:Hortobagy National Park Directorate, H-4002 Debrecen, POB. 216., Hungary 2Department of Ecology, University of Debrecen, H-4010 Debrecen, POB. 71., Hungary
A central issue in conservation biology and nature management is whether or not characteristic species of a given habitat type could be preserved by fragmented habitat patches or not. The classical theory of island biogeography predicts that the number of species supported by an island increases with the area of the island. However, there is a significant difference between real and habitat islands. In real islands, the surrounding habitat (ocean, sea, lake, river etc.) is usually inhospitable to organisms occurring on islands. In the case of habitat islands, the bordering habitat (the matrix) is usually less hostile. Consequently, species richness of real islands is not influenced notably by the surrounding habitat. This difference is increasingly emphasized when studying the predictions of island biogeography theory on habitat islands. Clear distinction should be drawn between specialist species that truly perceive the habitat patches as islands and are unable to survive in the surrounding matrix, and those species that occur in both the habitat patch and the matrix (generalist species).
In this case study, we demonstrated that depending on the ratio of specialist and generalist species in an assemblage, the species-area relationship may be positive or negative. Ground beetles (Coleoptera: Carabidae) of sandy grassland patches were studied in Eastern Hungary (Central Europe). The total number of ground beetle species correlated negatively with grassland area. Based on this result, one can draw the (seriously false) conclusion that it is sufficient to conserve small patches because they support most species. This negative relationship was due to the increasing ratio of generalist species with decreasing patch size. Analyzing the habitat specialist species (open-habitat species associated with sandy soils), the significant negative relationship turned over, and became significantly positive; i.e. the ratio of habitat specialist species increased with patch size, as predicted by the theory of island biogeography.
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