Reading Passage 2
You should spend about 20 minutes on Questions 14–26, which are based on the passage below.
Surviving the desert
A Deserts are among the harshest places on Earth. By day the sun beats down and temperatures soar; by night, with no clouds to hold the heat, they can plunge close to freezing. Above all, water is desperately scarce, with rain that may fall only a few times a year, or not at all. And yet deserts are far from empty. They teem with plants and animals that have evolved an extraordinary array of tricks for staying alive where, at first sight, nothing should be able to survive.
B For every desert organism, the central problem is water — how to find enough of it, how to store what little there is, and above all how to avoid losing it. Every adaptation of desert life, in plants and animals alike, can be understood as an answer to one or more of these three challenges. The creatures that flourish there are, in effect, master water managers.
C Plants have found several ingenious solutions. The cactus stores water in its thick, fleshy stems, swelling after rain and slowly shrinking as the water is used. Instead of broad leaves, which would lose precious moisture to the air, many desert plants bear only sharp spines, which lose almost no water and have the added benefit of protecting the plant from thirsty animals. Roots, meanwhile, may spread out just below the surface to catch the lightest shower, or plunge deep in search of hidden water. Some plants avoid the problem altogether: their seeds can lie dormant in the soil for years, springing to life and racing through their whole life cycle in the brief weeks after a rare rainfall.
D Desert animals must escape the heat as well as the drought. Many simply avoid the worst of the day: they are nocturnal, resting through the fierce heat and emerging to feed only in the cool of the night. During the day they shelter underground in burrows, where the temperature stays far lower and steadier than on the baking surface. A few take avoidance even further and sleep, or estivate, right through the hottest and driest part of the year, waiting for conditions to improve.
E Other adaptations help animals to save water directly. Many desert animals never drink at all, taking all the moisture they need from their food. The kangaroo rat, for example, can live its entire life without a single drink, extracting water from the dry seeds it eats. Such animals also waste as little water as possible, producing only tiny amounts of highly concentrated urine. The camel, the most famous desert animal of all, survives partly by tolerating swings in its body temperature that would kill other mammals, and by storing fat — not water, as is often believed — in its humps, which it can break down for energy when food is scarce.
F For all their toughness, desert ecosystems are surprisingly fragile, and finely balanced against the extremes they endure. They recover only slowly from damage, and they are now under growing pressure. A warming climate, together with human activities such as overgrazing by livestock and the pumping of underground water for farming, is degrading these lands and causing many deserts to spread into once-fertile regions — a process known as desertification. Protecting the desert's remarkable survivors will mean protecting the delicate balance on which they depend.