Directional changes in the composition and organization of ecologic communities over time are collectively known as succession. The process of succession is usually associated with a single local community: the initiation of a new community at an unoccupied site is referred to as primary succession; recovery following a disturbance is termed secondary succession. Secondary succession is one of the most thoroughly studied developmental phenomena in ecology, most of the studies having been carried out in temperate forests that featured localized disturbances and subsequent recoveries. The best-known descriptions are of "old field successions," in which plant communities recover from farming activity, involving abandonment of cleared patches and serial recolonization from a regional species pool. Despite the level of interest in succession, however, there is a continuing debate about deterministic versus probabilistic factors involved in the processes of change, the relative importance of internal biotic control ("autogenic succession") compared with abiotic external forcing ("allogenic succession"), and whether only local communities undergo succession or if the process can be generalized to larger, more inclusive systems. All true succession and successionlike processes make up the developmental properties of ecologic systems collected under the term temporal dynamics. The study of these directional changes in ecologic systems has led ecologists to speculate about general, lawlike characterizations of nature since the late nineteenth century, and the level of interest has never abated.
The temptation to extrapolate and over-generalize has been irresistible in studies of temporal dynamics. The urge to identify general processes that are like physical laws has usually led ecology into dead ends, because in many cases scaling considerations have been ignored. Some ecologists would use the term succession to refer to any directional changes occurring in various kinds of ecologic systems at any position in the so-called ecologic hierarchy: communities or local ecosystems, regional ecosystems, even the entire biosphere—all have been characterized as undergoing succession. Succession also is used to describe both biologically controlled transitions and those paced by outside, environmental changes. This has caused much confusion and is related to the inability of ecologists to settle on definitive concepts for the fundamental units of ecologic organization. In this summary, succession will be regarded as a developmental process restricted to local ecosystems and involving mostly internal dynamics.
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