Croplands have expanded dramatically during this century from 9.1 x 106 to lSxlO^km2 (Richards 1990). This expansion altered landscape heterogeneity in grasslands. Habitat selectivity by domestic livestock has differentially influenced riparian ecosystems and therefore altered landscape diversity. Domestic livestock, and especially cattle, tend to congregate in the topographically lowest portions of the landscape (Senft et al. 1985; Pinchak et al. 1991). Such habitat selectivity has negative effects on the plant and animal diversity of riparian ecosystems (Kauffman and Krueger 1984; Smith et al. 1992). The reduction in diversity of the stream-side vegetation and its productivity have negative effects on both physical and chemical indicators of water quality (Kauffman and Krucger 1984). Reduction in the diversity and productivity of the herbaceous vegetation layer can change the velocity and erosive energy of the stream flow. Losses of the woody overstory has large effects on water temperature. Both the overstory and understory vegetation layers have important effects on the rates and kinds of aquatic processes that occur in a stream (Kauffman and Krueger 1984). Diversity and productivity of invertebrates and fishes are profoundly influenced by the diversity of the stream-side vegetation.
6.5.6 Biotic linkages/species interactions
Invasions in grasslands are common and in some cases have been associated with changes in grazing regime. Examples of grasslands which have been invaded by exotic species are the California grasslands and the intermoun-tain west of North America, the Pampas in South America, and the savannas in tropic South America (Sala et al. 1986; D'Antonio and Vitousek 1992). Invasions in grasslands usually occurred in association with the increase in grazing intensity and/or a change in dominant grazer. Vulnerability to invasions associated with grazing appears to be related to moisture availability and the grazing history in evolutionary time (Milchunas et ai 1988). Grasslands which evolved under light grazing conditions and under mesic conditions are more vulnerable to invasions than those which evolved under heavy grazing in xeric environments. Semi-arid grasslands of northwest US and southwest Canada have a short evolutionary grazing history, and before the introduction of cattle they were dominated by perennial tussock grasses (Tisdale 1947: Daubenmire 1970). The inability of these grasses to cope with heavy grazing resulted in the invasion and dominance of many areas by Eurasian weeds (Daubenmire 1940, 1970; Ellison 1960; Mack 198); Mack and Thompson 1982). Invasions often disrupt competitive interactions (D'Antonio and Vitousek 1992), which results in changes in species composition with the ecosystem effects described above.
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