The animal life of the desert depends on its plants, which are essential stores of
nutrition and, more importantly, of water. Greenery—or as is more often the
case in desert environments, brownery—is essential for maintaining a constant
supply of water. Some of the world’s greatest diversity and sheer numbers of
creatures occur in the moistest environments of the rain forests. Humans, too,
tend to live in moderate climate zones with plentiful supplies of water; in normal
conditions the human body needs to take in about 2.3 liters (4 pt.) of water a
day, equivalent to some 4 to 5 percent of total body weight.
Desert environments cannot support such a luxurious fluid intake, and animals
in arid regions are adapted to reduce their water requirements to as little as 1
percent of total body weight. Such adaptations are generally one of three major
kinds. Behavioral adaptations include such strategies as nocturnal foraging,
which enables many small desert animals to reduce water loss by avoiding the
dehydrating effects of hot sun. Anatomical adaptations might include a scaly
covering to reduce moisture evaporating from the body, while a common
physiological adaptation is the production of small volumes of very concentrated
urine and dry, cakelike feces.
As in any habitat desert life organizes into systems of feeding links called
food chains, which interconnect as food webs. At the base of every chain are
plants—called primary producers by ecologists—which provide nutrients,
minerals, and energy for the next link or level, the herbivorous animals. In the
desert environment these vary from ants, beetles, and bugs to small rodents and
seed-eating birds and to larger mammals such as gazelles and asses. Herbivores
in their turn become meals for the carnivores—the third link in the food chain.
Smaller carnivores such as lizards, spiders, and insectivorous birds then become
prey for larger predators, known as the top carnivores, such as cheetahs and
hawks. These are not, however, the final link. All animals, even top carnivores,
eventually die and provide nourishment for the numerous desert scavengers in
what are called the detritivore links of the food chains. In this way nutrients and
minerals are passed on, either directly to other animals or by rotting back into
the ground as a natural compost for fresh plant growth. Even in the desert nature
endlessly recycles the raw materials of life—from the soil to plants to animals
and back into the soil. Nevertheless the harsh, sparse nature of the desert means
that food chains and webs are severely restricted, both in terms of length and the
number of interconnections. There are simply fewer species of living things;
there is, as scientists say, a low biodiversity.