Category Plants & Animals

More than a third of the Earth’s land is covered by desert, but very little of it has the sandy appearance that we usually think of when deserts are mentioned. Most of the world’s deserts are barren, stony places.

Most of the world’s surface is covered in water, in the form of oceans. The remaining landmass of Earth amounts to approximately 29 percent of the surface. Of this remaining 29 percent, deserts of all types constitute an estimated 33 percent, or one-third, of the Earth’s total landmass. This large percentage is due in part to the vastness of the world’s largest desert — Antarctica.

Though sand dunes and cacti might immediately spring to mind when people think about deserts, the term “desert” is actually more inclusive and refers to a variety of different land types, from arid canyons to frigid polar plains. Deserts, which make up a large amount of the Earth’s land mass, are home to a diverse collection of plants, animals and landforms.

Deserts are primarily defined by their dryness. A desert can be any land area that has an annual deficit of water — that is, an area where more moisture evaporates than is taken in through any form of precipitation. In more concrete terms, a desert is commonly defined as any area that receives less than 254 millimeters (10 inches) of precipitation (in the form of snow or rain) in a given year.

Because the definition of a desert is so broad and focused on aridness, there are many different and varied types of deserts. In general, deserts may be hot, like the Sahara, or cold, like Antarctica. Beyond that, deserts may be broken into several more categories, depending on their geographic and physical features and how they form. For example, rain shadow deserts are formed when landforms like mountains interrupt cloud cover and prevent precipitation on the protected, or leeward, side of a mountain range.

Antarctica averages less than 5 centimeters (2 inches) of precipitation in the form of snow each year. The icy nature of Antarctica is due to the accumulation of snow, which, despite its small amount, still falls faster in most regions than it can evaporate due to frigid temperatures. This massive polar desert accounts for approximately 14.2 million square kilometers (5.5 million square miles) of the Earth’s surface. The largest hot desert on Earth is Africa’s Sahara desert, which makes up 8.6 million square kilometers (3.3 million square miles) of the planet’s surface. This arid land receives an average of less than 25 millimeters (1 inch) of rain each year.

Camels are among the largest desert animals, but they are so well adapted to dry conditions that they have been domesticated for thousands of years by people living in desert areas. They are kept mainly as beasts of burden but are also eaten and used as racing animals! Camels’ feet are able to splay out to prevent them from sinking into loose sand. They are able to close their nostrils to keep out sand, and their eyes are also protected by long eyelashes. The fat in their humps is a food store. Camels very rarely sweat, so they are able to conserve the water in their bodies much more efficiently than human beings.

Desert adapted camels have evolved physiological adaptations that reduce the amount of water lost or are able to tolerate significant amounts of water loss [9]. Where green forage is available in mild climates, the camel may go several months without drinking. During the winter and cold seasons of the year camels can go without water for months. They do not even drink when offered water. Under very hot conditions, it may drink only every eight to ten days and lose up to 30 percent of its body weight through dehydration. When the mean temperature reach 30-35°C, camels can go 10-15 days without water but when the temperature exceeds 40°C, shorter periods between watering is necessary.

The digestive and urinary tracts are well specialized in water conservation. Cattle lose 20 to 40 liters of fluid daily through feces, whereas camels lose only 1.3 liters. This is one of the primary methods for resisting water deprivation in the desert. Fluid is absorbed in the end part of the intestines, where the small fecal balls are produced.

The rumen helps maintain water balance in two ways. First, the rumen of hydrated ungulates and the foregut of camels contain a large volume of water, approximately equal to 20% of body weight, and may buffer ungulates against short term water deprivation. During the first few days of dehydration, fluid contained in the rumen is used to maintain water balance of blood and body tissues and represents a large portion (50–70%) of the water lost during dehydration. Second, after dehydration in some species, the rumen plays a role in the prevention of hemolysis and osmotic tissue shock during rapid rehydration.

The kidney is an important organ involved in the removal of unwanted nitrogenous substances, excess water and relative maintenance of osmotic concentration of the blood. The camel’s kidneys play a major role in the process of conserving water through increasing the osmolality of urine. The kidney is characterized by a long loop of henle, and a well-developed medulla. During dehydration, the kidneys reduce water losses both by decreasing the glomerular filtration rate and by increasing the tubular re-absorption of water.

The long loops of henle, which are four to six times longer than in cattle, have the function of both concentrating urine and reducing its flow. A dehydrated camel urinates only drops of concentrated urine being shown by white stripes of salt crystals on the hind legs and tail. This concentrated urine not only serves to conserve water, but also allows camels to drink water which is more concentrated than sea water (above 3% NaCl), and to eat salty plants (halophytes) that would otherwise be toxic. Anti-diuretic hormone (ADH) is important in regulating the volume of urine excreted and its concentration. ADH is produced in the hypothalamus and is released into the blood stream in response to increased blood osmolarity. Larger release of ADH leads to a fast renal response that causes increased re-absorption of water. This leads to a smaller volume of more concentrated urine being excreted.

The body of camels can tolerate loss of water over 30% of body weight whereas most mammals die if they lose half of this value. Rehydration following a period of water deprivation is important for animal survival. A camel may drink more than one third of its body weight as it rehydrates. In terms of actual water intake reported 110 liters in 10 minutes. In other animals rehydration at these levels would lead to over hydration and possibly death. The camel is able to do this as large amounts of water can be stored for up to 24 hours in the gut to avoid a rapid dilution of the blood.

No plants or animals can survive if they have no water at all for a long period, but in the desert regions of the world many living things have adapted so that they can thrive with very little water. Deserts are places with very little rainfall, but they are not always hot. Some are very cold at night or in the winter. Animals and plants have to be able to deal with extreme temperatures as well as a lack of water.

On average, a human can only go for an average of three days without water depending on the climate because the human body loses a substantial amount of water through sweating, breathing, and excretion. Every drop of sweat, exhaled breath, and wet excrete increases the chances of death of any desert animal through dehydration. There are a few animals that can survive for years without drinking any water. Some of the popular examples include the desert tortoise, kangaroo rat, the thorny devil, water-holding frog, African lungfish, and desert spade-foot toads.

Kangaroo Rat

The kangaroo rat of North America is one of the most specialized animals living in the desert environments and can go for its entire life without water. The small rodent gets its name from is long hind legs (similar to the Australian Kangaroo) that enable it to leap long distances as they search for food and water. Some of the adaptations to the arid environment include large cheek pouches that are lined with fur as opposed to saliva which enables the rodent to carry seeds without losing much-needed moisture. Other adaptations, such as highly specialized kidneys with additional tubules, help them in the conservation of water in the body through the extraction of water from urine. The urine of a kangaroo rat is about five times as concentrated as human urine. The kangaroo rat has an oily coat and does not sweat which goes a long way in conserving water in the body. In addition to that, kangaroo rats feed on seeds that are safely hidden in burrows. The seeds once consumed are metabolized to yield energy and water.

Water Holding Frog

The water holding frog (Cyclorana platycephala) is commonly found in Australian desert areas and has truly unique adaptation mechanisms to the harsh arid environment. During wet seasons, the water holding frog lives like ordinary frogs and then burrow into the soil when dry conditions set in to escape the hostile conditions. The frog has the unique capacity to absorb significant amounts of water through its skin which is then stored in its bladder and body tissues. Once the frog burrows into the soil, it encloses itself in a cocoon made up of its skin to keep from losing water. While in this condition the frog feeds on its skin and can stay in this condition for several years.

Thorny Devil

The thorny devil (Moloch horridus), which is also commonly referred to as the thorny dragon, is typically found in desert areas in Central Australia. The thorny devil captures rainfall and dew during drier conditions through layered scales on its body that have a hinge which enables it to trap moisture and water droplets between the scales. The collected water is then transported under the skin to the mouth. The process is enabled by tongue movements that create the needed pressure to draw water to the back of the mouth.

The Desert Spadefoot Toad

The spadefoot toad (Scaphiopus couchi), which was recently discovered in the Colorado Desert, has some of the greatest adaptations to desert conditions. These animals have managed to survive in a few isolations such as the edge of dunes and dry washes due to their unique environmental conditions. Spadefoot toads living in the dunes burrow into the permanently wet layer in the sand and remain there for the whole dry period. Others bury themselves beneath dense vegetation. Adult toads retain several layers of partially shed skin which reduce moisture loss by forming semi-impermeable membranes and can remain in that condition for years. The high osmotic concentration greatly increases its ability to retain water and perhaps even extract moisture from damp conditions. These toads also exhibit an extremely accelerated growth rate. Their eggs take less than 48 hours to hatch, and within ten days the tadpoles develop legs. In less than three months, the young toads grow to half the adult size.

As in most other habitats, the colouring of animals in the rainforest is very varied. Some are brilliantly coloured, to attract mates or to warn predators that they are poisonous. Other creatures have green or dark colouring to camouflage them amongst the vegetation. This hides them from their enemies and enables them to creep up on their prey unseen.

Parrots, macaws, parakeets… these rain-bowed tropical birds put to shame the brown and gray birds that are so common in Illinois and Chicago. Even Chicago’s brightest birds—cardinals, blue jays, gold finches—are vibrant, but single-colored. Why are bright and multi-colored birds so common in tropical rain forests, and nowhere to be found in temperate climates like Chicago? Do the changing seasons make bright birds sitting ducks in the winter? Do jungle birds eat bright berries and fruits instead of brown and black seeds? What gives?

We can eliminate one option right away: a parrot’s color has nothing to do with its diet. While a flamingo gets its pink color from the food it eats (brine shrimp and blue-green algae) and a cardinal is red in part because of the seeds in its diet, a parrot’s color is determined by its genes. The incredible colors of the blue-and-yellow macaw do not come from tropical mangoes and imported blueberries.

It must be some other quality of the tropics that creates brighter birds: is it the rainfall? The year-round high temperatures.

The truth is that tropical birds don’t tend to be more colorful. Dr. Nicholas Friedman of the Okinawa Institute of Science and Technology explains, “If you look at birds in the tropics, there are a lot of colorful birds that stand out. But there are really more species in general there and there are just as many more of the little brown ones”.

In other words, the tropics are much more diverse in general than temperate or dry climates. The rainfall and year-round high temperatures contribute to rainforests having many more animal and plant species than other places. Of these many more animal species, some are brightly colored birds, but there are even more species that are plainly colored. The birds that are exported from the rainforests for zoos or as pets are the brightest birds, and these are the tropical birds that we in Chicago are familiar with. This leads to the overall impression that birds from the rainforest are more colorful as a rule.

The many habitats to be found in rainforests can be thought of as layers. In real forests, of course, these layers overlap each other a good deal.

Emergent Layer

These giant trees thrust above the dense canopy layer and have huge mushroom-shaped crowns. These trees enjoy the greatest amount of sunlight but also must endure high temperatures, low humidity, and strong winds.

The emergent layer consists of the tallest trees, with umbrella-like branches poking through the mass of leaves below. In this layer live free-flying birds and bats, including birds of prey.

Canopy Layer

The broad, irregular crowns of these trees form a tight, continuous canopy 60 to 90 feet above the ground. The branches are often densely covered with other plants (epiphytes) and tied together with vines (lianas). The canopy is home to 90% of the organisms found in the rain forest; many seeking the brighter light in the treetops.

The tree canopy consists of the leaves of mature trees. Their tops spread out to reach as much of the light as possible. As well as birds and fruit bats, monkeys and squirrels live in this layer, feeding on the fruits, nuts and leaves of the trees in the canopy.

Understory
Receiving only 2-15% of the sunlight that falls on the canopy, understory is a dark place. It is relatively open and contains young trees and leafy herbaceous plants that tolerate low light. Many popular house plants come from this layer. Only along rivers and roadways and in tree-fall and cut areas are sunlight sufficient to allow growth to become thick and impenetrable. 

Very little light filters through the leaves of the canopy. In the mid-zone, creepers called lianas hang in great ropes among the trees. Here there are monkeys, squirrels, birds and bats again, but also some snakes and tree frogs.

Forest Floor

The forest floor receives less than 2% of the sunlight and consequently, little grows here except plants adapted to very low light. On the floor is a thin layer of fallen leaves, seeds, fruits, and branches that very quickly decomposes. Only a thin layer of decaying organic matter is found, unlike in temperate deciduous forests.

The forest floor is very dark. Larger mammals, such as deer, tapirs, elephants, jaguars and bush pigs, forage among the fallen leaves or prey on each other or smaller animals.

Rainforests are being cut down at an alarming rate for two main reasons. Both large commercial farming companies and individual families clear the forest to gain land to cultivate and graze animals, although the rainforest soil is not suitable for this use. Secondly, forests have been felled to supply tropical hardwoods for furniture-making and building. Woods such as mahogany have been highly prized in wealthy countries for hundreds of years.

More than half of Earth’s rain forests have already been lost due to the human demand for wood and arable land. Rain forests that once grew over 14 percent of the land on Earth now cover only about 6 percent. And if current deforestation rates continue, these critical habitats could disappear from the planet completely within the next hundred years.

The reasons for plundering rain forests are mainly economic. Wealthy nations drive demand for tropical timber, and cash-strapped governments often grant logging concessions at a fraction of the land’s true value. “Homesteader” policies also encourage citizens to clear-cut forests for farms. Sustainable logging and harvesting rather than clear-cutting are among the strategies key to halting rain forest loss.

• Logging interests cut down rain forest trees for timber used in flooring, furniture, and other items.
• Power plants and other industries cut and burn trees to generate electricity.
• The paper industry turns huge tracts of rain forest trees into pulp.
• The cattle industry uses slash-and-burn techniques to clear ranch land.
• Agricultural interests, particularly the soy industry, clear forests for cropland.
• Subsistence farmers’ slash-and-burn rain forest for firewood and to make room for crops and grazing lands.
• Mining operations clear forest to build roads and dig mines.
• Governments and industry clear-cut forests to make way for service and transit roads.
• Hydroelectric projects flood acres of rain forest.