10.2.6.5. Impacts of Desertification
Desertification reduces soil fertility, particularly base cation content, organic
matter content, pore space, and water-retention capacity. Desertification also
reduces vegetative productivity, leading to long-term declines in agricultural
yields, livestock yields, plant standing biomass, and plant biodiversity. These
changes reduce the ability of the land to support people, often sparking an
exodus of rural people to urban areas. Breaking the strong connection of people
to the land produces profound changes in social structure, cultural identity,
and political stability.
In Niger, on farmed land where organic carbon in the top 10 cm of the soil
has fallen from 0.3 to 0.2% 4 years after coming out of fallow, millet yields
fell from 280 to 75 kg ha-1 (Bationo et al., 1993). Modeling
of the 4,000 km2 Mgeni River watershed in South Africa showed that
conversion of more than one-quarter of the watershed from forest and rangeland
to agriculture and exotic tree plantations since the area was colonized would
double mean annual runoff in urban areas and other areas of reduced land cover
(Schulze, 2000).
In the Senegal Sahel, the densities of trees with a height of >3 m declined
from 10 trees ha-1 in 1954 to 7.8 trees ha-1 in 1989;
the species richness of trees and shrubs fell from 16 species per 4 km-2
around 1945 to about 11 per 4 km-2 in 1993 (Gonzalez, 1997, 2001).
These changes have caused a 25-30 km shift of the Sahel, Sudan, and Guinean
vegetation zones in half a century, proceeding at an average rate of 500-600
m yr-1. Arid Sahel species expanded in the northeast, tracking a
concomitant retraction of mesic Sudan and Guinean species toward areas of higher
rainfall and lower temperature to the southwest.
In the Senegal Sahel, human carrying capacity in 1993 stood at approximately
13 people km-2 at observed patterns of resource use, compared to
an actual 1988 rural population density of 45 people km-2 (Gonzalez,
1997, 2001). This means that people with no other alternatives need to cut into
their natural resource capital to survive. Such changes across Africa have pushed
a rural exodus that may have displaced 3% of the population of Africa since
the 1960s (Westing, 1994).
Desertification also will cause conversion of perennial grasslands to savannas
dominated by annual grasses. Such changes have occurred in the Kalahari Gemsbok
National Park in South Africa, where Landsat imagery showed increases in exposed
soil surface (Palmer and van Rooyen, 1998). Such declines often are irreversible
(Schlesinger et al., 1990).
10.2.6.6. Vulnerability and Adaptationa
The tragic death of as many as 250,000 people in the Sahel drought of 1968-1973
(UNCOD, 1977) demonstrates the vulnerability of humans to desertification. As
desertification proceeds, agricultural and livestock yields decline, reducing
people's options for survival. Furthermore, not only do local people lose
the vital ecosystem services that dead trees and shrubs had provided; the loss
of firewood, traditional medicine species, and emergency food species render
them more vulnerable to future environmental change.
Adaptations by farmers and herders in Africa to climate change and desertification
have involved diversification and intensification of resource use (Davies, 1996;
Downing et al., 1997). Resourceful diversification responses by women
in Bambara and Fulbe households in Mali (Adams et al., 1998) reflect
the importance of women in guiding adaptation strategies across Africa. In southern
Kenya, Maasai herders have adopted farming as a supplement to or replacement
for livestock herding (Campbell, 1999). In Kano, Nigeria, peri-urban vegetable
gardening has expanded (Adams and Mortimore, 1997), revealing a common diversification
trend in small cities across west Africa. In northern Cameroon, Fulbe herders
have increased the number of herd displacements between pasture areas and even
resorted to long-distance migration, sometimes introducing significant changes
to their way of life (Pamo, 1998).
In the future, seasonal climate forecasting (NOAA, 1999; Stern and Easterling,
1999) may assist farmers and herders to know times of higher probability of
success of resource diversification or intensification. Seasonal forecasts for
Africa currently exhibit moderate skill levels (Thiaw et al., 1999) but
skill levels and user communications are not yet high enough to permit users
to confidently implement field applications (UNSO, 1999; Broad and Agrawala,
2000). Neither trade nor technology will likely avert the widespread nutritional
and economic effects of desertification through the 2020s (Scherr, 1999).
Other adaptations to desertification involve more efficient management of resources.
In Niger, farmers with access to credit will adopt low-cost, appropriate technologies
for wind erosion control, including windbreaks, mulching, ridging, and rock
bunds (Baidu-Forson and Napier, 1998). Across Africa, farmers traditionally
have adapted to harsh environmental conditions by promoting natural regeneration
of local trees and shrubs. Natural regeneration is a practice whereby farmers
and herders seek to reconstitute vegetative cover by setting aside parcels of
land or by selecting valued trees in their fields, pruning them, straightening
them, and raising them to maturity. The Sereer in Senegal (Lericollais, 1973)
and the Mossi in Burkina Faso (Kessler, 1992) have achieved doubling of tree
densities in certain semi-arid areas with Acacia albida and Butyrospermum parkii,
respectively.
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