The basic materials for a good life for most dryland people have their origin in biological productivity. More people in drylands than in any other ecosystem depend on ecosystem services for their basic needs. Crop production, livestock and dairy production, growth of fuelwood, and construction materials all depend on plant productivity, which in drylands is constrained by water availability. Thus it is the dryland climate that constrains viable livelihood opportunities. Practices like intensified cultivation in areas that do not have an adequate level of supporting services (soil fertility, nutrients, and water supply) thus require adjustments in management practices or costly imports of nutrients and water (C22.5).
Fluctuation in the supply of ecosystem services is normal, especially in drylands, but a persistent reduction in the levels of all services over an extended period constitutes desertification. Large inter-annual and longer-term climatic variations cause ﬂuctuations in crop, forage, and water yields. When the resilience of a dryland ecosystem is impaired and it does not return to the expected levels of service supply after the stress is removed, a downward spiral of degradation—in other words, desertification—may occur. Many mechanisms linked to this phenomenon have been documented for drylands: excessive loss of soil, change in vegetation composition and reduction in vegetative cover, deterioration of water quality and reduction in available quantity, and changes in the regional climate system. A schematic description of the pathways that lead to desertification is provided on the left side of Figure 1.1.. The intensity and impact of these mechanisms vary from place to place and change over time; they depend on the level of aridity and the varying pressure exerted by people on the ecosystem’s resources (C22 Figure 22.7; SAfMA).
Measurement of persistent reduction in the capacity of ecosystems to supply services provides a robust and operational way to quantify land degradation and desertification.The international community, through the United Nations Convention to Combat Desertification, agreed to define desertification as land degradation in arid, semiarid, and dry subhumid lands. Land degradation is in turn defined as a persistent reduction of biological and economic productivity. It is therefore logical to measure productivity in terms of the “things that ecosystems provide that matter to people”—that is, ecosystem services. (See Table 1.1. Key Dryland Ecosystem Services for a list of key dryland ecosystem services.) Doing so makes degradation quantifiable in an operational way, since many of the ecosystem services are measurable and some are routinely monitored. Furthermore, such an approach is robust, because it is based on ﬂow of services to a broad spectrum of people rather than a narrow range of beneficiaries (CF2, SAfMA).
The coping capacity of the affected population and the resilience of the ecosystem on which it depends determine the duration beyond which impaired services cause irreversible consequences. Dryland people have found ways of coping with periods of scarcity lasting up to several years. However, periods significantly longer than this can overwhelm their resources and adaptation strategies. Their capacity to cope with a shortage of services for extended periods can be increased by many factors, including demographic, economic, and policy factors (such as the ability to migrate to unaffected areas) and the time that has elapsed since the last stress period (C6).
A downward spiral of desertification may occur but is not inevitable, as shown on the right side of Figure 1.1. Understanding the location-specific interaction of socioeconomic and biophysical processes is critical. Some earlier explanations of irreversible desertification may have their origin in two fallacies. First, the time scale over which desertification evaluations are conducted is often too short, and reliable long-term extrapolations cannot be obtained. It is also important to consider continuous changes in dryland processes resulting from climatic factors and human intervention. Second, the spatial scale of assessments is either too large to effectively capture local phenomena or too local to provide a regional or global perspective. For example, desertification assessments rely on evaluation of national, regional, and continental soil surveys, on models of carrying capacity, on experimental plot studies, on expert opinion, and on nutrient balance models. While each of these methods is sound in its own right, the findings cannot simply be scaled up or down in time and space (C22.4.1).
Degradation is possible and observed in hyper-arid areas, which are not formally included within the UNCCD. The hyper-arid zone does not fall within the scope of the convention based on the argument that deserts are naturally low in productivity and cannot be further desertified. However, even hyper-arid areas have measurable levels of ecosystem service provision and support a human population with low density but significant numbers. Desertification has also been observed in hyper-arid areas, where mechanisms of degradation are similar to those in other dryland areas (C22.4.1).
Inland water, urban, cultivated, and other systems are integral parts of drylands and thus are critically linked to desertification processes. There are many systems embedded within drylands that are essential for the viability of the system as a whole and for livelihoods based on drylands. (In the MA, “system” is used to describe reporting units that are ecosystem-based but at a level of aggregation far higher than usually applied to ecosystems. The system also includes the social and economic elements. For example, the MA refers to “forest systems,” “cultivated systems,” “mountain systems,” “urban systems,” and so on. Systems thus defined are not mutually exclusive, and are permitted to overlap spatially or conceptually.)
Table 1.1. Key Dryland Ecosystem Services
In particular, the inland freshwater ecosystems within drylands—rivers, lakes, impoundments, wetlands, and so on— with their high potential for providing ecosystem services are of critical importance. Cultivated lands are a substantial part of the dryland landscape; about 44% of all cultivated systems world-wide are located within drylands, especially in the dry subhumid areas. (See Figure 1.2) Conversion of rangelands to cultivated lands, especially in arid and semiarid drylands, leads to trade-offs in long-term sustainability of services and livelihood generation for people. Although urban systems occupy a relatively small fraction (about 2%) of the area of drylands, they contain a large and rapidly increasing fraction (nearly 45%) of the dryland population. Significant fractions of coastal (9%) and mountain (33%) systems are classified as drylands, highlighting the need for integrated land and water management that gives due consideration to dryland perspectives (C26.1.2., C27).