Desertification: Reversibility vs. Irrecuperability
For the Sudan, Helldén (1991) summarizes Lund University's 15-year study, which combined remote-sensing techniques, air photos and reconnaissance flights, national demographic, production, and climate statistics, extensive field observations, historical accounts from the beginning of the century, and GIS-based spatial modeling. The whole country was observed by satellite from 1980 - 1987, and 7-km resolution normalized difference vegetation indices (NDVI) calculated for each 10-day dekad. Not only did the desert margin march south and retreat, but the southerly high-productivity zone also marched in step, retreating southward during drought years and marching northward during wet years. In concurring with the Tucker et al. (1991) study, Helldén concludes that “most of the negative and positive annual deviations from the mean annual production of major natural and rainfed agricultural production systems of the Sudan can be explained by climatic variations, without considering possible adverse impacts on the environment by man.”
However, there is not necessarily a uniform consensus among the members of the Lund University research group. Olsson and Rapp (1991) address the northward movement of grassland following the 1982-84 drought. “Recovery can be attributed in part to increases in rainfall, but it is important to note that rainfall during the period 1985 - 1987 remained below the long-term average for the region. Thus, it seems that an important contributor to the recovery has been the low level of exploitation in 1985 - 1986, owing to the large numbers of people and animals that had been wiped out during 1983 - 1984.... [T]he desertification that occurred in the study area [Kordofan Province] during 1982 - 1984 was sufficiently severe to require for its recovery a period of several years characterized by a combination of improved rainfall and reduced exploitation.”
Satellite Imagery and Ecosystem Processes
A basic contradiction exists when using remotely-sensed data for analysis of vegetation, and information derived from the analysis may be flawed because of misapplied assumptions. For example, arid and semiarid pastures may be more productive when they are grazed low. Up to a point, plants produce new and fresh growth if they are “encouraged” by being grazed, and may also be less vulnerable to drought if they are kept small (Skarpe, 1991). In other words, reducing vegetative ground cover might be interpreted as “desertification” when viewed from space, but paradoxically, at ground level, it is not. Conversely, the invasion of thorn scrub into heavily-grazed areas is regarded as land degradation; because of the increase in biomass, however, this appears on satellite images as an increase in vegetative cover (Pearce, 1992). This would therefore lead remote-sensing analysts to believe that the land degradation is being retarded or reversed, when it might actually be proceeding.
As Schlesinger et al. (1990) point out in reference to southern New Mexico, the desertification process at the study site has witnessed the replacement of native grasslands with invasive shrubs of similar net primary productivity. However, it's not a question of quantity, but of quality. The change in the quality of net primary production has lowered the economic potential of the landscape for pastoralism. Comparisons of overstocked versus less-intensely utilized savanna grazing systems in Zimbabwe indicated lower herbaceous basal cover, a larger proportion of bare ground, and higher plant mortality under overstocked conditions. Soil nutrients and water infiltration rates were also lower in the overstocked area, and soil erosion doubled (Skarpe, 1991). In the short term, provided that anthropogenic pressures are removed from such degraded areas, a single major rain event will often cause an ecosystemic rebound if the soil base has not been too badly damaged. But subsequent drought will see the dieback of the regrowth before it may have had the opportunity to go to seed. Such cycles of torrential rainfall followed by drought will deplete the soil seedbank irreversibly from a human perspective, though not from ecosystemic timescales. More typically, a regrowth flush will be soon grazed or browsed by livestock, often prior to setting seed (Sollod, 1990). As human population densities increase, therefore, so too will the number of livestock, and the probability of ongoing land degradation leading to desertification will concomitantly rise.
The 1968 - 1973 Sahelian drought was as destructive to livestock as the 1982 - 1984 drought that Olsson and Rapp (1991) refer to; Mattsson and Rapp (1991) state that millions of cattle died in the Sahelian zone. The figure below shows the typical pattern of livestock recovery after major die-offs; note that recovery of small livestock flocks precedes that of cattle herds. Pastoralist societies do not recover quite as quickly as some might assume, and neither does anthropogenic forcing of land degradation. It will take longer than Lund University's 15-year study to corroborate their assertion that desertification is a myth.
Standard deviations of Ngorongoro Crater Conservation Area livestock census data around a mean of 123,680 head for cattle and 113,795 head for small stock. Major drought occurred in this area of Tanzania from 1970 - 1973, 1975 - 1976, and 1983 - 1984. After major livestock die-offs following severe drought, households typically reacquire small stock before investing in cattle. Data source: McCabe, 1990.
Helldén, U., 1991. Desertification—time for an assessment? Ambio 20:8, 372-383.
Mattsson, J.O. and A. Rapp, 1991. The recent droughts in Western Ethiopia and Sudan in a climatic context. Ambio 20:5, 172-175.
McCabe, J.T., 1990. Turkana pastoralism: a case against the tragedy of the commons. Human Ecology 18:1, 81-101.
Olsson, K. and A. Rapp, 1991. Dryland degradation in Central Sudan and conservation for survival. Ambio 20:5, 192-195.
Pearce, F., 1992. Mirage of shifting sands. New Scientist, 12 December, 38-42.
Schlesinger, W.H., J.F. Reynolds, G.L. Cunningham, L.F. Huenneke, W.M. Jarrell, R.A. Virginia, and W.G. Whitford, 1990. Biological feedbacks in global desertification. Science 247, 1043-1048.
Skarpe, C., 1991. Impact of grazing in savanna ecosystems. Ambio 20:8, 351-356.
Sollod, A.E., 1990. Rainfall, biomass and the pastoral economy of Niger: assessing the impact of drought. Journal of Arid Environments 18, 97-107.
Tucker, C.J., H.E. Dregne, and W.W. Newcomb, 1991. Expansion and contraction of the Sahara Desert from 1980 to 1990. Science 253, 299-301.
The correct citation for this page is:
Milich, L., 1997. Desertification. http://ag.arizona.edu/~lmilich/rev.html.
The Table of Contents of my work on desertification and food security is available.
This site last updated August 10, 1997.