TheImprovement ofTropical and Subtropical Rangelands

INTRODUOTION 17
Moreover, grain crops deplete soil nutrients at a rate thirty times
greater than the rate of nutrient lou in a properly stocked range
ecosystem (Heady, 1975). The cost of replacing the lost phosphorus,
potassium, nitrogen, and other nutrients is generally prohibitive.
In many regions, high levels of sustained use pressure have eliminated
the more palatable plant species (species referred to as "decreasers"
in range science). In dryland ecosystems, plant growth is
relatively slow. When aerial biomass is consumed by foraging livestock,
many plants respond by transferring nutrients from their roots
in order to produce new leaves. This results in reduced rooting. Reduced
rooting, in turn, reduces the ability of the plant to absorb
moisture and nutrients even during rains. As the more palatable
species are weakened, less palatable species ("increasers") become
dominant. With continuing high levels of use pressure, increasers
give way to undesirable shrubs, grasses, and forbs ("invaders"). As
these species are overgrazed, the land surface is exposed to further,
more severe, degradation. In the drylands of Africa and Asia,
cattle have been particularly destructive. Unlike camels and goats
and most native herbivores, which are predominantly browsers, cattle
are grazers. Cattle therefore increue pressure upon perennial
grasses and often eliminate them, causing ecological deflections toward
ephemeral annual grasses and relatively unproductive trees and
shrubs, such as Co.lotrop" procera (Gaston and Dulieu, 1976).
. The reduction or elimination of vegetative cover, in combination
with trampling and the compaction of the surface by livestock,
reduces infiltration and permits the mobilization ofsoil particles subject
to transport by overland flow. This results in depressed groundwater
tables and increased soil erosion. Surface exposure and the
reduced organic content of soils also result in altered soil-water relationships
and greater amplitude in soil temperatures. This altered
soil ecology adversely affects important soil microorganisms, such as
the rhizobial bacteria responsible for nitrogen fixation in acacias and
other leguminous genera. This, in turn, affects nutrient regimes and
results in a further lou of soil structure. Altered soil ecology directly
eliminates additional plant species and frustrates regenerative
processes in others. Further losses occur through disruptions in various
biological dependency and affinity relationships. Environmental
degradation both reduces range carrying capacity for livestock and
affects wildlife populations through habitat modification. Rangeland
conditions in selected countries of Africa, Asia, and Western Asia are
described in tables 3, 4, and 5.