TheImprovement ofTropical and Subtropical Rangelands

6 OVERVIEW
adjacent to and within dry rangelands, the encroachment of agriculture,
or drought. Improper grazing usually follows these events on
constrained range resources. Continued high animal density accelerates
the removal of palatable species and the lack of competition
permits the growth of species that are less palatable or less capable
of supporting livestock on a sustainable basis. In many areas of the
tropics and subtropics, the pressure of heavy use has caused ecological
deflections from predominantly perennial grasses to annuals,
resulting in a sharp reduction of late dry season carrying capacity.
Woody shrubs, whose roots compete for soil moisture with the remaining
grasses, also increase in number. In many instances, the
woody vegetation is cut for fuel and shelter, thereby limiting the
fodder contributions of the vegetation. Wind and water assault the
exposed soil, causing cyclical flooding and erosion, eventually leaving
the land to become bare and sandy. The overuse of vegetation
by man and his livestock reduces the possibility of natural reseeding.
The complex mixture of native species that are palatable or desirable
becomes locally simplified as species become "extinct." In naturally
semiarid regions, this leads to what is becoming known commonly
as desertification-the simplification of the ecosystem to the point
where its biological productivity and diversity are reduced to a minimum,
and it can no longer sustain man and his animals in more than
a casual, seasonal, and very limited fashion.
In areas such as the West African Sahel, there has been speculation
that a progressive ecological downturn in productivity may
be occurring in a highly variable climatic region that until recently
experienced unusually favorable rainfall when viewed on an annual
basis. However, the consensus of informed opinion is that to date
there is insufficient evidence for attributing recent droughts to a major
climatic change. These climatic cycles are very long term, and
reconstruction of paleoclimate is difficult in the absence of historical
records of rainfall fluctuation. Climatic cycles are perhaps 400-500
years long, with internal shorter cycles on the order of 70 years, and
phases of 25-40 years (National Research Council, 1983b).
On the other hand, there is short-term evidence that the relationship
between land abuse by man and his livestock, resulting in
ecological deterioration, is more direct and irreversible than is generally
appreciated. There is also evidence of low soil fertility, which
continues to keep productivity low when combined with low rainfall
(Breman and deWit, 1983). Historical review of former levels of
vegetation, productivity, and land-use indicators shows that much