Background Essay: Desert Biome
The desert biome is characterized by low precipitation, a high rate of evaporation (seven to fifty times as much as precipitation), and a wide daily range in temperature. The dramatic temperature fluctuations are the result of low humidity, which allows up to 90 percent of solar radiation to penetrate the atmosphere and heat the ground during the day, then for this accumulated heat to be released back into the atmosphere at night.
Precipitation in deserts, unlike other biomes, is highly irregular. In the Sonoran Desert, rain usually comes in short, sporadic clusters of rainy days three to fifteen times a year. On average, only one to six of these rainfalls is large enough to stimulate plant growth. Thus, Sonoran plants experience long periods of inactivity broken by periods of rapid growth and reproduction.
Plant production, given these extremely limited water resources, depends on the efficiency with which plants can absorb and use available water. Desert plants have evolved a wide variety of structural characteristics that limit the amount of water they lose to the atmosphere -- from dense spines on some cactuses that create shade for the plant underneath, to a waxy coating on the surfaces of leaves. In addition, many desert plants have evolved a functional strategy to limit water loss: They perform the processes of transpiration and photosynthesis separately instead of concurrently as in most plants. They do this by fixing large amounts of CO2 throughout the night, then storing it until daytime when they can use it to photosynthesize carbohydrates.
Most desert animals are what ecologists call generalists and opportunists, animals that eat whatever they find, whenever they can find it. Some small desert mammals, however, are seed-eating specialists. One study conducted in the Sonoran Desert showed that while these small herbivores (especially jackrabbits and kangaroo rats) consumed only 2 percent of all the leaves and stems produced by plants in their area in a given year, they ate 87 percent of all seeds produced. Further, the researchers found that the preferences these animals had for some varieties of seeds over others had a pronounced effect on plant populations and species composition.