Biome Background:  Deserts

Where Are the Deserts?

Deserts, like forests and grasslands, occur all around the world. Symmetrical clusters of deserts are found around the Tropic of Cancer and the Tropic of Capricorn - the two latitudes that define the area where the sun's angle at solar noon is closest to ninety degrees overhead at the Equinox.

Some famous deserts are the Gobi and Taklamakan deserts in Asia, the Sahara in northern Africa, and the Great Sandy and Simpson deserts in Australia. Deserts can come and go with changes in climate. The Sahara was once green and filled with animals, as portrayed in wall paintings in its sandstone monuments. But when the ice glaciers Sahara is passed over by strong winds from the north that pick up any available moisture on their way to the equator.
 

What Are Deserts?

There are both cold and hot deserts. Both kinds of deserts receive very little rainfall and both get very cold at night, but hot deserts bake during the day. The vegetation typically is ground cover, stunted bushes, or occasional trees and is specially adapted for life in such difficult circumstances.

Key Characteristics of Deserts

- Deserts receive less than 25 cm of rainfall each year and rainfall is very irregular. Parts of the Sahara can go years without any rain at all.
- Deserts experience a wide range of temperature from day to night. There is little to absorb the sun's radiation, so it all goes to the soil or sand. Similarly, there is nothing  -- cloud cover, vegetation, water -- to hold the warmth, so it quickly goes away.
- The soil is very dry (sand) and is low in organic nutrients, as few plants live, die and decay there.
- Desert animals and plants are adapted to reproduce quickly during the brief moist period.

Why Are There Deserts?

Two key factors in the creation of deserts are the rain shadows of mountains and the big circulation pattern of global wind. In the case of mountains, as water-filled air is forced up a mountain slope, it cools and dumps its water (precipitation) on that side of the mountain. If the mountain range is large enough, little water gets to the other side. Global wind patterns are complicated, but worth understanding. The winds that circle the globe are caused by the difference between warm equatorial temperatures and cooler polar temperatures. When air is warmed at the equator, it rises. It then moves north and south towards each of the poles, where it cools, loses moisture, sinks, and returns toward the equator. On its return, the air can hold more water, so it picks up any available moisture over the desert areas near the equator.  

Adaptations

In a desert, water is all-important. Adaptations to the scarce rainfall typical of deserts include:

- Annual plants have seeds that are able to stay dormant until there is sufficient rainfall to support the young plant.
- Succulent plants, like cacti, store water in residual leaves that are spines. Photosynthesis takes place on the stem and there are pleats on the stem that can quickly expand when a torrent of rare water hits the desert.
- Shrubs have sunken stomata and evergreens waxy cuticles that keep water from escaping. The holly plant holds its leaves at seventy degrees so that the sun hits only its sides. When the sun is low, it hits the full leaf. It also has a covering of fine salt to reflect heat away from the plant. The creosote bush lives on dew, using a fine network of rootlets to obtain water. It grows in expanding rings and nothing next to it can compete for water.
- Many animals live in burrows, only go out at night, and generally try to avoid the heat. Even lizards, which hunt in the day, avoid the heat of noon. Some desert animals have large ears (desert fox, jack rabbit, hedgehog, and bandicoot) to radiate heat away from their body. The desert squirrel holds its tail over its head like an umbrella to stay cool.