development, a paper that was highly influential to conservation and environmental restoration. Odum argued that ecological succession was an orderly progression toward a climax state where “maximum biomass and symbiotic function between organisms are maintained per unit energy flow."[17] Odum highlighted how succession was not merely a change in the species composition of an ecosystem, but also created change in more complex attributes of the ecosystem, such as structure and nutrient cycling.[18]
Modern era
A more rigorous, data-driven testing of successional models and community theory generally began with the work of
More recent definitions of succession highlight change as the central characteristic.[17] New research techniques are greatly enhancing contemporary scientists' ability to study succession, which is now seen as neither entirely random nor entirely predictable.[18]
Factors
Ecological succession was formerly seen as having a stable end-stage called the
The development of some ecosystem attributes, such as
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
Though the idea of a fixed, predictable process of succession with a single well-defined climax is an overly simplified model, several predictions made by the classical model are accurate. Species diversity, overall plant biomass, plant lifespans, the importance of decomposer organisms, and overall stability all increase as a community approaches a climax state, while the rate at which soil nutrients are consumed, rate of biogeochemical cycling, and rate of net primary productivity all decrease as a community approaches a climax state.[23]
Communities in early succession will be dominated by fast-growing, well-
Some of these trends do not apply in all cases. For example, species diversity almost necessarily increases during early succession as new species arrive, but may decline in later succession as competition eliminates opportunistic species and leads to dominance by locally superior competitors. Net Primary Productivity, biomass, and trophic properties all show variable patterns over succession, depending on the particular system and site.
Types
Primary succession
Successional dynamics beginning with colonization of an area that has not been previously occupied by an ecological community are referred to as primary succession.[1] This includes newly exposed rock or sand surfaces, lava flows, and newly exposed glacial tills.[1] The stages of primary succession include pioneer microorganisms,[24] plants (lichens and mosses), grassy stage, smaller shrubs, and trees. Animals begin to return when there is food there for them to eat. When it is a fully functioning ecosystem, it has reached the climax community stage.[25]
Secondary succession
Secondary succession follows severe disturbance or removal of a preexisting community that has remnants of the previous ecosystem.[1] Secondary succession is strongly influenced by pre-disturbance conditions such as soil development, seed banks, remaining organic matter, and residual living organisms.[1] Because of residual fertility and preexisting organisms, community change in early stages of secondary succession can be relatively rapid.[1]
Secondary succession is much more commonly observed and studied than primary succession. Particularly common types of secondary succession include responses to natural disturbances such as fire, flood, and severe winds, and to human-caused disturbances such as logging and agriculture. In secondary succession, the soils and organisms need to be left unharmed so there is a way for the new material to rebuild.[9]
As an example, in a fragmented old field habitat created in eastern Kansas, woody plants "colonized more rapidly (per unit area) on large and nearby patches".[26]
Secondary succession can quickly change a landscape. In the 1900s, Acadia National Park had a wildfire that destroyed much of the landscape. Originally evergreen trees grew in the landscape. After the fire, the area took at least a year to grow shrubs. Eventually, deciduous trees started to grow instead of evergreens.[25]
Secondary succession has been occurring in
Seasonal and cyclic dynamics
Unlike secondary succession, these types of vegetation change are not dependent on disturbance but are periodic changes arising from fluctuating species interactions or recurring events. These models modify the climax concept towards one of dynamic states.
Causes of plant succession
Autogenic succession can be brought by changes in the soil caused by the organisms there. These changes include accumulation of organic matter in litter or humic layer, alteration of soil nutrients, or change in the pH of soil due to the plants growing there. The structure of the plants themselves can also alter the community.[28] For example, when larger species like trees mature, they produce shade on to the developing forest floor that tends to exclude light-requiring species. Shade-tolerant species will invade the area.
Allogenic succession is caused by external environmental influences and not by the vegetation. For example, soil changes due to erosion, leaching or the deposition of silt and clays can alter the nutrient content and water relationships in the ecosystems. Animals also play an important role in allogenic changes as they are pollinators, seed dispersers and herbivores. They can also increase nutrient content of the soil in certain areas, or shift soil about (as termites, ants, and moles do) creating patches in the habitat. This may create regeneration sites that favor certain species.
Climatic factors may be very important, but on a much longer time-scale than any other. Changes in temperature and rainfall patterns will promote changes in communities. As the climate warmed at the end of each ice age, great successional changes took place. The tundra vegetation and bare glacial till deposits underwent succession to mixed deciduous forest. The greenhouse effect resulting in increase in temperature is likely to bring profound Allogenic changes in the next century. Geological and climatic catastrophes such as volcanic eruptions, earthquakes, avalanches, meteors, floods, fires, and high wind also bring allogenic changes.
Mechanisms
In 1916,
- Nudation: Succession begins with the development of a bare site, called Nudation (disturbance).[14]
- Migration: refers to arrival of propagules.[14]
- Ecesis: involves establishment and initial growth of vegetation.[14]
- Competition: as vegetation becomes well established, grows, and spreads, various species begin to compete for space, light and nutrients.[14]
- Reaction: during this phase autogenic changes such as the buildup of humus affect the habitat, and one plant community replaces another.[14]
- Stabilization: a supposedly stable climax community forms.[14]
Seral communities
A seral community is an intermediate stage found in an ecosystem advancing towards its
Changes in animal life
Succession theory was developed primarily by botanists. The study of succession applied to whole
Animal life also exhibits changes with changing communities. In the lichen stage, fauna is sparse. It comprises a few mites, ants, and spiders living in cracks and crevices. The fauna undergoes a qualitative increase during the herb grass stage. The animals found during this stage include nematodes, insect larvae, ants, spiders, mites, etc. The animal population increases and diversifies with the development of the forest climax community. The fauna consists of invertebrates like slugs, snails, worms, millipedes, centipedes, ants, bugs; and vertebrates such as squirrels, foxes, mice, moles, snakes, various birds, salamanders and frogs.
Microsuccession
Succession of
Climax concept
According to classical
Climax community
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.[1] 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 productionand 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 compositionmaintains equilibrium.
- It is an index of the climate of the area. The life or growth forms indicate the climatic type.
Types of climax
- Climatic Climax
- If there is only a single climax and the development of climax community is controlled by the climate of the region, it is termed as climatic climax. For example, development of Maple-beech climax community over moist soil. Climatic climax is theoretical and develops where physical conditions of the substrate are not so extreme as to modify the effects of the prevailing regional climate.
- 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 (disturbance climax) or anthropogenic subclimax (man-generated). 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 (1916) and recognizes only one climax whose characteristics are determined solely by climate (climatic climax). 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 (1935). 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 (1953). 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.
The theory of
Succession by habitat type
Forest succession
Forests, being an ecological system, are subject to the species succession process.
An example of pioneer species, in forests of northeastern North America are Betula papyrifera (
Things in nature are not black and white, and there are intermediate stages. It is therefore normal that between the two extremes of light and shade there is a gradient, and there are species that may act as pioneer or tolerant, depending on the circumstances. It is of paramount importance to know the tolerance of species in order to practice an effective silviculture.
Wetland succession
Since many types of wetland environments exist, succession may follow a wide array of trajectories and patterns in wetlands. Under the classical model, the process of secondary succession holds that a wetland progresses over time from an initial state of open water with few plants, to a forested climax state where decayed organic matter has built up over time, forming peat. However, many wetlands are maintained by regular disturbance or natural processes at an equilibrium state that does not resemble the predicted forested "climax."[35] The idea that ponds and wetlands gradually fill in to become dry land has been criticized and called into question due to lack of evidence.[5]
Wetland succession is a uniquely complex, non-linear process shaped by
Grassland succession
For a long time, grasslands were thought to be early stages of succession, dominated by weedy species and with little conservation value. However, comparing grasslands that form after recovery from long-term disruptions like agricultural tillage with ancient or "old-growth" grasslands has shown that grasslands are not inherently early-successional communities. Rather, grasslands undergo a centuries-long process of succession, and a grassland that is tilled up for agriculture or otherwise destroyed is estimated to take a minimum of 100 years, and potentially on average 1,400 years, to recover to its previous level of biodiversity.[37] However, planting a high diversity of late-successional grassland species in a disturbed environment can accelerate the recovery of the soil's ability to sequester carbon, resulting in twice as much carbon storage as a naturally recovering grassland over the same period of time.[38]
Many grassland ecosystems are maintained by disturbance, such as fire and grazing by large animals, or else the process of succession will change them to forest or shrubland. In fact, it is debated whether fire should be considered disturbance at all for the North American prairie ecosystems, since it maintains, rather than disrupts, an equilibrium state.[39] Many late-successional grassland species have adaptations that allow them to store nutrients underground and re-sprout rapidly after "aboveground" disturbances like fire or grazing. Disturbance events that severely disrupt or destroy the soil, such as tilling, eliminate these late-successional species, reverting the grassland to an early successional stage dominated by pioneers, whereas fire and grazing benefit late-successional species.[37] Both too much and too little disturbance can damage the biodiversity of disturbance-dependent ecosystems like grasslands.[40]
In North American semi-arid grasslands, the introduction of livestock ranching and absence of fire was observed to cause a transition away from grasses to woody vegetation, particularly mesquite.[41] However, the means by which ecological succession under frequent disturbance results in ecosystems of the sort seen in remnant prairies is poorly understood.[42][40]
See also
- Connell–Slatyer model of ecological succession
- Cyclic succession
- Ecological stability
- Intermediate disturbance hypothesis
References
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- ^ a b Budowski G (1965). "Distribution of tropical American rain-forest species in the light of successional processes". Turrialba. 15 (1): 40–42.
- ^ a b c Moseley, Kendra. "Wetland Ecology- Basic Principles" (PDF). United States Department of Agriculture.
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- ^ Evans, E.W.; Briggs, J.M.; Finck, E.J.; Gibson, D.J.; James, S.W.; Kaufman, D.W.; Seastedt, T.R. "Is Fire a Disturbance in Grasslands?" (PDF).
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Further reading
- S2CID 3587878.
- Frouz J, Prach K, Pižl V, Háněl L, Starý J, Tajovský K, et al. (2008). "Interactions between soil development, vegetation and soil fauna during spontaneous succession in post mining sites". European Journal of Soil Biology. 44: 109–121. .
External links
- Science Aid: Succession Explanation of succession for high school students.
- Biographical sketch of Henry Chandler Cowles.
- Robbert Murphy sees a significantly ideological, rather than scientific, basis for the disfavour shown towards succession by the current ecological orthodoxy and seeks to reinstate succession by holistic and teleological argument.