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

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Grasslands of the world
Patagonia, transitions in the plant community are frequently close-to-irreversible
and involve not only a reduction in forage biomass for livestock
but also a decrease in water-use efficiency that leads to an overall decline in
ANPP (Aguiar et al., 1996). In some instances, degradation involves permanent
physical changes in soils, resulting in shifts in the soil texture of superficial
layers (Oliva, Bartolomei and Humano, 2000).
Although alternative steady states of vegetation in Patagonia have been
described with a reasonable level of detail, the factors that trigger transitions
from one state to another have not been tested under controlled experimental
conditions (Bertiller, 1993b). Overestimation of carrying capacity , uneven
distribution of sheep in large pastures and year-long continuous grazing have
been suggested as possible factors responsible for vegetation degradation over
the last 50 years (Golluscio, Deregibus and Paruelo, 1998).
An analysis of peak sheep numbers in the province of Santa Cruz shows
that pioneering sheep farmers did overestimate the carrying capacity of
the system. This is especially true of operations on the semi -deserts of
the central plateau (Unit Kg 11 in Figure 4.5), where stocking rates were
consistently 60 percent above the estimated carrying capacity (Oliva et al.,
1996) and the current sheep population has fallen well below the expected
carrying capacity of the system (Cibils, 2001). Most of the variability in
sheep numbers between 1931 and 1960 on the drier grass steppes north of
the Straits of Magellan can be explained by the variation in MAP (r2 = 0.97,
p = 0.02; Cibils and Coughenour, 2001), a variable that has been shown to
relate linearly to ANPP (Sala et al., 1988; Paruelo and Sala, 1995). After
1970, this relation disappears, giving way to a second period in which
sheep numbers decrease despite an apparent phase of overall increase in
MAP (Cibils and Coughenour, 2001). Interestingly, 1960–1970 was the
driest decade in the century; there could have been a breaking point in the
system sometime around or after this decade as a consequence of excessive
stocking combined with prolonged droughts that caused a significant shift
in the relation between rainfall (or ANPP) and sheep numbers (Cibils and
Coughenour, 2001).
The spatial distribution of sheep grazing in very large pastures is very
uneven and primarily related to the distribution of watering points (Lange,
1985). Sheep densities at a given point within a pasture can vary from 8 to
0.02 times the mean density assigned to the whole pasture (Lange, 1985).
Although there have been few efforts to measure grazing distribution in
Patagonian-type pastures, there is circumstantial evidence that confirms the
occurrence of the grazing patterns described by Lange (1985). Heterogeneous
grazing distributions generate local areas of degradation that can trigger
severe erosion, particularly when highly-affected areas are sensitive sites,
such as riparian meadows (Borrelli and Oliva, 1999; Golluscio, Deregibus
and Paruelo, 1998).