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close this bookThe Improvement of Tropical and Subtropical Rangelands (BOSTID)
close this folderPart I
close this folderSite evaluation
View the document(introduction...)
View the documentAn ecosystem perspective
View the documentA systems approach to site evaluation
View the documentEvaluation of abiotic and biotic components
View the documentIntegrated evaluations
View the documentReferences

An ecosystem perspective

A site for a proposed range improvement project or program is a microcosm of a larger ecosystem. Regardless of how it is delineated, an ecosystem is the basic unit of ecology, typically a complex system, comprising the physical setting, plants, animals, and its human population. Compounding this complexity is the fact that an ecosystem is almost always changing, even in semiarid and arid rangelands.

The natural process of change in the composition of an ecosystem is referred to as succession. Successional changes take place in response to natural or man-made influences in the environment. So-called primary succession happens on newly exposed areas, such as landslides or sand dunes, whereas secondary succession occurs after the previous vegetation has been destroyed or disturbed by fire or agricultural practices, for example. In many areas of Africa and Asia, a disclimax (a climax maintained through disturbance) has been established through savanna burning and heavy use pressure by livestock. In any case, natural ecosystems evolve from essentially bare areas to more or less stabilized types of dominant vegetation through a series of successional stages.

The current successional stage of a site being considered for improvement should be characterized. Some individual plant species grow better when in competition with existing vegetation on sites in the early stages of succession. Other plant species survive and grow with existing vegetation on sites in later stages of succession. Through recognition of the current successional stage, the species to be planted and managed can be better matched with the successional condition of the site, thereby enhancing the probability of continued growth.

Knowledge of successional patterns is gained, in general, from analyses of systematic, long-term observations of cyclical processes by astute ecologists. These analyses are difficult where successional change is orderly, and they are next to impossible where the changes are erratic. Nevertheless, the potential for the improvement of a site is put in a proper ecological perspective when analyses provide approximations of the current successional condition with respect to the range of successional stages that characterize a site.