General patterns of ecosystem change were acknowledged by early Roman writers and were described by many naturalists in the 18th and 19th centuries (Spurr 1952). The earliest scientific work was conducted in sand dunes by Cowles (1899), and patterns described by naturalists and Cowles were formalized by Clements in a series of papers during the early part of the 20th century. Clements' 1916 book on succession was particularly influential in establishing a paradigm for ecosystem dynamics. Clements promoted the view that ecosystems are organic
units; in fact, the Clementsian view of ecosystem dynamics is often termed "organismic" and ecosystems or plant communities are called "superorganisms." The organismic concept is regarded by most contemporary ecologists as an inappropriate view (but see Wilson and Sober 1989; Wilson 1997). Nonetheless, this view remains the dominant paradigm promoted in ecology texts and it forms the basis for site classification in land management agencies (i.e., delineation of range sites and habitat types). The Clementsian view will, therefore, be briefly described (Figure 4.2).
Clements (1916) described succession as a sequence of identifiable stages, which he termed nudation, ecesis, competition, reaction, and stabilization. Nudation is the process which creates a patch of bare soil, and it is said to initiate succession. Ecesis is the successful establishment of plants, coming either from propagules remaining in the soil (i.e., seeds, root fragments, or whole plants) or migrating from elsewhere. Ecesis is said to be controlled by environmental conditions and the characteristics of plant species available at the site. Competition among established plants then leads to the elimination of some species. Reaction is the change in the physical environment that results from the growth and death of plants, and it contributes to continual change of resource availability. Finally, stabilization occurs as long-lived species dominate a site.
Clements indicated that this phase rarely, if ever, occurs. It is synonymous with "climax."
Clements' conceptual model of succession is descriptive, but it is not explanatory or mechanistic. Thus, Clements simply formalized the process of species' replacement described by naturalists during the preceding two centuries.
The traditional view that developed as a result of Clements' work was that of "relay floristics." According to this view, species prepare an area to make it more suitable for other species. Because ecosystems and communities are superorganisms, they are capable of employing a strategy, including the strategy of site preparation. This idea has been repeated in dozens of papers and books, including relatively recent textbooks. For example, Odum (1983) discussed succession within a section titled "The Strategy of Ecosystem Development:"
[S]uccession is an orderly process of community development; it is reasonably directional and, therefore, predictable ... it results from modification of the physical environment by the community; that is, succession is community-controlled . . . Species replacement in the sere occurs because populations tend to modify the physical environment, making conditions favorable for other populations until an equilibrium between biotic and abiotic is achieved.
Thus, the prevailing paradigm in ecology from the early 1900s until relatively recently was that ecosystems facilitate the development of other ecosystems by altering site conditions. This implies that late-successional species could not occupy the site without earlier occupation by earlier-successional species.
This traditional Clementsian explanation of vegetation succession is the most familiar model to wildlife managers, and represents the only model with which many managers are familiar. Undergraduate wildlife biology curricula usually include only two to three botany courses, one to two of which are often plant taxonomy, with only one course in plant ecology. Thus, wildlife biology majors are not exposed to a deep understanding of the modern views of vegetation succession. Yet, vegetation manipulation via alteration of vegetation succession - using techniques such as timber harvesting, prescribed burning, plowing, or disking - to manipulate animal populations is a frequent approach used by wildlife managers. The following sections offer alternative views of succession; the underlying processes and mechanisms have numerous implications for the research, management, and conservation of plant and animal communities.
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