- Drylands are areas which face great water scarcity. They cover over 40% of the earth's land surface, and are home to more than two billion people.
- They are highly adapted to climatic variability and water stress, but also extremely vulnerable to damaging human activities such as deforestation, overgrazing and unsustainable agricultural practices, which cause land degradation.
- Land degradation in drylands is known as desertification, and is the loss of the biological or economic productivity of land.
- Desertification reduces agricultural output, contributes to droughts and increases human vulnerability to climate change.
- The loss of biodiversity in drylands, including bacteria, fungi and insects living in the soil, is one of the major causes and outcomes of land degradation.
- Restoring rangelands and sustainable land management practices can preserve drylands biodiversity, restore ecosystem functions, and halt land degradation.
Drylands support an impressive array of biodiversity. This includes wild endemic species – such as the Saiga Antelope in the Asian steppe and American bison in the North American grasslands that do not occur anywhere else on earth – and cultivated plants and livestock varieties known as agrobiodiversity. Biodiversity in drylands also includes organisms which live in the soil, such as bacteria, fungi and insects – known as soil biodiversity – which are uniquely adapted to the conditions. Soil biodiversity comprises the largest variety of species in drylands – determining carbon, nitrogen and water cycles and thereby, the productivity and resilience of land. The loss of biodiversity in drylands is one of the major causes and outcomes of land degradation.
Food and water provision
Low precipitation and prolonged dry seasons in drylands can lead to water scarcity, and limit agricultural productivity and output. Drylands biodiversity maintains soil fertility and moisture to ensure agricultural growth, and reduces the risk of drought and other environmental hazards. For example, vegetation is decomposed in the stomachs of large herbivores in the drylands, after which the dung is transformed into nutrients by bacteria in the soil, which are absorbed by plants. Bacteria and other microbes also break down plants and animals into decomposing residues – soil organic matter, which helps the soil easily absorb rainwater and retain moisture. Each gram of organic matter can increase soil moisture by 10-20 grams, and each millimetre of additional infiltration of water into the soil represents one million additional litres of water per square kilometre.
Poor crop and soil management, and habitat destruction undermine the ability of drylands biodiversity to perform nutrient recycling, and water storage and filtration services. On severely degraded land – devoid of biodiversity – as little as 5% of total rainfall may be used productively. An estimated 20 million hectares of fertile land is degraded every year, and in the next 25 years global food production could fall by up to 12% as a result of land degradation – threatening the food and water security of the rising human population.
Climate change mitigation and adaptation
The world’s soils contain 1,500 billion tons of carbon in the form of organic matter – two to three times more carbon than is present in the atmosphere. The carbon stored in soil is released into the atmosphere when land is degraded, and about 60% of the earth’s organic carbon has been lost through land degradation. This represents a significant contribution to man-made greenhouse gas emissions. Increasing the quantity of carbon contained in soil, for example through agriculture and pasture management practices which increase soil organic matter, can reduce the annual increase in carbon dioxide in the atmosphere. It is estimated that improved livestock rangeland management could potentially sequester a further 1,300-2,000 million metric tons of carbon dioxide by 2030.
Climate change will also impact drylands, with models predicting even more climatic variability and extreme temperatures. Biodiversity in drylands has adapted over millennia to the seasonality, scarcity and variability of rainfall, and can be useful in helping people adapt to climate change. For example, the unique species in drylands provide a genetic reservoir for new varieties of cultivated plants and livestock breeds, which are resilient to the climatic variations.