Up to 44% of all the world's cultivated systems are in the drylands. Plant species endemic to the drylands make up 30% of the plants under cultivation today. Their ancestors and wild relatives still grow here. Traditionally, drylands have been largely used for livestock, but they are increasingly being converted into cropland. Rangelands support 50% of the world livestock and are habitats for wildlife. Livestock production is dominant in the more arid zones. Cropland dominates in the dry sub-humid areas.
The livelihoods of more than 1 billion people in some 100 countries are threatened by desertification. Nearly1billion of the poorest and most marginalized people, who live in the most vulnerable areas, may be the most severely affected by desertification. The Millennium Assessment found that in general, the human well-being of dryland peoples is lower than that of people in other ecological systems. For example, compared to other ecosystems, infant mortality rates are highest in drylands and gross national product (GNP) per capita lowest. This implies that drylands are home to populations with comparatively low levels of well-being.
The total drylands population is 2.1 billion, meaning they are the home to one in three people in the world today. According to UN the Habitat, the 18.5% population growth rate in the drylands was faster than that of any other ecological zone. Population density increases as aridity decreases. It ranges from 10 people per square kilometer in the deserts to 71 people in the dry sub-humid (rangelands) areas.
When land degradation occurs in the drylands, it is referred to as desertification. It is generally reasoned that desertification is most likely to occur in regions about halfway between the high- and low-density areas. A 2009 study by the International Food Policy Research Institute reveals wide scale restoration of forests in densely populated regions of Burkina Faso and Niger through low-cost farmer-managed restoration activities.
Aridity is associated with water availability or scarcity. It matters for human well-being and the two key functions of land, namely, primary production and nutrient recycling. Water scarcity, the gap between its demand and supply, is highest in the drylands. Water scarcity increases with an increase in aridity. For basic well-being, each person requires a minimum of 2,000 cubic meters of water per year. Drylands people have access to 1,300 cubic meters only, and availability is projected to decrease. Today, water scarcity affects between 1-2 billion people, most of them in the drylands. Under the climate change scenario, nearly half of the world's population in 2030 will be living in areas of high water stress. In some arid and semi-arid areas, it will displace up to between 24 million and 700 million people.
The status of species in the drylands remains unknown, as no assessment exists to date. About 8% of the drylands are protected, which is comparable to an average of about 10% in other ecosystems. The Millennium Assessment reports that 8 of the 25 global hotspots are in the drylands. These are areas where 0.5% of the plant species are endemic to the region but habitat loss exceeds 70%.
Drylands play a vital role in local, but also global climate regulation. Land use change releases greenhouse gases into the atmosphere, while soil improvement is essentially a reverse process. It is sucking excess atmospheric carbon into the soil. Drylands store approximately 46% of the global carbon share. Its soils contain 53% of global soil carbon and its plants 14% of the global biotic carbon. Land rehabilitation practices such as mulching, composting, manuring and mixed cropping and reforesting, which increase carbon stocks in the soil, contributing directly to soil carbon sequestration. These techniques are also part of the technology toolbox known as sustainable land management (see below).
There is no linear cause-effect process that leads to land degradation in the drylands, but its drivers, which interact in complex ways, are known. They change over time and vary by location. The direct drivers are climatic, especially low soil moisture, rainfall patterns and evaporation. The indirect drivers are mostly human derived, and include poverty, technology used, global and local market trends and sociopolitical dynamics. Poverty is both a cause and consequence of land degradation.
Applying sustainable land management practices helps to combat desertification and to recover and rehabilitate land; soil, water and vegetation. Sustainable land management refers to the multi-functional use of land, and is contrasted to mono-functional land uses. The application of SLM has been shown to increase yields by between 30-170%. The land lost annually could produce 20 million tons of grain. Desertification and degradation represents an income loss of US$42 billion per year.
source : UN-DDD - Decade for Deserts and the fight against Desertification