Walia Special Edition on the Bale Mountains (2011) - Zoologische ...

evidence that low levels of grazing can be beneficial to vegetation growth and can maintain higher
vegetation diversity (Harper 1969). For instance, although grazers can cause the local extinction of
those species on which they preferentially feed, thereby reducing species diversity (Harper 1969;
Waser and Price 1981), grazers can potentially increase species diversity by reducing the densities
of competitively dominant keystone species where the reduced abundance of a superior competitor
allows competitively inferior species to coexist (Harper 1969; Samson, Philippi et al. 1992). With
increasing livestock grazing intensity, a shift from shrubs and perennial grasses to annual grasses and
forbs is frequently observed, and removal of perennial plants is usually associated with increased
soil erosion (Mwendera et al. 1997). Pasture land properties are also altered by livestock grazing
since the reduction of vegetation cover below a critical level can increase the impact of raindrops,
decrease soil organic matter and soil aggregrates, increase surface soil crusts and decrease water
infiltration rates (Humphreys 1991).
Exploitative competition between livestock and native ungulates and/or small mammals like
rodents often takes place and increases if diets or habitats overlap (Steen et al. 2005). If competition
for habitat and food is important, and particularly if resources are limited, rodent population growth
should decrease with increasing ungulate densities (Tokeshi 1999). Decreases in rodent populations
have been attributed to reduction in vegetation cover (Grant et al. 1982; Keesing and Crawford 2001)
resulting in an increased predation risk (Flowerdew and Ellwood 2001) and decrease in suitable food
resources (Steen et al. 2005). On the other hand, increases in rodent populations under moderate and
light grazing regimes have been attributed to grazing-induced increased plant growth and prolonged
growing season (Rhodes and Sharrow 1990; Jones and Longland 1999). Scattered observational
evidence in the literature from salt deserts to high arctic ecosystems report both increases (Jones
and Longland 1999; Keesing and Crawford 2001) and decreases in the activity or abundance of
rodents when subject to ungulate grazing (Jones and Longland 1999; Weickert et al. 2001). In some
instances, respective increases or decreases depended on the habitat type (Hanley and Page 1980;
Hewson 1982) or rodent species involved (Hanley and Page 1980; Jones and Longland 1999).
As a result it is often difficult to predict accurately how grazers will affect species diversity
in any given situation (Huntly 1991) because in many systems the influence of grazers on species
diversity depends largely on the density and activity of grazers (Harper 1969; Paine and Vadas 1969).
This suggests that several mechanisms are operating within such systems and that an experimental
approach beyond a comparison of ‘‘grazed versus ungrazed’’ is needed to test these hypotheses.
Overgrazing in BMNP
Overgrazing in Ethiopia is partly the result of the transition of transhumant and nomadic pastoralists
to a sedentary lifestyle. This transition progressively took place at a time when the pastoralists had
been prohibited for a long time from travelling large distances in search of forage as a result of
ethnic conflicts and the Derg government’s attempt to restrict the movement of the population within
the country (Jacobs and Schloeder 2001). Many significant and widespread environmental changes
<strong>Walia</strong>-<strong>Special</strong> <strong>Edition</strong> on the Bale Mountains 199