Drylands are expansive ecosystems covering a significant portion of Earth’s land surface. These environments are globally distributed and defined by specific climatic conditions that shape their features. Understanding these regions is important due to their vastness and the ways life adapts within them.
Defining Drylands and Their Global Presence
Drylands are defined by an aridity index (AI), the ratio of average annual precipitation (P) to potential evapotranspiration (PET). An AI of less than 0.65 classifies an area as a dryland, indicating water loss through evaporation and plant transpiration significantly exceeds rainfall. This negative water balance results in water scarcity, often coupled with high evaporation rates and extreme temperatures.
These regions span diverse biomes, including deserts, steppes, savannas, grasslands, and scrublands, covering over 40% of Earth’s land surface. Major dryland areas are found across all continents, with extensive stretches in Africa, Asia, Australia, and parts of North and South America. Drylands are further categorized into hyper-arid (AI < 0.05), arid (AI 0.05–0.20), semi-arid (AI 0.20–0.50), and dry sub-humid (AI 0.50–0.65) zones, reflecting a gradient of moisture availability. Hyper-arid regions, like the Sahara Desert, are the driest and generally do not support crop growth without irrigation.
Life Thrives in Drylands: Adaptations and Biodiversity
Life in drylands demonstrates adaptations to water scarcity, intense heat, and limited nutrients. Plants, known as xerophytes, employ strategies to conserve water, reduce loss, and absorb available moisture. Many succulents, such as cacti and agaves, store water in their fleshy stems or leaves, while others, like the saguaro cactus, develop extensive shallow root systems to quickly capture infrequent rainfall. Some plants also exhibit Crassulacean Acid Metabolism (CAM), opening their pores for gas exchange only at night to minimize water loss through transpiration.
Animals in dryland environments also display physiological and behavioral adaptations. Many desert rodents, for example, have low resting metabolic rates, which helps them conserve energy during food shortages. Animals like the greater roadrunner possess specialized nasal glands to remove excess salt from consumed water, and some species, such as the sidewinder snake, modify their behaviors to cope with extreme temperatures and find shelter. These adaptations contribute to the rich biodiversity found in drylands, supporting species evolved for these challenging conditions.
Human Interaction and Ecological Significance
Drylands have a long history of human habitation, supporting diverse communities for thousands of years. Traditional livelihoods in these regions often involve pastoralism, where communities manage livestock like cattle, sheep, and camels, relying on rangelands for forage. These communities have developed intricate resource management strategies, including drought-resistant crop cultivation, water harvesting techniques, and nomadic movements in response to climatic shifts. Such practices highlight the deep understanding and resilience of dryland populations.
Drylands provide ecological services that extend beyond their borders, supporting global well-being. They play a role in carbon sequestration, storing carbon in their soils and vegetation, which helps regulate global climate. These areas also harbor unique genetic resources, including wild relatives of many cultivated plants, contributing to global biodiversity. Furthermore, drylands support a substantial portion of the world’s livestock and agricultural production, making them important for global food security.
Environmental Changes Affecting Drylands
Drylands face environmental challenges, with desertification being a major concern. Desertification refers to the degradation of land in arid, semi-arid, and dry sub-humid areas, reducing biological productivity. This process is driven by a combination of factors, including land mismanagement and climate variability. Poor agricultural practices, such as over-cultivation, lack of crop rotation, and reliance on chemical fertilizers, can deplete soil fertility and increase erosion.
Overgrazing by livestock can also erode land and prevent vegetation regeneration, leaving soil exposed to wind and water erosion. Climate change exacerbates these issues, contributing to rising temperatures and altered rainfall patterns. Warmer temperatures increase evaporation, making it harder for plants to retain moisture, while changes in precipitation can lead to more intense droughts or torrential downpours that wash away unprotected topsoil. These shifts can lead to habitat loss and threaten dryland communities.