Events in biology have been somewhat the reverse of those in physics. Whereas physics began with directionless laws and was confronted with exceptions, biologists had originally thought that phylogeny took a progressive direction over the eons, culminating in the appearance of humankind at the apex of the natural order—the so-called "natural chain of being." Evolutionary biologists, however, have sought to disabuse other biologists of such directional notions (Gould, 1994). At each turn in its history, a biotic system is subject to random, isotropic influences. What looks in retrospect like a progression has been merely the accumulation of the results of chance influences. Complexity simply accrues until such time as a chance catastrophe prunes the collection back to a drastically simpler composition.
We thus encounter a strong bias at work within the community of scientists to deny the existence of bias in nature (a statement which makes sense only because humanity has been postulated to remain outside the realm of the natural). Physicists and (perhaps by virtue of "physics envy") evolutionary theorists appear keen to deny the existence of direction anywhere in the universe, preferring instead a changeless Eleatic world-view. It is against this background that the notion of direction in ecology takes on such importance.
Directionality, in the form of ecological succession, has been a key phenomenon in ecology from its inception (Clements, 1916). By ecological succession is meant "the orderly process of community change" (Odum, 1959) whereby communities replace one another in a given area. Odum (ibid.) do not equivocate in saying, "The remarkable thing about ecological succession is that it is directional." In those situations where the process is well known, the community at any given time may be recognized and future changes predicted. That is, succession as a phenomenon appears to be reproducible to a degree.
Of course, it was not long after the ideas of community succession came into play that the opinion arose that its purported direction was illusory. Gleason (1917) portrayed succession in plant communities as random associations of whatever plant species happened to immigrate into the area. Others have pointed out that "seres" of ecological communities almost always differ in terms of the species observed (Cowles, 1899). The ecosystem ecologist takes refuge in the idea that the functional structure nonetheless remains predictable (Sheley, 2002).
The question thus arises as to whether ecological succession is orderly in any sense of the word, and, if so, what are the agencies behind such order? We begin by noting that the directionality of ecosystems is of a different ilk from those mentioned in the opening of this chapter. With regard to all three of those examples, the direction of the system in question was determined by sources exterior to the system—by the colliding billiard ball in the first instance, and by the sun as perceived by the sunflower and copepod. It will be argued below, however, that the directionality of an ecosystem derives from an agency active within the system itself. Surely, external events do impact the system direction by providing constraints, but any one event is usually incremental in effect. On rare occasions an external event can radically alter the direction and the constitution of the system itself (Prigogine, 1978; Tiezzi, 2006b), but this change is every bit as much a consequence of the system configuration as it is of the external event (Ulanowicz, 2006a). The direction an ecosystem takes is both internal and constitutional. Most change seen elsewhere is neither.
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