11-4: Deserts

Water Balance Adaptations

Animals have many different adaptations that help them keep enough water in their bodies. Some amphibians and mammals, like this camel, physically store water in their bodies for later use. Many reptiles have thick, scaly skins that prevent water loss. Most desert animals conserve water by excreting very minimal waste, especially in the form of urine.

Coastal

Can a desert exist near an ocean? Yes, some coastal lands are classified as deserts. One example is the Atacama Desert of Chile, shown in this image. Because this area is in a rain shadow, it receives relatively little precipitation each year. Coastal deserts get about 10 cm (about 4 inches) of rainfall per year, usually during the winter. Temperatures are much more moderate than either hot and dry deserts or semiarid deserts. They range from about 40 degrees F (5 degrees C) to about 75 degrees F (24 degrees C.)

Behavioral Adaptations

Many plants exhibit special behaviors that help them survive and reproduce successfully in the desert. For example, some plants go dormant (the root -dorm- means "to sleep") by shedding their leaves and becoming otherwise inactive during the very driest parts of the year. The ocotillo shown here becomes active only for a short period of time after being exposed to precipitation. Then it reproduces by quickly making flowers, and after its cycle of reproduction, it falls back into dormancy.

Which of these is the world's largest desert?

Sahara (Northern Africa)

Semiarid

Semiarid (semi- means "part," and -arid means "dry") deserts are found in the United States in Utah and Montana. Other semiarid deserts exist in Greenland, Newfoundland, Northern Europe, and Russia. Because these deserts are more northern, they tend to be cooler than hot and dry deserts, and as their name suggests, they tend to have more rainfall. Temperatures rarely climb over 100 degrees F (38 degrees C), and nights stay at around 50 degrees F (10 degrees C.) Because these temperatures are not as extreme, moisture can condense (change from vapor to liquid) on the ground in the form of dew. Rainfall is also higher in semiarid regions compared to hot and dry ones, averaging about 3 cm (1.3 inches) per year.

Rain Shadow

Some deserts form in what is called a rain shadow. Deserts formed by rain shadow are usually next to mountains. Warm, moist air rises as it tries to cross over a mountain range. As this air gets higher, the temperature cools, causing the vapor in the air to condense (turn from a gas into a liquid) into precipitation that falls near the mountain top. The dry air then moves forward, leaving a desert on the other side of the mountain.

Which of these describes a phreatophyte?

a plant with very long roots

What is the BEST definition of a desert biome?

an area or region where more water moves from the land to the atmosphere than falls to the ground as precipitation

Cold Ocean Currents

1. Cool water flows in the ocean. 2. This causes the water vapor in the air above it to condense and to fall into the ocean as precipitation. 3. The air that moves onto land is dry and unable to produce precipitation. This forms a desert.

Cold Deserts Adaptations

Although most plant life in cold deserts is aquatic (it lives in the water,) the animals that live in the Arctic and Antarctica have special adaptations that enable them to get enough water. Unlike most animals, marine birds like this penguin can drink saline (salty) water. Polar bears get most of their water from the fat of other animals. All animals in the Arctic have insulating layers of fur and/or fat to help maintain their body temperatures.

It may seem strange, but penguins and polar bears technically live in deserts. They are not found in the same desert, but they are found in similar deserts. Where do you think these deserts are located?

Both the Arctic (where polar bears live) and the Antarctic (where many species of penguins dwell) are deserts.

Cold

Cold deserts are found in places like Greenland, the Arctic, and the Antarctic, as shown in this image. Precipitation, when it comes, is usually in the form of snow, sleet, or ice. Winters are long and cold, and summers tend to be short. Precipitation averages at about 20 cm (about 8 inches) a year, and temperatures usually don't climb above 77 degrees F (25 degrees C.)

Body Temperature Adaptations

For animals that live in hot and dry deserts, temperature regulation is a major concern. As such, animals like the fennec fox, shown here, have large ears that act like radiators, spreading heat away from the animal's body. Other animals adapt to the heat by changing their behaviors. For example, they may sleep during the daytime, and hunt and do other activities during the cooler nights.

In addition to farming crops and building cities, what other ways might humans influence the desert biome?

Humans raise animals for food, and grazing these animals on already barren lands can affect the desert biome. Humans also change water courses by building dams, causing some areas to flood and others to be deprived of water.

Root Adaptations

Phreatophytes (phreato- means "underground") have extremely long roots that go deep into the ground to find water. The mesquite tree in this picture is one example of a phreatophyte. Many phreatophytes also have long roots that travel radially (outward) from the plant to cover as much ground as possible in the search for water.

Equatorial Winds

The areas in yellow on this map are deserts. Deserts can be found all over the world, but notice that many of them are located around two geographical markers called the Tropic of Cancer and the Tropic of Capricorn. The tilt of Earth's axis exposes the planet's equator to almost constant sunlight, making the air at the equator very warm. This warm air at the equator rises, creating winds that blow to both the north and the south away from the equator. These winds cool off and start to fall at the Tropic of Cancer and the Tropic of Capricorn, but they rarely bring any precipitation with them. The world's largest desert, the Sahara exists because of these winds.

What do you think is the greatest challenge to survival faced by both plants and animals that live in the desert?

The biggest challenge for organisms that live in the desert is getting and keeping enough water in their bodies.

Why did most of Earth's major deserts, including the Sahara, form?

The winds blowing away from the equator are very dry and rarely bring precipitation.

Hot and Dry

When most people think of a desert, they think of the first type--hot and dry. Hot and dry deserts exist in North America (like the American Southwest's Mojave, shown here), South America, Asia, Africa, and Australia. Temperatures remain consistently warm throughout the year with highs of over 100 degrees F (38 degrees C.) However, the temperatures can drop to 0 degrees F or less! Precipitation comes in the form of rain. Desert rains are very heavy over a short period of time, and they are sporadic (irregular.) Rainfall can total less than 1.5 cm (less than an inch) per year.

Stem and Leaf Adaptations

Xerophytes are plants that have special stem and leaf structures that allow them to survive in very dry conditions. (Xero- means "dry" in Greek, and -phyta means "plants," so xerophytes are literally "dry plants.") The barrel cacti shown here have thick stems to store water and a waxy cuticle (outer layer) to prevent water from evaporating too quickly. Most plants constantly lose water to the air by transpiration, which is the evaporation of water from leaves to the atmosphere. In many xerophytes, leaves are either absent or modified into spines, which discourage thirsty animals from taking the plants' precious water.

Most adaptations of organisms in a desert biome help an animal do which of these?

keep a water balance

Which answer describes an adaptation used by plants to minimize transpiration?

small or absent leaves

What is a rain shadow?

the area found next to a mountain where very little rain occurs

Which adaptation would you expect to see in a xerophyte?

thick, waxy stem to help store water