figure 21-7

All of the organisms of the desert biome are adapted to endure dry, often hot conditions by conserving energy and water. This saguaro cactus is full of water, which it has stored from the infrequent rains in the desert.

Deserts are areas that receive an average of less than 25 cm (9.9 in.) of rainfall per year. Large parts of North Africa, central Australia, southwestern North America, and eastern Asia are desert. Contrary to popular belief, deserts are not hot all the time. So-called cold deserts, such as the Great Basin in the western United States and the Gobi in eastern Asia, are hot in summer but cold in winter. Even in hot deserts, temperatures may fall by as much as 30°C (54°F) at night because the dry air is a poor insulator, allowing the heat that builds up during the day to escape.

Desert vegetation is often sparse and consists mainly of plants that have adapted to the dry climate. The leaves of the creosote bush of the United States and Mexico, for example, have a waxy coating that reduces evaporation. Plants in moist environments normally allow a great deal of water to escape through evaporation and transpiration from stomata, tiny openings in their leaves. In contrast, desert plants may have few stomata or may open the stomata only at night. A common type of desert plant is a cactus, such as the saguaro cactus shown in Figure 21-7. It has an expandable body that can store water and leaves that have evolved into sharp spines that protect the plant from herbivores. A single saguaro can hold about 1,000 kg (2,200 lb) of water. The saguaro cactus is found in Arizona, California, and Mexico.

Like desert plants, desert animals must conserve water. Many animals avoid the heat of the day by hiding in small spots of shade or by burrowing into the ground. Others, such as kit foxes and some kinds of lizards and snakes, are active only at night, when loss of water to evaporation is low.

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