Succession

views updated

Succession

Succession is a fundamental process of ecological change (which is why it is sometimes called ecological succession), involving the progressive replacement of earlier biotic communities with others over time. The concept of succession was first scientifically considered in the nineteenth century. American plant ecolo-gist Frederic Edward Clements (1874−1945) and American botanist and ecologist Henry Chandler Cowles (1869−1939) are recognized as early developers of theories on succession. In fact, the research on Indiana Dunes of Lake Michigan (United States) were instrumental in the early studies of Cowles on succession.

Succession usually begins with the disturbance of a pre-existing ecosystem, followed by recovery. In the absence of further stand-level disturbance, succession culminates in a stable climax community, the nature of which is determined by climate, soil, and the nature of the local biota. Primary succession occurs on bare substrates that have not been previously modified by biological processes. Secondary succession occurs on substrates that have been modified biologically, and it follows disturbances that have not been so intense as to eliminate the regenerative capacity of the vegetation.

Disturbance, stress, and succession

Disturbance is an episodic stress that causes substantial changes in the structure and function of ecosystems. Depending on its severity and extent, disturbance can influence individual organisms or entire stands, disrupting relationships among individuals within the community, and affecting ecological processes such as productivity, nutrient cycling, and decomposition.

Natural disturbances can be caused by intense events of physical disruption associated with, for example, a hurricane, tornado, oceanic tidal wave, earthquake, the blast of a volcanic eruption, or over geological time spans, the advance and retreat of glaciers. Natural disturbances are also associated with wildfire, and with biological events, such as an irruption of defoliating insects that can kill a mature forest. Human activities are also important sources of ecological disturbances, as is the case of agricultural practices such as plowing, the harvesting of trees from forests, construction activities, and explosions associated with military activities. There are numerous other examples of disturbances caused by human activities.

Note that all of these disturbances can operate at various spatial scales. In some cases, they can result in extensive disturbances over very large areas, for example, when a very severe wildfire consumes millions of hectares of forest. In other cases a disturbance may not cause a stand-level mortality, as when a lightning strike kills a single mature tree in a forest, creating a gap in the overstory, and initiating a microsuccession that culminates in occupation of the gap by another mature tree. Scale is a very important aspect of succession.

Once the intense physical, chemical, or biological stress associated with an event of disturbance is relaxed, succession begins, and this process may eventually restore an ecosystem similar to that present prior to the disturbance. However, depending on environmental circumstances, a rather different ecosystem may develop through post-disturbance succession. For example, if climate change has resulted in conditions that are no longer suitable for the establishment and growth of the same species of trees that dominated a particular stand of old-growth forest prior to its disturbance by a wildfire, then succession will result in the development of a forest of a different character.

Succession can also follow the alleviation of a condition of longer-term, chronic environmental stress. For example, if a local source of pollution by poisonous chemicals is cleaned up, succession will occur in response to the decreased exposure to toxic chemicals. This might occur, for example, if emissions of toxic sulfur dioxide and metals from a smelter are reduced or stopped. This abatement of longer-term environmental stress allows pollution-sensitive species to invade the vicinity of the source, so that succession could proceed. Similarly, removal of some or all of a population of cattle that are chronically overgrazing a pasture would allow the plant community to develop to a more advanced successional stage, characterized by greater biomass and possibly more biodiversity.

Succession can also be viewed as a process of much longer-term ecological change occurring, for example, as a lakebasin gradually in-fills with sediment and organic debris, eventually developing into a terrestrial habitat such as a forest. This particular succession, which occurs through the development and replacement of a long series of community types, can take thousands of years to achieve its progression through the aquatic to terrestrial stages. The series of ecological changes is still, however, properly viewed as a successional sequence (or sere).

Primary succession

Primary successions occur after disturbances that have been intense enough to obliterate any living organisms from the site, and even to have wiped out all traces of previous biological influences, such as soil development. Natural disturbances of this intensity are associated with glaciations, lava flows, and very severe wildfires, while human-caused examples might include the abandonment of a paved parking lot, or an above-ground explosion of a nuclear weapon. In all of these cases, organisms must successfully invade the disturbed site for succession to begin.

A well-known study of primary succession after deglaciation has been conducted in Alaska. This research examined a time series of substrates and plant communities of known age (this is called a chronosequence) at various places along a fiord called Glacier Bay. In this study, historical positions of the glacial front could be dated by various means, including written records and old photographs. The primary succession begins as the first plants colonize newly deglaciated sites and, in combination with climatic influences, begin to modify the post-glacial substrate. These initial plants include mosses, lichens, herbaceous dicotyledonous species such as the river-beauty (Epilobium latifolium ), and a nitrogen-fixing cushion plant known as mountain avens (Dryas octopetala ). These pioneers are progressively replaced as succession continues, first by short statured species of willow (Salix spp.), then by taller shrubs such as alder (Alnus crispa, another nitrogen-fixing plant), which dominates the community for about 50 years. The alder is progressively replaced by sitka spruce (Picea sitchensis ), which in turn is succeeded by a forest of western hemlock (Tsuga heterophylla ) and mountain hemlock (T. mertensiana ). The primary succession at Glacier Bay culminates in the hemlock forest—this is the climax stage, dominated by species that are most tolerant of stresses associated with competition, in a mature habitat in which access to resources is almost fully allocated among the dominant biota.

The plant succession at Glacier Bay is accompanied by other ecological changes, such as soil development. The mineral substrate that is initially available for plant colonization after the meltback of glacial ice is a fine till, with a slightly alkaline pH of (on average) about 8%, and as much as 10%, carbonate minerals. As this primary substrate is progressively leached by percolating water and modified by developing vegetation, its acidity increases, reaching pH 5 after about 70 years under a spruce forest, and eventually stabilizing at about pH 4.7 under a mature hemlock forest. The acidification is accompanied by large reductions of calcium concentration, from initial values as large as 10%, to less than 1%, because of leaching and uptake by vegetation. Other important soil changes include large accumulations of organic matter and nitrogen, due to biological fixations of atmospheric carbon dioxide (CO₂) and dinitrogen (N₂).

Other studies of primary succession have examined chronosequences on sand dunes of known age. At certain places on the Great Lakes, sandy beach ridges are being slowly uplifted from beneath the lake waters by a ponderous rebounding of the land. The process of uplifting is called isostasy, and it is still occurring in response to meltback of the enormously heavy glaciers that were present only 10,000 years ago. The initial plant colonists of newly exposed dunes that were studied on Lakes Michigan and Huron are short-lived species, such as sea rocket (Cakile edentula ) and beach spurge (Euphorbia polygonifolia ). These ephemeral plants are quickly replaced by a perennial dune-grass community, dominated by several grasses (Ammophila breviligulata and Calamovilfa longifolia ). With time, a tall-grass prairie develops, dominated by other species of tall grasses and by perennial, dicotyledonous herbs. The prairie is invaded by shade-intolerant species of shrubs and trees, which form nuclei of forest. Eventually a climax forest develops, dominated by several species of oaks (Quercus spp.) and tulip-tree (Liriodendron tulipifera ). This series of successional plant communities is accompanied by soil development, with broad characteristics similar to those observed at Glacier Bay (although, of course, the rates of change are different).

Secondary succession

Secondary succession occurs after disturbances that are not intense enough to kill all plants, so that regeneration can occur by re-sprouting and growth of surviving individuals, and by the germination of preexisting seeds to establish new plants. This regeneration by surviving plants and seeds is supplemented by an aggressive invasion of plant seeds from elsewhere. Another characteristic of secondary succession is that the soil still retains much of its former character, including previous biological and climatic influences on its development.

Secondary successions are much more common than primary successions, because disturbances are rarely intense enough to obliterate previous ecological influences. Most natural disturbances, such as wind-storms, wildfires, and insect defoliations, are followed by ecological recovery through secondary succession. The same is true of most disturbances associated with human activities, such as the abandonment of agricultural lands, and the harvesting of forests.

Secondary succession can be illustrated by an example involving natural regeneration after the clear-cutting of a mature, mixed-species forest in northeastern North America. In this case, the original forest was dominated by a mixture of angiosperm and coniferous tree species, plus various plants that are tolerant of the stressful, shaded conditions beneath a closed forest canopy. Some of the plants of the original community survive the disturbance of clear-cutting, and they immediately begin to regenerate. For example, each cut stump of red maple (Acer rubrum ) rapidly issues hundreds of new sprouts, which grow rapidly, and eventually self-thin to only one to three mature stems after about 50 years. Other species regenerate from a long-lived seed bank, buried in the forest floor and stimulated to germinate by the environmental conditions occurring after disturbance of the forest. Pin cherry (Prunus pensylvanica ) and red raspberry (Rubus strigosus ) are especially effective at this type of regeneration, and these species are prominent during the first several decades of the secondary succession. Some of the original species do not survive the clear-cutting in large numbers, and they must re-invade the developing habitat. This is often the case of coniferous trees, such as red spruce (Picea rubens ). Another group of species is not even present in the community prior to its disturbance, but they quickly invade the site to take advantage of the temporary conditions of resource availability and little competition immediately after disturbance. Examples of these so-called ruderal species are woody plants such as alders (in this case, Alnus rugosa ) and white birch (Betula papyrifera ), and a great richness of herbaceous perennial plants, especially species in the aster family (Asteraceae), such as goldenrod (Solidago canadensis ) and aster (Aster umbellatus ), along with various species of grasses, sedges, and other monocotyledonous plants.

This secondary succession of plant communities is accompanied by a succession of animal communities. For example, the mature forest is dominated by various species of warblers, vireos, thrushes, woodpeckers, flycatchers, and others. After clear-cutting, this avian community is replaced by a community made up of other native species of birds, which specialize in utilizing the young habitats that are available after disturbance. Eventually, as the regenerating forest matures, the bird species of mature forest re-invade the stand, and their community re-assembles after about 30 to 40 years has passed.

Mechanisms of succession

As noted previously, succession generally begins after disturbance creates a situation of great resource availability that can be exploited by organisms, but under conditions of little competition. The classical explanation of the ecological mechanism of community change during succession is the so-called facilitation model. This theory suggests that the recently disturbed situation is first exploited by certain pioneer species that are most capable of reaching and establishing on the site. These initial species modify the site, making it more suitable for invasion by other species, for example, by carrying out the earliest stages of soil development. Once established, the later-successional species eliminate the pioneers through competition. This ecological dynamic proceeds through a progression of stages in which earlier species are eliminated by later species, until the climax stage is reached, and there is no longer any net change in the community.

Another proposed mechanism of succession is the tolerance model. This concept suggests that all species in the succession are capable of establishing on a newly disturbed site, although with varying successes in terms of the rapid attainment of a large population size and biomass. In contrast with predictions of the facilitation model, the early occupants of the site do not change environmental conditions in ways that favor the subsequent invasion of later-successional species. Rather, with increasing time, the various species sort themselves out through their differing tolerances of the successionally increasing intensity of biological stresses associated with competition. In the tolerance model, competition-intolerant species are relatively successful early in succession when site conditions are characterized by a free availability of resources. However, these species are eliminated later on because they are not as competitive as later species, which eventually develop a climax community.

A third suggested mechanism of succession is the inhibition model. As with the tolerance model, both early- and later-successional species can establish populations soon after disturbance. However, some early species make the site less suitable for the development of other species. For example, some plants are known to secrete toxic biochemicals into soil (these are called allelochemicals), which inhibit the establishment and growth of other species. Eventually, however, the inhibitory species die, and this creates opportunities that later-successional species can exploit. These gradual changes eventually culminate in development of the climax community.

All three of these models, facilitation, tolerance, and inhibition, can be supported by selected evidence from the many ecological studies that have been made of succession (especially plant succession). Although these models differ significantly in their predictions about the organizing principles of successional dynamics, it appears that none of them are correct all of the time. Facilitation seems to be most appropriate in explaining changes in many primary successions, but less so for secondary successions, when early

KEY TERMS

Canopy closure —A point in succession where the sky is obscured by tree foliage, when viewed upward from the ground surface.

Chronosequence —A successional series of stands or soils of different age, originating from a similar type of disturbance.

Competition —An interaction between organisms of the same or different species associated with their need for a shared resource that is present in a supply that is smaller than the potential, biological demand.

Cyclic succession —A succession that occurs repeatedly on the landscape, as a result of a disturbance that occurs at regular intervals.

Ruderal —Refers to plants that occur on recently disturbed sites, but only until the intensification of competition related stresses associated with succession eliminates them from the community.

Sere —A successional sequence of communities, occurring under a particular circumstance of types of disturbance, vegetation, and site conditions.

Successional trajectory —The likely sequence of plant and animal communities that is predicted to occur on a site at various times after a particular type of disturbance.

post-disturbance site conditions are suitable for the vigorous growth of most species. The relatively vigorous development of ecological communities during secondary succession means that competition rapidly becomes an organizing force in the community, so there is an intensification of interactions by which organisms interfere with and inhibit each other. The tolerance and inhibition models more readily explain aspects of these interactions. Overall, it appears that successions are idiosyncratic—the importance of the several, potential mechanisms of succession varies depending on environmental conditions, the particular species that are interacting, and the influence of haphazard events, such as which species arrived first, and in what numbers.

Climax—the end point of succession

The climax of succession is a relatively stable community that is in equilibrium with environmental conditions. The climax condition is characterized by slow rates of change in an old-growth community, compared with more dynamic, earlier stages of succession. The climax stage is dominated by species that are highly tolerant of the biological stresses associated with competition, because access to resources is almost completely allocated among the dominant organisms of the climax community. However, it is important to understand that the climax community is not static, because of the dynamics of within-community microsuccession, associated, for example, with gaps in a forest canopy caused by the death of individual trees. Moreover, if events of stand-level disturbance occur relatively frequently, the climax or old-growth condition will not be achieved.

Resources

BOOKS

Mayhew, Peter J. Discovering Evolutionary Ecology: Bringing Together Ecology and Evolution. Oxford, UK, and New York: Oxford University Press, 2006.

Herrel Anthony, Thomas Speck, and Nicolas P. Rowe, eds. Ecology and Biomechanics: A Mechanical Approach to the Ecology of Animals and Plants. Boca Raton, FL: CRC/Taylor & Francis, 2006.

Herrera, Carlos M., and Olle Pellmyr. Plant-animal Interactions: An Evolutionary Approach. Malden, MA: Blackwell Science, 2002.

Smith, Thomas Michael. Elements of Ecology. San Francisco, CA: Benjamin Cummings, 2006.

Stiling, Peter D. Ecology: Theories and Applications. Upper Saddle River, NJ: Prentice Hall, 2002.

Bill Freedman

The Gale Encyclopedia of Science