Succession

In ecology, succession is the process in which the species structure of a biological community changes over a period of time. It is an important ecological phenomenon because it allows a variety of species to occupy a given area through time, often progressing toward a more complex and diverse biological system. As succession proceeds, significant changes occur in species composition, nutrient cycling, energy flow, productivity, and stratification. In some cases this leads to the establishment of a relatively ecologically stable climax community that represents the most complex and diverse biological system possible, given existing environmental conditions and available energy input. While it was once thought that all communities progress toward climax conditions, modern ecologists avoid the assumption that an ideal climax community will emerge in every ecosystem given enough time. Instead, it is widely believed that succession itself occurs in a diverse number of ways even within a given ecosystem, without necessarily reaching equilibrium, especially when seen across different time scales.

Despite controversy over the ultimate result of succession, the process generally follows stages that highlight important concepts in ecology. Immature communities tend to have high populations of a few species that are relatively small and simple. Biomass (weight of living material) is low, and nutrient conservation and retention are poor. Food chains are short, and available energy is shared by few species. Community structure is simple and easily disrupted by external forces. As communities mature, larger and more complex organisms appear, and there is a higher species diversity (number of different species). Biomass increases, and nutrients are retained and cycled within the community. The greater number of species results in more species interactions and the development of complex food webs. Community productivity (conversion of solar energy to chemical energy), initially high in immature communities, becomes balanced by community respiration as more energy is expended in maintenance activities.

Stages of Succession

The entire sequence of communities is called a sere, and each step or community in the sequence is a seral stage. According to traditional model, the climax community is in balance, or equilibrium, with the environment. Although this view has become widely disregarded for most, if not all, ecosystems, mature communities do often display greater stability, more efficient nutrient and energy recycling, a greater number of species, and a more complex community structure than that of each preceding seral stage.

Each seral stage is characterized by its own distinctive forms of plant and animal life, which are adapted to a unique set of chemical, physical, and biological conditions. Excepting the climax community, change is the one constant shared by all seral stages. Changes can be induced by abiotic factors, such as erosion or deposition, and by biotic factors, modification of the environment caused by the activities of living organisms within the community.

These self-induced factors bring about environmental changes detrimental to the existing community but conducive to invasion and replacement by more suitably adapted species. For example, lichens are one of the first colonizers of barren rock outcrops. Their presence acts to trap and hold windblown and water-carried debris, thereby building up a thin soil. As soil depth increases, soil moisture and nutrient content become more optimal for supporting mosses, herbs, and grasses, which replace the lichens. These species continue the process of soil-building and create an environment suitable for woody shrubs and trees.

In time, the trees overtop the shrubs and establish a young forest. These first trees are usually shade-intolerant species. Beneath them, the seeds of the shade-tolerant trees germinate and grow up, eventually replacing the shade-intolerant species. Eventually, what is considered a climax forest community develops on what once was bare rock, and remains relatively stable. However, the climax community itself continues to undergo change, and new species can become dominant whether due to relatively short-scale disruption or through long-term environmental changes.

Primary and Secondary Succession

The sere just described—from barren rock to climax forest—is an example of primary succession. In primary succession, the initial seral stage, or pioneer community, begins on a substrate devoid of life or unaltered by living organisms. Succession that starts in areas where an established community has been disturbed or destroyed by natural forces or by human activities (such as floods, windstorms, fire, logging, and farming) is called secondary succession.

An example of secondary succession occurs on abandoned cropland. This is referred to as old-field succession and begins with the invasion of the abandoned field by annual herbs such as ragweed and crabgrass. These are replaced after one or two years by a mixture of biennial and perennial herbs, and by the third year the perennials dominate. Woody shrubs and trees normally replace the perennials within ten years. After another ten or twenty years have passed, a forest is established, and ultimately, after one or two additional seral stages in which one tree community replaces another, a climax forest emerges.

Both primary and secondary succession begin on sites typically low in nutrients and exposed to extremes in moisture, light intensity, temperature, and other environmental factors. Plants colonizing such sites are tolerant of harsh conditions, are characteristically low-growing and relatively small, and have short life cycles. By moderating the environmental conditions, these species make the area less favorable for themselves and more favorable for plants that are better adapted to the new environment. Such plants are normally long-lived and relatively large. Secondary succession usually proceeds at a faster rate than primary succession, because a well-developed soil and some life are already present.

Aquatic Environments

Succession can also take place in aquatic environments, such as a newly formed pond. The pioneer community consists of microscopic organisms that live in the open water. Upon death, their remains settle on the bottom and join with sediment and organic matter washed into the pond. An accumulation of sediment provides anchorage and nutrients for rooted, submerged aquatic plants such as pondweeds and waterweeds. These add to the buildup of sediment, and as water depth decreases, rooted, floating-leaved species such as water lilies prevent light from reaching the submerged aquatics and eliminate them.

At the water’s edge, emergent plants rooted in the bottom and extending their stems and leaves above water (cattails, rushes, and sedges) trap sediment, add organic matter, and continue the filling-in process. The shallow margins fill first, and eventually the open water disappears and a marsh or bog forms. A soil rich in partially decomposed organic matter and saturated with water accumulates. As drainage improves and the soil becomes raised above the water level, trees and shrubs tolerant of wet soils invade the marsh. These act to lower the water table and improve soil aeration. Trees suited to drier conditions move in, and once again a climax community characteristic of the surrounding area develops.

Influence of Climate

The American ecologist Frederic E. Clements (1874–1945) believed that the characteristics of a climax community were determined solely by regional climate. According to Clements, all communities within a given climatic region, despite initial differences, eventually develop into the same climax community. Some seral stages might be abbreviated or skipped entirely, while others could be lengthened or otherwise modified; however, the end result would always be a single climax community suited to the regional climate. This phenomenon is called convergence, and Clements’s single-climax concept is known as the monoclimax theory.

Most ecologists have found the monoclimax theory to be simplistic and have offered other theories. One of these, the polyclimax theory, holds that, within a given climatic region, there could be many climaxes. It was noted that in any single climatic region, there were often many indefinitely maintained communities that could be considered separate and distinct climaxes. These developed as a result of differences caused by soil type, soil moisture, nutrients, slope, fire, animal activity (grazing and browsing), and other factors. Clements countered that these would eventually reach true climax status if given enough time and proposed terms such as subclimax (a long-lasting seral stage preceding the climax) and disclimax (a nonclimax maintained by continual disturbance) to describe such situations.

A third theory, the climax pattern concept, views the climax as a single large community composed of a mosaic or pattern of climax vegetation instead of many separate climaxes or subclimaxes. Numerous habitat and environmental differences account for the patterns of populations within the climax; no single factor such as climate is responsible. Many ecologists have moved away from the climax concept altogether, suggesting that no ecosystem is ever in true equilibrium. This view has been reinforced by the fact that even seemingly stable factors such as climate can change significantly as the time scale is increased.

Regardless of which theory is accepted, it is clear that climate does play a role in succession. The succession process occurs with different species depending on the broader ecosystem conditions in different places. For this reason, climate change can have a significant impact on succession, and therefore ecological diversity. For example, desertification induced by global warming can disrupt formerly non-desert environments, upsetting the historical pattern of succession in a region. Climate change can also restrict the range of certain species and increase the range of others, introducing new species into ecosystems, where they may compete with climax species. This phenomenon of invasive species can also be directly introduced by humans, who may intentionally or unintentionally bring species into a new ecological community and thereby alter the system. Realization of these and other large-scale disruptive forces is another factor leading many ecologists to move away from the traditional focus on supposed climax communities in succession.

While there is little doubt about the reality of succession in general, it is apparently not a universal phenomenon. For example, disturbed areas within tropical rain forests do not undergo a series of seral stages leading to reestablishment of the climax community. Instead, the climax is established directly by the existing species. Nevertheless, in most regions succession is the mechanism by which highly organized, self-maintained, and ecologically efficient communities are established.

Bibliography

Bazzaz, F. A. Plants in Changing Environments: Linking Physiological, Population, and Community Ecology. New York: Cambridge University Press, 1996. Discusses how disturbance changes the environment, how species function, coexist, and compete for resources, and how species replace one another over time.

Brewer, Richard. The Science of Ecology. Fort Worth, Tex.: Saunders College Publishers, 1994. A clearly written text suitable for high school reference and college use. Includes a chapter on community change and succession. Contains biographical sketches of ecology pioneers Frederic E. Clements and Henry Chandler Cowles. Discusses ecosystem development through the earth’s history. Illustrated with black-and-white photographs and line drawings. Includes glossary, bibliography.

"Ecological Succession." Khan Academy, 2017, www.khanacademy.org/science/biology/ecology/community-structure-and-diversity/a/ecological-succession. Accessed 23 Oct. 2017.

Perry, David A. Forest Ecosystems. Baltimore: Johns Hopkins University Press, 1994. Textbook for students, land managers, scientists, and policymakers. Topics range from the world’s forest types and subdisciplines of ecology to more complex discussions of such things as productivity, succession, nutrient cycling, and stability.

Smith, Robert L. Ecology and Field Biology. 6th ed. San Francisco: Benjamin Cummings, 2001. The chapter “Succession” is a clear and informative treatment of the subject. Good discussion of land use and effects on succession as well as succession of human communities. The latter part of the chapter discusses ecosystem development through time with emphasis on continental drift. Glossary and extensive bibliography. Illustrated with black-and-white photographs and line drawings.