True deserts are found on all continents except the European subcontinent (see Figure 1). Altogether about 20% of landmass can be classified as desert, making it the largest biome on Earth. Table 1 gives an overview of the largest deserts. In addition to these major deserts, many smaller, separately named deserts exist; many of these can be classified as local rain shadow deserts. All desert areas of the world border on land semiarid zones. These are either Mediterranean-type climate and vegetation, or dry temperate or tropical grasslands/savannas. The vicinity to these areas is important as many desert organisms are either shared with these transitional biomes or evolved from similar more mesic organisms.
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