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Box 5-9. Impacts of Some Pressures on Rangelands
Land-Use Change
Major factors in land-use change are conversion of rangelands to croplands
(Allen-Diaz, 1996; WRI, 2000) and increased human settlements, especially
urbanization (Gitay and Noble, 1998), which lead to fragmentation (WRI,
2000). There have been large-scale changes in land use: For example, in
the South Platte Basin in the United States, 40% of the land cover has
been converted from rangelands to croplands. This can alter carbon stores,
sometimes leading to soil carbon loss of as much as 50% (Allen-Diaz, 1996),
but it also can lead to increased plant productivity through irrigated
grain production (Baron et al., 1998). Baron et al. (1998) conclude that
subsequent impacts on the biogeochemical cycles of the basin and on land-atmosphere
interactions can affect many rangelands. In many rangelands, native species
that occur at low density are used for fuelwood. In some cases, fuelwood
collection can lead to decreases in woody vegetation cover and possible
land degradation.
Livestock Production
Some rangelands have high densities (>100 km-2) of livestock,
with livestock being moved to take advantage of the periodic growth, especially
after rain and/or fires (WRI, 2000). Modeling studies show that increased
grazing pressure (i.e., overgrazing) would cause grass and herbaceous
productivity to fall below a certain threshold, resulting in increased
and rapid rates of land degradation especially under drier and/or hotter
climate conditions (Abel, 1997).
Competition for Land and Water
There is increased demand for water for direct human consumption and
for irrigation (WRI, 2000). Food production obviously is positively affected
by increased water use (see Section 5.3 and Chapter
4), but this water use is an added pressure on many rangeland ecosystems.
Nomadic pastoralism, which was common in many rangelands until recently,
allowed pastoralists to cope with the variable climate of the rangelands
they inhabited. Land-use changes (to permanent agriculture, urban areas,
conservation, and game reserves that have included loss of sources of
permanent water) have led to overall loss of land available. Together
with increased human population, this has led to competition for land
and changes from pastoral communities to more market-orientated and cash-based
economies (Allen-Diaz, 1996).
Altered Fire Regimes
The SAR did an extensive review of fires and rangelands. The projected
increase in variability in climate led to the conclusion that the frequency
and severity of fires will increase in rangelands (Allen-Diaz, 1996),
provided that drought and grazing do not lead to a reduction in vegetation
biomass. There is historical evidence that fire frequency has changed
(increased and decreased, depending on vegetation biomass) in recent decades
(see, e.g., Russell-Smith et al., 1997), leading to changes in vegetation
composition. Fires also are started by humans; for example, in Africa
25-80% of rangelands are burned every year, often to induce new plant
growth (WRI, 2000). This has implications for short-term productivity
but possibly long-term land degradation.
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