In speaking of the resilience of an arid ecosystem from the perspective of sustainability, we must consider the following components: (1) the capability of soil particles to bind rather than become dustlike (a function of: soil organic matter, clay fraction, vegetation cover, human technological prowess); (2) the ability of the land surface to resist water erosion (a function of: slope, vegetation cover, human technological prowess); (3) the ability of microorganisms and plant propagules to estivate or lay dormant during times of adverse conditions (a function of: species diversity, genetics, vegetation cover, human technological prowess); and (4) human pressure. Periods of drought stress arid ecosystems, but species inhabiting these regions have adapted to this stress. Even the human species has adapted; cultural ecologists have investigated the various coping mechanisms of many arid-lands indigenous peoples, and found that they are ubiquitously well-adapted and resilient to perturbations of various sorts (see, e.g., Netting, 1986).
Dryland ecosystems and production systems use various strategies – especially the spatial and social spread of risk – to track climatic variations. Thus, the resilience of arid ecosystems and production systems is definable simply as the ability to track rainfall, and thus to rebound to full bloom from apparent lifelessness when the rains are adequate. Any less than a return to “full bloom” status implies some fragility within the system, and suggests some degree of irreversibility.
Schlesinger et al. (1990) suggest how overgrazing can disrupt the pulse-reserve strategy that enables arid ecosystems to resiliently survive extreme variability. In the U.S. Southwest's undisturbed perennial grasslands, the ground surface generally has vegetation cover of some kind, thus slowing runoff from rain events and enhancing infiltration. Relatively uniformly in a spatial sense, soil reserves of water and nutrients slowly build up. Intensive grazing not only removes vegetation selectively, but changes the soil surface: trampling compacts the soil, thus increasing runoff, which in turn results in increased soil erosion and nutrient removal. The net effect is to reduce the spatial homogeneity of soil moisture and nutrients. Shrubs, rather than grasses, are better adapted to exploit spatially heterogeneous moisture and nutrients, which at this stage are often found deeper in the soil profile. The biogeochemical cycling of plant nutrients is subsequently and increasingly restricted to the less-harsh environment directly beneath the shrubs, leading to the development of “islands of fertility” in between which nothing grows. In southern New Mexico, the net primary productivity of desert shrubland is similar to that of grassland.
Hutchinson (1996) suggests that as soil moisture and nutrient reserves are drawn down or eliminated in the Sahel, plants and animals (including humans) develop an increasing dependence on the infrequent and sporadic pulses that accompany rainfall. The system becomes ever-more reliant on annual grass species, which appear only after rainfall events. Demands for woody shrubs for browse and fuelwood in the Sahel accelerate the conversion process. Reduction in overall soil organic matter now feeds forward into concomitant decrease in soil water-retention capacity and nutrient recycling, which increases the spatial heterogeneity of these reserves. It is difficult to conceive how to avoid labeling such a downward spiral in the drylands as “desertification.”
DeAngelis, D.L., S.M. Bartell, and A.L. Brenkert, 1989. Effects of nutrient recycling and food-chain length on resilience. The American Naturalist 134:5, 778-805.
FEWS, 1994. Vulnerability Assessment (July). Washington, D.C.: U.S. Agency for International Development, Bureau for Africa, Disaster Response Coordination, Famine Early Warning System.
Helldén, U., 1991. Desertification—time for an assessment? Ambio 20:8, 372-383.
Hutchinson, C.F., 1996. The Sahelian desertification debate: A view from the U.S. Southwest. Journal of Arid Environments 33:4, 519-524.
Kumar, M. and M.M. Bhandary, 1993. Impact of human activities on the pattern and process of sand dune vegetation in the Rajasthan Desert. Desertification Control Bulletin 22, 45-54.
Netting, R.M., 1986. Cultural Ecology (second edition). Prospect Heights, IL: Waveland Press.
Parker, K.C. 1993. Climatic effects on regeneration trends for two columnar cacti in the northern Sonoran Desert. Annals of the Association of American Geographers 83:3, 452-474.
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.
Sears, P.B., 1959. Deserts on the March (third edition, revised). Norman: University of Oklahoma Press.
UNEP, 1993. Agenda 21, Chapter 12: Managing fragile ecosystems: combating desertification and drought. Desertification Control Bulletin 22, 9-18.
The Table of Contents of my work on desertification and food security is available.
This site last updated August 10, 1997.