How does ecological succession work

Especially in primary succession, this leads to more stable, less severe environments. At the same time interactions between species of plant tend to intensify competition for basic resources such as water, light, space, and nutrients. Successional change results from the normal complex interactions between organism and environment which lead to changes in overall species composition.

Whether succession is promoted by changing environmental factors or competitive interactions, species composition alters in response to availability of niches. Populations occurring in the community at a point in succession are those able to provide propagules such as seeds to invade the area, being sufficiently tolerant of current environmental conditions, and able to withstand competition from members of other populations present at the same stage Fig.

Species lacking these qualities either become locally extinct or are unable to enter and survive in the community. In some cases, successional sequences may take hundreds of years to complete, and direct observation at a given site is not possible.

Adjacent sites may be identified as successively older stages of the same successional sequence, if it is assumed that conditions were similar when each seral stage was initiated. Early stages of succession tend to be relatively rapid, whereas the rates of species turnover and soil changes become slower as the community matures.

Eventually an approximation to the steady state is established with a relatively stable community, the nature of which has aroused considerable debate. Earlier, the so-called climax vegetation was believed to be determined ultimately by regional climate and, given sufficient time, any community in a region would attain this universal condition.

This unified concept of succession, the monoclimax hypothesis, implies the ability of organisms progressively to modify their environment until it can support the climatic climax community. Although plants and animals do sometimes ameliorate environmental conditions, evidence suggests overwhelmingly that succession has a variety of stable end points. This hypothesis, known as the polyclimax hypothesis, suggests that the end point of a succession depends on a complex of environmental factors that characterize the site, such as parent material, topography, local climate, and human influences.

Actions of the community on the environment, termed autogenic, provide an important driving force promoting successional change, and are typical of primary succession where initial environments are inhospitable. Alternatively, changes in species composition of a community may result from influences external to the community called allogenic.

Whereas intrinsic factors often result in progressive successional changes, that is, changes leading from simple to more complex communities, external allogenic forces may induce retrogressive succession, that is, toward a less mature community.

For example, if a grassland is severely overgrazed by cattle, the most palatable species will disappear. As grazing continues, the grass cover is reduced, and in the open areas weeds characteristic of initial stages of succession may become established.

In some instances of succession, the food web was based on photosynthetic organisms and there was a slow accumulation of organic matter, both living and dead. This is termed autotrophic succession. In other instances, however, addition of organic matter to an ecosystem initiates a succession of decomposer organisms which invade and degrade it. Such a succession is called heterotrophic. See also: Eutrophication ; Productivity ; Food web.

Following the partial or complete destruction of an established community by disturbing events, such as fire or the removal of all trees clearfelling , and similarly on the cessation of grazing or tillage, a sequence of species invasion and replacement ensues. Compared to the slow initial progress of primary succession in which amelioration of the environment plays an important part, secondary succession is characterized initially by rapid turnover of typically opportunist species that invade relatively congenial habitats.

Observed changes in the structure and function of seral communities result from natural selection of individuals within their current environment. Three mechanisms by which species may replace each other have been proposed; the relative importance of each apparently depends on the nature of the successional sequence and stage of development.

The facilitation hypothesis states that invasion of later species depends on conditions created by earlier colonists. Earlier species modify the environment so as to increase the competitive ability of species which are then able to displace them. Succession thus proceeds because of the effects of species on their environment.

The tolerance hypothesis suggests that later successional species tolerate lower levels of resources than earlier occupants and can invade and replace them by reducing resource levels below those tolerated by earlier occupants.

Succession proceeds despite the resistance of earlier colonists. The inhibition hypothesis is that all species resist invasion of competitors and are displaced only by death or by damage from factors other than competition. Succession proceeds toward dominance by longer-lived species. Siskin are partial to birch seeds, and birch also supports large numbers of insects, including aphids , which are themselves an important food source for other wildlife. Succession is a key process in a healthy forest, and given a chance woodland can regenerate itself.

As the vegetation reaches its climax, earlier stages may die out due to shade, but generally speaking, habitats get more diverse over time, and climax woodlands are especially rich ecosystems. In a really old and unmanaged woodland there is a lot of dead and dying wood which is used by a whole host of species including fungi, insects such as the pine hoverfly, and hole nesting birds including the crested tit.

Twinflower and other pinewood plants replace moorland species on the ground layer. Humans have had a huge influence on succession for millennia, and if our influence were removed, woodland would spring up over a lot of the Highlands.

Activities such as eliminating top predators, overgrazing and excessive burning all prevent climax vegetation from developing; this is known as arrested succession. This is why fences and deer control play an important role in encouraging natural regeneration. If the site is too remote for natural regeneration, it can be helpful to make informed predictions about the woodland type that might develop in an area.

By working out what the climax vegetation would be, based on, for example, the existing vegetation and soil conditions, we can choose appropriate species to plant. For example, the presence of heather and blaeberry on mineral soils are good indicators that pine woodland would develop under natural conditions. However, to have really healthy, dynamic ecosystems with all stages of succession present, we would need to establish large areas of forest, ideally with a range of large herbivores and carnivores, so that a wider range of natural processes can take place.

This page has been archived and is no longer updated. This change is due to shifts in the presence and relative abundance of different species as time passes over years to centuries. While succession is most often thought about in terms of the plant community, shifts in the populations of other organisms also need to be considered.

The process of succession can be seen in many different systems, ranging from the establishment of grasslands after a volcanic eruption, to the re-establishment of forests after agricultural fields have been abandoned. Succession is one of the longest-studied ecological concepts. Henry Cowles was the first ecologist to thoroughly characterize successional patterns, which he did in his classic study of sand dunes along the shores of Lake Michigan Cowles Cowles described the chronosequence of vegetation along sand dunes, moving from bare sand beach, to grasslands, to mature forests.

Figure 1: Chronosequences are often used to study succession A Typical chronosequence for sand dune succession. The concept of predictable change in vegetation time was next championed by Frederick Clements in the early s. He proposed the concept of a climax state for communities, which represented the final, or permanent, end-stage of succession Clements For Clements, climax communities were the assemblage of characteristic plants that define an ecosystem, such as tall grasses in a prairie, or mature trees in a forest.

Clements held that, after a disturbance, any given ecosystem would eventually return to its characteristic assemblage of species. For example, if an oak-hickory forest had a severe forest fire which destroyed most of the trees, that forest system would eventually return to the climax community, defined by oak- and hickory-dominated species.

This idea, that an ecosystem could self-form, or self-renew into a stable climax community, became very popular in the s. Figure 2: Two contrasting views of succession A The super-organism concept, where groups of species are tightly associated, and are supplanted by other groups of tightly associated species. B The individualistic concept, where individual species independently respond to environmental conditions. Each curve on the graphs represents the abundance of a single species.

While the concept of a climax community is still viable today, the super-organism concept was opposed by another ecologist, Henry Gleason. Gleason argued that communities were individualistic; that is, communities were only the fortuitous assembly of species, and that there was no such thing as a climax state for ecosystems. Figure 3: Changes over time in total plant species richness over time at select sites on Mount Saint Helens, WA Plant reestablishment 15 years after the debris avalanche at Mount St.

Helens, Washington. For example, species diversity tends to increase with the successional age of an ecosystem. After the eruption of Mount St. Helens in the United States in , ecologists monitoring the return of plant life to the mountain observed a steady increase in species diversity over time Figure 3. Eugene Odum, an ecosystem ecologist, described several predictable differences between early and late successional systems.

For example, early successional systems tend to have smaller plant biomass, shorter plant longevity, faster rates of soil nutrient consumption, a reduced role for decomposer organisms, more open and rapid biogeochemical cycling, higher rates of net primary productivity, lower stability, and lower diversity than late successional systems Odum Similarly, Fakhri Bazzaz characterized early and late successional systems based on the physiology of plants associated with these stages.

Early successional plants tend to have high rates of photosynthesis and respiration, high rates of resource uptake, and high light compensation points, whereas late successional plants often have opposite characteristics Bazzaz Facilitation is the most common mechanism proposed to explain succession.

However, other possible mechanisms included tolerance, inhibition, and random colonization. Since , the glacier filling Glacier Bay has steadily been retreating Figure 4a. Researchers have characterized primary succession in this system, where plant communities progress from pioneer species i.

Both facilitation and inhibition act as mechanisms regulating succession in this system Figure 4b. For example, both Dryas and alders increase soil nitrogen, which increases the establishment and growth of spruce seedlings. However, both Dryas and alders produce leaf litter which can inhibit spruce germination and survival. B Summary of facilitative and inhibitory effects of each successional stage of vegetation on spruce seedling growth.

Figure modified from Chapin et al. A classic study of secondary succession was conducted by Catherine Keever In this study, Keever characterized succession in an old field after agricultural use had ceased.

She observed a predictable shift in plant community composition following field abandonment, with horseweed Erigeron canadense dominating fields one year after abandonment, white aster Aster pilosis dominating in year two, and broomsedge Andropogon virginicus dominating in year three Figure 5. She found that life history strategies of individual species, seed dispersal, allelopathy biochemical production by a plant which alters growth and survival of other plants or itself , and competitive interactions among species, led to this predictable pattern of succession.

Figure 5: Keever's observed pattern of succession in North Carolina agricultural old fields Figured modified from Keever Bazzaz, F. Physiological ecology of plant succession.

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Mechanisms of succession in natural communities and their role in community stability and organization. American Naturalist , Cooper, W. The recent ecological history of Glacier Bay, Alaska: the present vegetation cycle. Ecology 4, Cowles, H. The ecological relations of the vegetation on the sand dunes of Lake Michigan. Botanical Gazette 27 , , , , Gleason, H.