must be compromised or alternative treatments must be administered to patches within the grassland.
Light to moderate levels of grazing usually result in a richer diversity of plant species than do heavy levels of grazing or no grazing at all, especially in the more humid grasslands such as the tall-grass prairie in the United States (Risser et al., 1981). Presumably, this increased diversity is caused by opening the vegetation canopy and allowing more species to compete successfully. Thus, species diversity is maximized by light to moderate grazing intensities—no grazing by domestic herbivores reduces diversity because of the thick vegetation canopy and heavy litter layer, and heavy grazing reduces species diversity by selectively eliminating the more palatable species.
The southwestern grasslands of the United States are dominated by warm-season perennial species. Since these species mature later in the growing season, many ranching operations include pastures of cool-season species, such as crested wheatgrass (Agropyron cristatum) to serve as livestock forage early in the year. These crested wheatgrass fields contain very little species diversity.
In the eastern tall-grass prairie, where remaining grasslands are relegated primarily to small patches in an otherwise agricultural landscape, there are several threats to biodiversity. One is the intrusion of several aggressive alien plant species, which are invading and replacing native species. In Illinois, more than a dozen species have invaded prairies to such a degree that the prairies themselves are now threatened.
The small, isolated prairie remnants are unable to support the normal complement of either native flora or native fauna. In Missouri, only 0.5% of the original tall-grass prairie remains, mostly in isolated prairie islands within a matrix of improved pastures and croplands. Sampson (1980) compared prairie sizes with the presence and absence of the greater prairie chicken (Tympanucus cupido) and found that grasslands without prairie chickens averaged 172 hectares, but those without prairie chickens averaged only 33 hectares. Furthermore, those grassland remnants without prairie chickens were isolated from other grasslands by 81 kilometers, whereas those with prairie chickens were, on average, only 14 kilometers from other grasslands. Sampson concluded that Missouri grasslands capable of supporting the greater prairie chicken should be about 300 hectares larger and within 20 kilometers of other grasslands.
Sampson (1980) also computed the probability that a given habitat size will annually contain a breeding population of selected native grassland bird species. The minimum habitat sizes were calculated as follows: for the eastern meadowlark (Sturnella magna), less than 1 hectare; for the horned lark (Eremophila alpestris) and grasshopper sparrow (Ammodramus sarannarum), more than 1 hectare; for Henslow’s sparrow (A. henslowii), the upland sandpiper (Bartramia longicanda), and the greater prairie chicken, more than 10 hectares. In general, the size and not the habitat heterogeneity had a significant influence on the number of breeding prairie bird species. In Illinois, Graber and Graber (1976) also found that the size of the grassland had a major influence on the number of bird species and that in small patches over a 20-year period, the number of bird species decreased at a much faster rate than the simple reduction in the total area of grassland.