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grassland becomes converted to a relatively sparse shrubland composed of less-palatable herbaceous or woody species. This type of conversion can be found, for example, along the India-Pakistan borders in the Sind-Kutch region, throughout much of the Sahelian area of Africa, and in parts of the southwestern United States (Brown, 1950; Buffington and Herbel, 1965; Howard-Clinton, 1984). An obvious consequence of this impact is a loss of the native biodiversity of grasslands throughout the world.
Many grasslands, especially those in relatively humid environments, produce large amounts of below-ground growth consisting primarily of roots and rhizomes. As these plant parts naturally die, the organic matter is incorporated into the soil. These enriched soils are prime agricultural soils, and as a result, the grasslands are quite vulnerable to conversion to croplands. An obvious example is the prairie peninsula (Transeau, 1935) in the east central part of the United States—an area that is now almost completely cropland rather than the original tall-grass prairie with its deep, organic-rich melanized soils.
Well over 100 species of native plants commonly grow in prairie remnants smaller than 2 hectares. Within the Central Plains and tall-grass prairie, between 250 and 300 species are usually found in remnants with areas of approximately 250 hectares (Steiger, 1930). Although the loss of grassland habitat has been calculated for selected states and specific grassland types (Risser, 1986), there are no general figures on the loss of grassland species.
MORE SUBTLE IMPACTS
Although overgrazing and conversion to croplands represent the most obvious impacts on the native biodiversity of grasslands, a true diagnosis requires a more refined analysis. For example, relatively recent widespread overgrazing and resultant major changes in species composition of grassland habitats has occurred in developing countries where there is and has been enormous pressure to produce food. Conversion of grassland to agricultural cropland has taken place primarily in humid grasslands. Thus, in the United States, most dry western grasslands, or steppes, can now be adequately managed to remain as perpetuating rangelands. And although there have been misguided efforts to plow the rangelands and some cases of grassland abuse by overstocking with grazing animals, most of these western grasslands now remain intact. In the eastern prairie region, most of the grasslands have been converted to cropland, but important preservation and restoration efforts are now under way (Risser, 1986).
Prairie fires have been a persistent characteristic of grasslands that produce enough fuel for them (Daubenmire, 1968). In fact, in the tall-grass prairie, periodic burning increases the species diversity above that found on an unburned prairie, especially one that is not grazed. Burning is routinely used intentionally in grassland management to reduce invasion by woody shrub species and to encourage native perennial species. However, these burning practices are frequently not beneficial to insects (Cancaledo and Yonke, 1970) or to the small mammals and birds. Thus, to attain optimum biological diversity, either the scheduling of prescribed burning