ABSTRACT
Drylands cover about 41% of the earth’s land surface, comprising hyperarid to dry subhumid climate zones that are defined by low mean annual precipitation amounts compared to potential evaporation, that is, a ratio of mean precipitation to potential evaporation less than 0.65 (Thomas and Middleton, 1994; Safriel et al., 2005; see Figure 17.1). They include a large number of ecosystems that belong to the four broad biomes, forests, Mediterranean, grasslands, and deserts (Safriel et al., 2005), and are home to about one-third of the global population, with many residents directly depending on dryland ecosystem services including the provision of food, forage, water, and other resources (Millennium Ecosystem Assessment, 2005a). Drylands also provide ecosystem services of global significance, such as climate regulation by sequestering and storing vast amounts of carbon due to the large areal extent (Lal, 2004) (Table 17.1). <target id="page_419" target-type="page">419</target>The spatial extent of drylands based on the aridity index (AI equals ratio of rainfall (P) and potential evapotranspiration (PET) for the period 1951–1980). Hyperarid: P/PET < 0.05; arid: 0.05 ≤ P/PET < 0.20; semiarid: 0.20 ≤ P/PET < 0.50; and dry subhumid: 0.50 ≤ P/PET < 0.65. Goode homolosine projection. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429089435/9c570d80-7548-4c77-8c7f-d7be50eaec81/content/fig17_1.jpg" xmlns:xlink="https://www.w3.org/1999/xlink"/> (From UNEP (United Nations Environment Programme) The UNEP Environmental Data Explorer, as compiled from UNEP/DEWA/GRID, Geneva, Switzerland, 2014, https://geodata.grid.unep.ch, ESRI Data & Maps.) Key Dryland Ecosystem Services https://www.niso.org/standards/z39-96/ns/oasis-exchange/table">
Supporting Services: Services That Maintain the Conditions for Life on Earth
Soil development (conservation, formation)
Primary production
Nutrient cycling
Biodiversity
Provisioning Services: Goods Produced or Provided by Ecosystems
Regulating Services: Benefits Obtained from Regulation of Ecosystem Processes
Cultural Services: Nonmaterial Benefits Obtained from Ecosystems
Provision derived from biological productivity: food, fiber, forage, fuelwood, and biochemicals
Water purification and regulation Pollination and seed dispersal
Recreation and tourism Cultural identity and diversity
Fresh water
Climate regulation (local through vegetation cover and global through carbon sequestration)
Cultural landscapes and heritage values Indigenous knowledge systems
Spiritual, aesthetic, and inspirational services
Source: After Millennium Ecosystem Assessment, Ecosystems and human well-being: Desertification synthesis, World Resources Institute, Washington, DC, 2005a.
