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Although (by definition) primary succession occurs in unvegetated areas, dispersal distances may not be great enough to limit the ability of particular species to colonize a disturbed area. At Mount St. Helens and Kilauea, remnants of the prior vegetation are scattered throughout the disturbed sites, where they serve as important sources of propagules. Most other primary succession, such as that on new beaches and sand bars in rivers, occurs on smaller disturbed sites, where dispersal distances may be short.
Climax Revisited
The concept of the climax—the hypothesized static, deterministic end point of succession—once dominated ecologists’ thinking about succession. According to this idea, once a community has reached the climax state, it stops changing, unless a disturbance occurs that resets the community back to an earlier seral stage. There are serious difficulties with this concept, which have become increasingly apparent over the years.
The notion of a successional climax was devised by Hult (1885) and developed over the next several decades (Clements 1916). One of its proponents was Frederick Clements. His ideas may have been influenced by classical Greek philosophy, a major part of the intellectual tradition of his day, which emphasized idealized types in nature. Ecologists of the time viewed the natural state of the community as being unchanging and disturbance as an unnatural, external process. As we have seen in this chapter, however, various kinds of disturbances are intrinsic to most communities. As plant ecologists have recognized this fact in recent decades, many have called into question the entire concept of the climax as the end point of succession.
The result was a conceptual shift to considering patterns of stasis and change at different spatial scales. Consider a landscape dotted with many different communities, each containing many different patches. A single small patch may always be in a state of flux as populations change and one species replaces another (as in van der Maarel’s carousel model; see Chapter 10). At this scale, change may seem directionless and at the constant mercy of unpredictable factors such as climate and herbivory. In contrast, the community as a whole may be undergoing a slow successional process of gradual replacement of one set of species by another. At this larger scale, disturbances may still occur every century or so, resetting the successional cycle. Finally, the entire landscape may consist of a mosaic of communities at different points along the successional cycle. Although each community is changing, the proportion of the entire landscape at each successional stage remains about the same. Thus, a dynamic equilib-
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rium may exist at the scale of the landscape, even though no single community is at equilibrium. We will return to this issue of hierarchies of scales when we discuss landscapes in more detail in Chapter 17.
Today, plant ecologists recognize that communities and landscapes never attain a constant, unchanging state. Rather, communities may attain a dynamic equilibrium, if an equilibrium is reached at all. Long-term climate changes (see Chapter 21), introduction or evolution of new species, and landform changes such as erosion, mountain building, and continental movements mean that the world is always in flux. At best, these changes occur slowly enough that communities or landscapes are at quasi-equilibrium. Thus, one’s view of change and equilibrium depends on the scale considered.
Summary
In this chapter, we have explored the process of succession and the implications of that process for our understanding of the nature of communities. Succession starts with disturbance. All communities experience disturbances. Those disturbances can be small (a limb falling from a tree, a badger creating a mound) or large (a forest fire, a volcano erupting). They can occur every year, or once in a millennium. They can result from a multiplicity of factors—fire, wind, rain, snow, animals, disease. After a disturbance, new species may colonize the site. These species then interact with one another so that the community changes over time, resulting in the process of succession. Finally, if disturbances are relatively infrequent, the community may reach a state of dynamic equilibrium. Earlier ecologists thought that a static end point to succession, called the climax, was the usual state of affairs in plant communities. There are now strong reasons to think that even a state of longterm dynamic equilibrium may be exceptional.
In this chapter and the previous ones, we addressed a number of questions about the nature of communities. We began by asking whether communities are entities in their own right, or merely collections of populations that happen to co-occur. We described a number of ways in which species interact. These interactions can be direct or indirect, positive or negative. For example, competition for common nutrients is a direct, negative interaction. In contrast, the stabilization of a sand dune by Elymus canadensis allowing colonization by other species is an indirect, positive interaction. These interactions help to shape communities. Succession occurs, in part, because of these interactions. Thus, a community is more than simply the sum of its constituent species. How those species interact is also important in shaping community composition and structure.
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Additional Readings
Classic References
Cowles, H. C. 1899. The ecological relations of the vegetation on the sand dunes of Lake Michigan. Bot. Gaz. 27:95–117, 167–202, 281–308, 361–391.
Cooper, W. S. 1923. The recent ecological history of Glacier Bay, Alaska. II. The present vegetation cycle. Ecology 4:223–246.
Oosting, H. J. 1942. An ecological analysis of the plant communities of Piedmont, North Carolina. Am. Midl. Nat. 28:1–126.
Contemporary Research
Chapin, F. S., L. R. Walker, C. L. Fastie, and L. C. Sharman. 1994. Mechanisms of primary succession following deglaciation at Glacier Bay, Alaska. Ecol. Monogr. 64:149–175.
Halpern, C. B., P. M. Frenzen, J. E. Means, and J. F. Franklin. 1990. Plant succession in areas of scorched and blown-down forest after the 1980 eruption of Mount St. Helens, Washington. J. Veg. Sci. 1:181–195.
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Keeley, J. E., C. J. Fotheringham, and M. Morais. 1999. Reexamining fire suppression impacts on brushland fire regimes. Science 284:1824–1832.
Phillips, D. L. and D. J. Shure. 1990. Patch-size effects on early succession in southern Appalachian Forests. Ecology 71:204–212.
Additional Resources
Johnson, E. A. 1992. Fire and Vegetation Dynamics. Cambridge University Press, Cambridge, UK.
Pickett, S. T. A., and P. S. E. White. 1985. The Ecology of Natural Disturbance and Patch Dynamics. Academic Press, Orlando, FL.
Pyne, S. J., P. L. Andrews, and R. D. Laven. 1996. Introduction to Wildland Fire, 2nd ed. Wiley, New York.