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18.1: Introduction

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    92889
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    Ecological succession is the process of change in the species composition of an ecological community over time. It is a process by which an ecological community undergoes more or less orderly and predictable changes following a disturbance or the initial colonization of a new habitat. Succession may be initiated either by formation of new, unoccupied habitat, such as from a lava flow or a severe landslide, or by some form of disturbance of a community, such as from a fire, severe windthrow, or logging. Succession that begins in new habitats, uninfluenced by pre-existing communities is called primary succession, whereas succession that follows disruption of a pre-existing community is called secondary succession.

    History of succession

    Succession was among the first theories advanced in the ecological literature. Ecological succession was first documented in the Indiana Dunes of Northwest Indiana and remains an important ecological topic of study.1

    Between 1899-1910, Henry Chandler Cowles, at the University of Chicago, developed a more formal concept of succession. Inspired by studies of Danish dunes by Eugen Warming, Cowles studied vegetation development on sand dunes on the shores of Lake Michigan (the Indiana Dunes). He recognized that vegetation on dunes of different ages might be interpreted as different stages of a general trend of vegetation development on dunes (an approach to the study of vegetation change later termed space-for-time substitution, or chronosequence studies).

    From about 1900 to 1960, however, understanding of succession was dominated by the theories of Frederic Clements, a contemporary of Cowles, who held that seres were highly predictable and deterministic series of successional plant communities that converged on a climatically determined stable climax community regardless of starting conditions. Clements explicitly analogized the successional development of ecological communities with ontogenetic development of individual organisms, and his model is often referred to as the pseudo-organismic theory of community ecology. Clements and his followers developed a complex taxonomy of communities and successional pathways.

    Henry Gleason offered a contrasting framework as early as the 1920s. The Gleasonian model was more complex and much less deterministic than the Clementsian. It differs most fundamentally from the Clementsian view in suggesting a much greater role of chance factors and in denying the existence of coherent, sharply bounded community types. Gleason argued that species distributions responded individualistically to environmental factors, and communities were best regarded as artifacts of the juxtaposition of species distributions. Gleason's ideas, first published in 1926, were largely ignored until the late 1950s.

    Two quotes illustrate the contrasting views of Clements and Gleason. Clements wrote in 1916:

    The developmental study of vegetation necessarily rests upon the assumption that the unit or climax formation is an organic entity. As an organism the formation arises, grows, matures, and dies. Furthermore, each climax formation is able to reproduce itself, repeating with essential fidelity the stages of its development.

    — Frederic Clements2

    while Gleason, in his 1926 paper, said:

    An association is not an organism, scarcely even a vegetational unit, but merely a coincidence.

    — Henry Gleason3

    Gleason's ideas were, in fact, more consistent with Cowles' original thinking about succession. About Clements' distinction between primary succession and secondary succession, Cowles wrote (1911):

    This classification seems not to be of fundamental value, since it separates such closely related phenomena as those of erosion and deposition, and it places together such unlike things as human agencies and the subsidence of land.

    — Henry Cowles4

    Succession theory was developed primarily by botanists. The study of succession applied to whole ecosystems initiated in the writings of Ramon Margalef, while Eugene Odum's publication of The Strategy of Ecosystem Development is considered its formal starting point.5

    Animal life also exhibits changes with changing communities. In the lichen stage fauna is sparse. It comprises a few mites, ants and spiders living in cracks and crevices. The fauna undergoes a qualitative increase during the herb grass stage. The animals found during this stage include nematodes, insects larvae, ants, spiders, mites, etc. The animal population increases and diversifies with the development of the forest climax community. The fauna consists of invertebrates like slugs, snails, worms, millipedes, centipedes, ants, bugs; and vertebrates such as squirrels, foxes, mice, moles, snakes, various birds, salamanders and frogs.

    A more rigorous, data-driven testing of successional models and community theory generally began with the work of Robert Whittaker and John Curtis in the 1950s and 1960s. Succession theory has since become less monolithic and more complex. J. Connell and R. Slatyer attempted a codification of successional processes by mechanism. Among British and North American ecologists, the notion of a stable climax vegetation has been largely abandoned, and successional processes have come to be seen as much less deterministic, with important roles for historical contingency and for alternate pathways in the actual development of communities. Debates continue as to the general predictability of successional dynamics and the relative importance of equilibrial vs. non-equilibrial processes. Former Harvard professor Fakhri A. Bazzaz introduced the notion of scale into the discussion, as he considered that at local or small area scale the processes are stochastic and patchy, but taking bigger regional areas into consideration, certain tendencies can not be denied.6

    Contributors and Attributions

    Modified by Castilleja Olmsted (University of Pittsburgh) and Kyle Whittinghill (University of Vermont) from the following sources:

    1.  Smith, S., & Mark, S. (2009). The historical roots of the nature conservancy in the Northwest Indiana/Chicagoland region: From science to preservation. The South Shore Journal, 3"Archived copy". Archived from the original on 2016-01-01. Retrieved 2015-11-22.
    2. Clements, F.E. (1916). Plant succession: an analysis of the development of vegetation.
    3. Gleason, H.A. (1926). The individualistic concept of the plant association. The Bulletin of the Torrey Botanical Club.
    4. Cowles, H.C. (1911). The causes of vegetational cycles. Annals of the Association of American Geographers, 1(1), pp. 3-20.
    5. The succession of forest trees, and wild apples. Archive.org. Retrieved on 2014-04-12.
    6. Bazzaz, F.A. (1996). Plants in changing environments. UK: Cambridge University Press. pp. 4–5. ISBN 9-780521-398435.

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