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

C.J. WILSON
The changing face of pastoral systems in grass-dominated ecosystems of eastern Africa 53
Plate 2.9
Heavily grazed grassland in the highlands near Bule, Ethiopia.
stability. It is considered as a primary tool in range management , and flexibility
of grazing intensity is critical to rangeland ecosystem health. Grazing intensity
has a major impact on range condition (Plate 2.9). In a recent study in Ethiopia,
range condition, based on the herbaceous layer, basal cover, litter cover, relative
number of seedlings, age distribution of grasses, soil erosion and soil compaction,
was higher in lightly grazed areas in the Rift Valley than in the heavily
grazed communal lands (Sisay and Baars, 2002). In Serengeti and Maasai Mara,
grazing was found to stimulate net primary productivity at most locations,
with maximum stimulation at intermediate grazing intensities and declines at
high levels of grazing. Stimulation was dependent upon soil moisture status at
the time of grazing (McNaughton, 1985).
There are many examples in the literature of the impact of management on
species composition and diversity (Herlocker, 1999). In a study in the rangelands
of southern Ethiopia, perennial grasses were relatively resilient in terms
of cover and productivity in response to grazing , while continuous grazing
encouraged forbs with lower grazing value for cattle (Coppock, 1994). Grazing
affects species diversity and richness in grasslands (Oba, Vetaas and Stenseth,
2001). Optimal conservation of plant species richness was found at intermediate
levels of biomass production and was found to decline if biomass increased
in ungrazed areas of arid -zone grazing lands in northern Kenya. These intermediate
levels can be achieved by manipulating management and grazing
pressure , although there will be seasonal fluctuations due to environment. In
the Serengeti Plains in Tanzania , elimination of grazing led to dominance by