Application of Steady-State Theory to Island Biogeography in Protected Areas
Today we live in managed landscapes with a high degree of fragmentation and therefore it appears obvious to apply island biogeography to the actual problems of protected areas. The relation of core to marginal zones in small habitats is important here. With a decrease in such habitats the diversity of conditions at the site increases linearly, but the quality of these conditions decreases exponentially (Mader 1983). Species with special demands - including demands for space - migrate, but broadly distributed species, well adapted to the site (euryoecic species), may remain established for a long time in the marginal zones, unless they lose their habitat by frequent interference. Therefore, application of the theory of island biogeography is, at the moment, only limited.
However, valid relations of size of area and distance (for mainland islands the degree of iso lation) have stimulated discussions on the minimum size of areas which should be sustained.
Of course, the size of protected areas should not be calculated according the rules of island bio-geography only, as strictly forest species would not be protected, because of the large demand for space. Habitats of the same or similar quality should be maintained not too far away as balancing spaces and (perhaps only intermittently required) refuge areas, pointing to the previously mentioned stepping stones by MacArthur and Wilson (1967). These stepping stones are an important component of the concept of interconnecting biotopes for protected areas. In our agriculturally managed landscape, such stepping stones could be small forest islands, as well as hedges, edges of fields, long-term fallow land, disused stone quarries or railway lines, etc. Concepts of nature protection must incorporate buffer zones and smaller areas of habitat fragments around the protected area, in addition to the closed protected areas which should be as large as possible. Number of species should not be the sole aim for protection. Large areas of mosaics of optimal and suboptimal habitats should be considered. There cannot be a generally applicable size for protected areas and stepping stones, nor for the spatial pattern of interconnected biotopes (particularly the distances between stepping stones and the protected area). These values depend on the communities to be protected. There are very few examples in which minimum areas and distances were established empirically for individual groups of organisms. Current, urgent attempts to create protected areas are decided on local conditions and considerations of plausibility. Increasingly, the more dynamic metapopulation concept by Gilpin and Hanski (1991) is becoming important.
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