Grasslands of the World.pdf - Disasters and Conflicts - UNEP

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Grasslands of the world
tall vegetatively propagated grass species, while the short sexually reproduced
species disappeared (Belsky, 1986). This implies that even though the greatest
number of species are found at intermediate grazing intensities, some species
are always lost when ungrazed pastures are grazed. Although there are more
species at intermediate levels of grazing, it is possible that any grazing negatively
affects rare plant species that are sensitive to grazing.
Desertification : driven by climate or overgrazing by livestock?
One of the most controversial and debated aspects of research about pastoral
systems is the existence and extent of overgrazing, desertification and land
degradation 4 in pastoral lands, particularly in Africa. Global assessments of
drylands maintain that much of the earth’s land surface is degraded (GLASOD,
1990) and that livestock are the principal global cause of desertification
(Mabbutt, 1984). Analysts suggest that African pastures are 50 percent more
degraded than those in Asia or Latin America (GLASOD, 1990). However,
other analyses show that livestock numbers only exceed likely carrying
capacities of arid and semi -arid rangelands in about 3–19 percent 5 of Africa
(Ellis et al., 1999). In addition, there is no sustained evidence for a reduction
in productivity , as measured by no change in the water-use efficiency of the
Sahelian vegetation over 16 years, suggesting that the extent of the Sahara
is more strongly influenced by drought than grazing (Tucker, Dregne and
Newcomb, 1991; Nicholson, Tucker and Ba, 1998).
However, these broad assessments are only correlative and can not assess
cause and effect rigorously, and can not measure the relative impacts of different
causative agents. Certainly, at more local scales, livestock impacts are
highly visible and persistent around towns, water points and along cattle
tracks (e.g. Georgiadis, 1987; Hiernaux, 1996). More illuminating – but much
more difficult to acquire – are two types of evidence: (1) remote sensing studies
at the landscape scale, tracking the extent of degradation or desertification
and of sufficient duration to cover drought and non-drought periods; and
4 Definitions of desertification, overgrazing and degradation are controversial and
problematic. We use the term desertification here to refer to the concepts used by the cited
global assessment (GLASOD, 1990). We use degradation to mean an irreversible change in
ecosystem state or function. We agree with de Queiroz (1993) that “degradation” has been
defined relative to human management objectives and thus is relative; for example, a change
from grassland to bushland is “degradation” to a cattle -keeper but may be “aggradation”
from the perspective of carbon sequestration . We would prefer a set of quantitative measures
that can assess the state of a particular piece of land and be used by land managers to assess
the desirability of different changes from the context of their own management objectives.
We define overgrazing here as any level of herbivore grazing that induces either temporary
or permanent changes in the species composition or function of a grassland.
5 Carrying capacity is exceeded in 19% of areas receiving 0–200 mm rainfall per annum, 15%
of the 200–400 mm zone, 3% of the 400–600 mm zone and 8.5% of the 600–800 mm zone
(Ellis et al., 1999).