In their study of grassland invertebrates, Whitcomb et al. (1986) found that more than 100 dominant grass species, and perhaps an equal number of forbs, are important contributors to the diversity of sap-sucking insects in North American grasslands. They reported that perennial and dominant (but not annual or sub-dominant) grass and forb species tended to have specific assemblages of cicadellids (leafhoppers) in a given geographic region but that the species composition of these assemblages varied geographically. Patch size and structure of the host vegetation stands were of considerable importance, and the rarity of these leafhoppers was directly attributable to the rarity of the host plant species. Even in host patches of sufficient size to support reasonably large numbers of cicadellids, insect populations were reduced by such disturbances as fire, drought, floods, predators, and, especially, parasites.
The origin of North American grasslands is relatively recent—they formed approximately 12,000 years ago (Dort and Jones, 1970). There is a low rate of vertebrate and plant endemism in these areas, and the origins of their flora and fauna are diverse. Therefore, despite the massive loss of grasslands in the United States and elsewhere, there are fewer than 15 true grassland species listed or proposed as federally threatened or endangered. However, as has been recognized for decades, grassland plant species have undergone a significant amount of ecotypic variation (Olmsted, 1945), and the reduction in grasslands has resulted in a reduction of genetic diversity—diversity losses that are not apparent in simple measures of species diversity.
Thus biodiversity in and among grasslands is complicated because of the rather subtle nature of the grassland ecosystem. Major, obvious impacts such as widespread overgrazing and conversion to agricultural croplands have significantly reduced the native biodiversity (Weaver, 1954). Among the more subtle impacts are the effects of reduced habitat size, the lack of endemic species, which are so easily recognized, and the highly developed ecotypic differentiation in grasslands, which is not detected in conventional measures of biodiversity.
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