Trends in Ecological Succession

Ecological succession is defined as a series of transitions in species composition over ecological time. The process of succession is termed primary succession when it involves establishment of a community in a newly-formed habitat where even soil does not exist, such as on a volcanic island or on the rubble left behind by a retreating glacier - imagine Long Island just after the last glaciers receded - the process of a community of organisms becoming established on the pile of bare rock that formed the island would be an example of primary succession.

Secondary succession occurs when an existing community has been cleared by some disturbance that leaves the soil intact and a new community gradually develops in the area opened by the disturbance. The re-growth of plant communities following forest fires is one example of secondary succession. The development of a community of species following human disturbances like construction or logging would be another good example.

Some common trends in succession:

1. The species composition changes continuously during the succession, but change is usually more rapid in the earlier stages than in the later ones.
2. The total number of species represented increases initially, then sometimes declines slightly, and finally becomes more or less stabilized in the older stages. This trend applies particularly to the heterotrophs, whose variety is usually much greater in the later stages of the succession.
3. Net primary productivity (the amount of energy converted into products of photosynthesis by autotrophs, and available to heteroptrophs) increases until it reaches a stable high level.
   

   Change in net primary productivity during plant succession on an area cleared of an oak-pine forest in Brookhaven, New York. The first rise represents the invasion of the area by herbs. The later rise (after ~14 years) reflects the entrance of larger woody plants into the community.

4. The store of inorganic nutrients held in the organisms and soil of the ecosystem increases, and an increasing proportion of this store is held in the tissue of plants.
5. Both the total biomass in the ecosystem and the amount of nonliving organic matter increase during the succession until a more stable stage is reached.
6. The height and massiveness of the plants in the community increase and lead to greater differentiation of vertical strata.
7. The food webs become more complex, and the relations between species in them become better defined or more specialized. As a result, the efficiency of resource utilization at the various levels usually rises.

Pond succession. Sediments washed from the surrounding land begin to fill the pond, and the dead bodies of planktonic organisms add organic material. Soon pioneer submerged vascular plants appear in the shallower water near the margins of the pond. Their roots hold the silt, and the pond bottom is built up faster where they are growing. In addition, as these plants die, their bodies accumulate faster than decomposers can break them down. Soon the water is shallow enough for broad-leaved floating pondweeds to displace the submerged species, which now become established in a zone farther out in the pond, where conditions are more favorable for them. But as the bottom continues to build up, the floating pondweeds are in their turn diplaced by emergent species (plants that have their roots in the mud of the bottom, but their shoots extending into the air above the water), such as cattails, bulrushes, and reeds. These plants grow very close together and hold the sediment tightly, and their great bulk results in rapid accumulation of organic material. Soon conditions are dry enough for a few terrestrial plants to gain a foothold. Now an area that was formerly part of the pond is newly formed dry land. This entire sequence can sometimes be seen as a nearly continuous series of zones girdling a pond or lake. With the passage of the years, the pond becomes smaller and smaller as the zones move nearer and nearer its center. Eventually nothing of the pond remains.

How will these fields change if the farmer stops farming them? Hay moved and rolled after the wheat harvest in Western Australia.

Succession in abandoned croplands and harvested forests. Since the effects of previous communities are usually not wholly erased in these situations and the physical conditions are more favorable for life than a beach or bare rock surface, secondary succession's initial stages often proceed relatively quickly in these environments. An abandoned cornfield, for example, will be covered with annual weeds (ragweed, horseweed, crabgrass) during the very first year. In the next year ragweed, goldenrod, asters, and various grasses will become common. Grass will dominate for several years and then shrubs and seedlings will appear. Pines, which grow well in the unshaded fields, will be the first tree seedlings to establish themselves and eventually a pine forest will replace the grass and shrubs. Pine seedlinds cannot grow well in the shade of older pines though, so oak, hickory, and other shade-tolerant, deciduous trees will begin growing beneath the old pines, eventually replacing them. The deciduous forest thus formed is relatively stable and may maintain itself for a very long time.

SOURCE: Figs. 35.22 & 35.27 and text in Keeton, W.T. and J.L. Gould. 1986. Biological Science, 4th ed. W.W. Norton & Company, New York, NY. pp. 946-947, 949-950.

Fig 53-19. A glacial retreat in southeastern Alaska. The dated locations chronicle recession of the glacier since 1760. The broken lines show the approximate edge of the ice in 1760 and 1860, based on historical descriptions. As the ice retreats, it leaves moraines along the edges of the bay (left inset). Primary succession occurs on the moraines (right inset). / © Pearson Education, Inc.		Fig 53-18a. Soon after fire. As this photo taken soon after the Yellowstone National Park fire of 1988 shows, the burn left patchy landscape; note the unburned trees in the background.
		Fig 53-18b. One year after fire. This photo of the same general area taken the following year indicates how rapidly the community began to recover. A variety of herbaceous plants, different from the species that inhabited the floor of the former forest, cover the ground.
Fig 53-20. Change in soil nitrogen concentration during succession after glacial retreat in Glacier Bay, Alaska. Plant succession is shown clockwise from top left. Top left, alders and cottonwoods covering the hillsides; top right, spruce coming into the alder and cottonwood forest; bottom right, spruce and hemlock forest. / © Pearson Education, Inc.

Climax Communities

What is a climax community? A climax community is a late successional stage that, as mentioned in the trends section above, is far more stable than its predecessors. This means that the community observed at this stage will persist for longer and have less of a tendency to alter its environent in a manner injurious to itself. Climax communities are expected to persist so long as climate, geography, and other major environmental factors go unchanged and so long as it remains undisturbed by humans or other "unnatural" influences.

But do such communities really exist? In a strict sense, the answer is probably no. What are usually described as climax communities are not static and may change rapidly if there are major shifts in the physical environment or biota. In addition, antiquated views of succession and climax communities regarded whole communities of plants, for example, as distinct entities and believed that a given climatic region would converge on a given climax community no matter what. Modern ecologists recognize that each species' distribution is determined by its own idiosyncracies - environmental tolerances, developmental constraints, biogeographic history, etc. Since environmental variables vary continuously in space the distributions of individual species will also vary continuously, not discretely. In practice the concept of climax communities may be useful in describing stable communities that occur often while recognizing that the "climax community" for a given area can probably only be determined by observing the pattern of environmental change and succession in that specific place.

For more on climax communities and their place in modern ecological thought see the first few paragraphs below.

SOURCE: Adapted from Keeton, W.T. and J.L. Gould. 1986. Biological Science, 4th ed. W.W. Norton & Company, New York, NY. pp. 950-951.

Models of Succession

The nonequilibrial model views communities as mosaics of patches at different stages of succession

A focus on interspecific interactions producing a predictable progression of stages is a traditional view of ecological succession. in the early 1900s this general view led some ecologists to hypothesize that succession leads to a stable endpoint, a climax community. A stable climax was predicted to develop when a web of interspecific interactions became so intricate that the community was saturated. No additional species could "fit into" the community unless resources became available through the localized extinction of species that were already present.

Ecologists soon recognized that the notion of stable communities was seriously flawed and too simplistic to represent the wealth of variation in nature. Most communities are routinely disturbed by outside factors during the course of succession. Without fire, many grassland areas would develop into forest. We might say that forest is the climax community for such areas, but that would make little sense if the forest community never develops. In this case, periodic fires maintain the community at a stage that simply does not fit the idea of a climax state. Even communities that appear relatively stable change over long periods of time. Studies of pollen preserved in lake sediments provide evidence of community composition over thousands or even millions of years. These studies indicate that common see species sometimes disappeared from North American forests for hundreds of years, only to eventually return at a later time. The concept of climax communities is no longer considered useful in ecological research, although it remains with us in some popular literature.

In sharp contrast to the concept of succession as an orderly, linear progression driven mainly by interspecific interactions, a more current view emphasizes the inevitability of disturbance and nonequilibrium. Succession is seen as a variable, ongoing process. Most studies indicate that linear progression is not typical of succession, and mounting evidence supports the concept that disturbance is the main force that drives successional changes.

According to this nonequilibrial model, succession can take a variety of pathways, depending on the size, frequency, and severity of disturbance. By destabilizing existing community structures and favoring new ones, disturbances not only initiate succession, they also affect its course. The course of succession may vary, for example, with the identity of the particular species that happen to colonize a disturbed area first, and disturbance may also change the identities and numbers of species during any successional stages. Many ecologists posit that disturbance keeps communities in a continual state of flux, rendering them mosaics of patches at different successional stages and preventing, them from ever achieving a state of equilibrium.

What are the actual effects of disturbance in a community? Disturbances result in the loss of individuals, thus creating opportunities in the form of vacated niche spaces that other species may colonize. After a disturbance, some survivors may recolonize an area; grasses may regrow from unburned roots, and sees may sprout from partially burned stumps. Species may also recolonize a disturbed area by migrating from adjacent areas. In general, disturbances followed mainly by regrowth and migration from adjacent areas do not significantly alter community structure because species formerly present in the disturbed areas largely refill it.

Disturbances that trigger major changes in community structure generally arc those whose magnitude or frequency results in colonization of disturbed patches by recruitment. In this process species, from distant areas that are not directly associated with the disturbed patch or its immediate vicinity are the major colonizers. Examples of recruits include windblown and animal-borne seeds. The usual result of recruitment is a different mix of species than formerly present. Thus, a supply of distant recruits and vacated spaces that they can colonize are prerequisites to significant community alteration by disturbances. Alteration may affect a community's relative abundance of species, or its species richness, or both.

Several hypotheses have been proposed to explain why some communities are more species diverse than others. One hypothesis, the nonequilibrial model, proposes that high species diversity results mainly from environmental patchiness caused by frequent abiotic disturbances. Another idea, the dynamic equilibrium hypothesis, proposed in the late 1970s, maintains that species diversity depends mainly on the effect of disturbance on the competitive interactions of populations. According to this model, species diversity is enhanced when the frequency of disturbance prevents competitive exclusion, because poorer competitors persist in the community. The effects of disturbances caused by keystone species, such as elephants, are consistent with this hypothesis.

Another 1970s proposal, the intermediate disturbance hypothesis, posits that species diversity is greatest where disturbances are moderate in both frequency and severity, because organisms typical of different successional stages will be present. When disturbance is severe and frequent, the community may include only good colonizers typical of early stages of succession. If disturbances are mild and rare in a particular location, the late -success tonal species that arc most competitive will make up the community. Studies of species diversity in tropical rain forests provide some evidence for the intermediate disturbance hypothesis. Scattered throughout these forests are gaps where trees and the vines attached to them have fallen. In these disturbed areas, species from various Successional stages coexist within a relatively small space.

Clearly, all communities are subject to some disturbance. On a small scale, worms and rodents disturb soil by burrowing, through it. Storms create patches of various sizes in virtually all communities. However, communities are also characterized by interspecific interactions that may affect community structure, succession and species diversity. The challenge of future research is to sort out the relative importance of the disturbances that tend to destabilize communities and the interspecific interactions that tend to stabilize them. These issues are central to the current dilemma of species conservation.

Download Printable (PDF) Version

Back to Main Menu / Previous Objective / Next Objective /