Water is an essential ingredient for life on Earth and its scarcity drives extraordinary biological, and often cultural, diversification. Drylands are defined by water scarcity and characterised by seasonal climatic extremes and unpredictable rainfall patterns. Yet despite their relative levels of aridity, drylands contain a great variety of biodiversity, much of which is highly adapted to dryland ecology. As a result there are many animal and plant species and habitats found only in drylands: some semi-arid and dry sub-humid areas are among the most biodiverse regions in the world. Diversity is also great within drylands, for example between areas of different aridity, temperature or altitude. Species have adapted to these factors in many unique ways creating a variety of habitats that are essential to the survival of species as well as to the livelihoods of people. Wetlands, forests, mountains and other Habitat types provide vital resource patches within the drylands. Species richness is generally lower in the drylands than in tropical forests but within-species diversity may be higher due to this diversity of habitats and the isolation of populations. Some drylands, however, such as North America's tall grass prairie, are among the most productive vegetation types in the world.