A desert is a place that has few, or sometimes even no, life forms. Sometimes life forms adapt to living in deserts, but conditions tend to be extreme, and survival is challenging.
Some deserts can be visited but not lived in. Some deserts are so inhospitable that life as we know it cannot survive in them at all.
In terms of rainfall, areas that receive less than ten inches of rain a year are considered to be deserts.
Some deserts receive only three or four inches of rain a year. A few places do not receive any rain at all.
When we think about deserts, we think about limiting factors.
These factors include:
on earth, liquid water is necessary for life. Some life forms survive periods when water is not available by becoming spores or seeds, or by becoming dormant (hibernation or estivation). Some plants can survive for many years as seeds. Insects and unicellular life forms can also wait out drought. Sooner or later, however, liquid water is necessary. Survival is essential, but it is not all of life. Without growth and reproduction, life is on hold, not progressing.
Salinity can also interfere with an organism’s use of water. Fresh water fish cannot live in the ocean, and land plants watered with sea water will die. The excess salt in briny water pulls water out of the organism and dehydrates it. If you put a salt water fish in fresh water it will die, too, because the organism will retain too much water in its cells.
Sunlight: is the ultimate source of most of the energy used by living things on earth. Plants use sunlight for photosynthesis. Lack of light in caves and under deep water make these environments unsuitable for photosynthesizing plants.
Hydrogen Sulfide: a small set of life forms live around deep sea volcanic vents, using a process called chemosynthesis to extract energy from the mineral rich hot water. Without these chemicals, these cold, dark areas are almost lifeless.
Essential Elements: include nitrogen, calcium, and phosphorus. These are elements are captured by plants and circulated through food chains. Much of the open ocean is actually a desert because it lacks these nutrients and cannot support life forms. In places where cold water is upwelling from the ocean depths, some of these nutrients are carried up from the ocean bottom, and provide food for life forms at the surface.
Food: Food provides energy (fuel) and also amino acids and vitamins necessary for life. Animals need both fuel and vitamins. Without them, they do not thrive.
Life processes are chemical processes. Life processes operate most efficiently within a narrow temperature range. Think of how we put food in the freezer to keep it from spoiling: freezing temperatures stop those processes. Too much heat also kills: fevers of around 108 degrees Fahrenheit kill humans, because the proteins of which we are formed begin to break down at those temperatures. Both plants and animals have ways of protecting themselves from intermittent extremes of temperature, but need to stay within certain temperature ranges in their environments in order to survive.
Life forms are adapted to the atmospheric or aquatic pressures at which they live. People die at high altitudes because they cannot extract enough oxygen from the air. Life forms brought up from the deep ocean will explode as the gasses dissolved in their tissues expand.
We don’t have radiation deserts on earth (yet), but we know that living things need protection from ultra-violet radiation (protection currently provided by the ozone layer), radioactive radiation (high around Chernobyl), and gamma radiation out in space.
The food web in the hot desert biome is a simple one. Life in this hot, dry environment is challenging, requiring adaptations from both animals and plants. The soil is often dry, and desert winds carry fine dust particles away, leaving a stony landscape. Plants that live in the desert year round have evolved special adaptations for capturing and storing water. Adaptations include secreting a waxy substance to protect their leaves from drying out, thorns and spines to keep hungry animals at bay, and body shapes that can expand rapidly when water becomes available. Plants have large networks of roots that lie near the surface and can capture rain when it falls. One bush, the creosote bush, actually secretes a substance in its roots that keep other roots out of its feeding area.
Many desert plants no longer have leaves, or grow only very small ones. They have chlorphyll in their stems. Many cacti do not have leaves at all. Their rounded bodies have a low surface to volume ratio, and the spines that protect them also cast a little precious shade on their green bodies. Annual desert plants germinate, grow, and flower quickly when there is a rainy year. They make small, hard seeds that may not sprout for ten years or longer. Some perennial plants store moisture in underground tubers or bulbs.
Desert plants are the primary producers. Animals that live in the desert feed on the plants’ seeds, flowers, and juicy bodies and leaves.
The plant-eating animals are the primary consumers. These animals are small, and can get by on very little food. Some desert dwellers are insects, and some, such as snakes and lizards, are reptiles. Reptiles are “cold blooded” and they can survive on only a little food. The warmth of the desert sun heats their bodies so that they can move quickly. A few small warm-blooded animals, such as kangaroo also live here. They hide from the heat in burrows, and come out at night to feed.
The secondary consumers eat the plant eaters. Lizards eat insects: snakes eat lizards, insects, and little desert rodents such as deer mice and kangaroo rats. Scorpions and tarrantulas also eat insects. They have exoskeletons, which help them to conserve moisture.
All animals need protection from the sun during the heat of the day. There is no shade in the desert, but there are little crannies in the rocks where a small animal can find shelter. Some of the animals go into underground burrows, where the air is a little cooler.
Not all land classified as desert is equally arid. The driest parts may look very stony, but where there is more moisture there will be more plants, such as sage brush, seasonal grasses, and small shrubs. There may tree sized cacti, palo verde, and Joshua trees. These greener deserts may be home to quail, pygmy owls, and even desert foxes and hawks. A few tertiary consumers may be able to survive in these richer environments.
An interesting desert “extra” is the oasis, a place where springs of water flow to the surface, providing an environment where palm trees and shrubs may grow. Though rare, oases gladden the hearts of travelers, and provide a refreshing micro-world with its own ecology.
When we think of deserts, we often think of hot deserts with sand dunes. While it is true that some parts of deserts have dunes, desert landscapes vary widely. This picture shows a part of Monument Valley, which is an arid area. You can see sand in the foreground, but behind the sand there are some desert plants: probably rabbit brush. Notice that these plants are not close to each other — there is not enough moisture in the soil to support more plants. However, there is something here for animals to eat. There are probably some little rodents in this desert — mice or kangaroo rats, and some lizards. There will be a few snakes to eat the lizards and rodents. There will be insects, too. During the summer many of the animals will seek shelter from the heat by hiding in burrows underground. They will come out at night to hunt for food.
Deserts are described as areas that get an average of less than ten inches of rain a year. In the summer, temperatures may soar to over 120 degrees during the days, but will drop as soon as the sun sets. Winters may be cold. The dry desert air and clear skies do not hold onto the heat at night. The heat just radiates out into space.
Plants and animals make special adaptations to the desert environment.
Some plants are annuals. Their seeds wait in the ground for years until there is a rainy winter: then they sprout rapidly and grow as fast as they can. They race the sun to make seeds before the water evaporates and the heat dries them up. Some of these plants are so tiny that the whole plant could be hidden under a dime. Others spread colorful blossoms over the sand and rocks for a few brief days.
Some plants develop water-storing strategies. Cacti have chlorophyll in their fleshy stems, and they store the water that they gather in those stems. They defend themselves with sharp spines. They have large networks of roots that lie under the ground near the surface of the soil. If it does happen to rain, these roots can soak up the water quickly. Many cacti have corrugated stems which can expand quickly if water becomes available.
If you cut across one of these stems, you would see shapes like this. The first shape shows the cactus stem when the plant has plenty of water, and you can see that it is pretty full. The second shape shows how the sides fold in when the plant is dry.
Some parts of deserts are covered with stones. Sometimes the stones are huge and jumbled. Sometimes these areas are flat, with the stones set close together. Flat, stone-covered areas are called desert pavement. In the wet springs, tiny flowers may appear between the stones.
Hot Desert Animals
Animals in the hot desert also make special adaptations. Most of these animals are small, because food and water are scarce. Some are reptiles, lizards and snakes, animals who are cold-blooded ectotherms. These animals do not need as much food as warm-blooded animals, and can go weeks between meals if they have to. The sun provides heat for them, and they can find little niches in the rocks to hide in during the heat of the day and during the cold nights.
Warm-blooded rodents are also small, and can survive by eating seeds and nibbling on plants where there is enough plant growth. They have evolved to be able to live with very little water. They are preyed upon by snakes, and perhaps by desert foxes or coyotes or hawks if there is enough vegetation to support a good-sized colony of these plant eaters.