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Desert Ecology. Deserts or dry lands are areas that receive less than 250 mm of annual precipitation (rain-fall) and have little or no vegetation.
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Deserts or dry lands are areas that receive less than 250 mm of annual precipitation (rain-fall) and have little or no vegetation. • Deserts also have extreme temperatures (hot or cold deserts) or located in areas near large mountain ranges because mountains can block the passage of moisture-filled clouds, thus block the rain supply.
Types of deserts 1- Hot deserts (Subtropical deserts): Lie within latitudes 30˚S and 30˚N (on both sides of the equator). Have very high temperatures even in the shade during the day. e.g. The Sahara desert in Africa. 2- Cold deserts: Have cold weather most of the year and lie away from any marine influence (Interior continental deserts). e.g. The Gobi Desert in china.
3-Polar deserts: These are considered as deserts because they have little precipitation and receive water only in the form of ice (plants can not use it). e.g. Antarctica and Arctica.
4-Rain-shadow deserts: These deserts are often located on one side of mountains and these mountains block the rain supply.e.g. The Mojave desert and the Great Basin desert in North America.
Factors which affect different forms of life in the desert • Water is the most important limiting factor in deserts. • Precipitation (rainfall) is used to classify desert ecosystems into: 1- Extreme arid: have less than 60-100 mm annual precipitation. 2- Arid: from 60-100 mm to 150-250 mm annual precipitation. 3- Semiarid: from 150-250 mm to 250-500 mm annual precipitation. • Life in extreme arid ecosystems is rare, while semiarid ecosystems often have some features of grasslands or woodlands.
Rainfall in the desert is characterized by: (a) It is so low (water is the dominant controlling factor). (b) It is highly variable through the year and occurs in infrequent and discrete events. (c) It is unpredictable. • We can now define desert ecosystems as: "water-controlled ecosystems with infrequent, discrete, and largely unpredictable water inputs"
Other Climatic Factors • Evaporation (free-water evaporation). • High intensity of solar radiation. • Extreme temperature: Daily or annually. - Temperature often influences plant and animal activities in deserts. - Temperature effects are usually in close interaction with the water factor. - Rainfall seasonality in relation to temperature has a strong modifying effect on plant growth dynamics. • When rain occurs in a warm season (low-latitude or summer rainfall deserts) both soil moisture and temperature are optimal and this is followed by immediate and very rapid growth of plants. • When rain or snow fall in a cold season (high-latitude or winter rainfall deserts) plant growth are almost completely inhibited by low temperature, even though water is available
Low air humidity: Which can increase the rate of evaporation. • Wind: hot and strong winds, usually associated with sand storms. The known shape of whirlwind (dust devil) results from sudden irregular upward movement of hot air on a still day, this movement carries sand and dust over the soil surface.
Dew: • Condensation of atmospheric moisture as dew is common during night and early morning where rapid cooling of surfaces happens. • Some desert plants and animals depend completely on dew as water source.
Water infiltration or Runoff: • Most of the water input at any point either infiltrates the soil or runs off the surface within minutes to hours. • Runoff from sand and stone surfaces is usually lower than from clay and silt surfaces.
Some useful definitions • A wash or arroyo: A stream bed that most of the time is dry (dry stream bed). • Alluvial fan: fan-shaped deposit of sand, silt, and gravel that forms where a stream slows down at the base of a mountain.
Playa: A dry lake bed (temporary desert lake). • Erosion: movement of soil and other material by wind or water.
Aquifer: underground layer of soil or porous rock that yields water.
Sand dunes: - Hill of sand built by wind movement over soft sand. - Dunes occur in different forms and sizes. - Most dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the leeward side of the wind.
Water balance in animals • The sources of water for animals are: 1- Free drinking water. 2- Water in food. 3- Water resulting from metabolism. • Animals lose water through: 1- Expired air or panting. 2- Faeces (stool). 3- Urine. 4- Sweating or evaporation through the body surface.
Life of Invertebrates in deserts • Desert invertebrates can face problems of dry air, high temperature and scarcity of free water by certain adaptations. • These adaptations may be morphological, behavioral or physiological. 1- Morphological adaptations: • The body cover is impermeable to water. e.g. cuticle of insects and arachnids. • Loss of water through the cuticle of arachnids is reduced by the presence of wax which have high melting point. • Many body appendages of invertebrate animals help them to move fast or burrow holes in desert soils.
2- Behavioral adaptations: • Most desert invertebrates are nocturnal and pass the hot and dry times of the day in burrows, holes and other shelter places.
3- Physiological adaptations: • During the rainfall season, food resources are available thus insects, beetles, scorpions and spiders increase in number. • During dry and hot seasons the life cycles of these animals is continued in larval or pupal stages which have much lower metabolic rates and needs for food. • Aestivation: a dormancy state during summer where development stops and the metabolic needs decrease greatly. • Insects and arachnids excrete nitrogenous wastes in the form of uric acid and guanine in dry form thus no water is lost during excretion. Also water is reabsorbed from the faeces thus faeces are almost dry. • Most desert invertebrates can depend on water obtained from food only or depend on water resulting from metabolism. So some insects can live their whole life without using any free drinking water.
Adaptations of desert animals to extreme temperatures • Two strategies: • Evaders: • Avoid the extreme temperature conditions by using burrows or any shelter during the day. • During hot seasons, the soil surface temperature may reach 70˚C or more while the burrow temperature may be about 30˚C. Also, relative humidity inside the burrow is good when compared to dryness and direct sunlight outside the burrow. • During cold seasons, some rodents direct their burrow openings away from the cold winds and towards direct sunlight.
So evaders are able avoid high temperatures during hot seasons and low temperatures during cold seasons. • 2. Endurers: • - Tolerate (endure) extreme temperature conditions. • Evaporators: use evaporative cooling to decrease the body temperature during a hot day. Some of the evaders or endurers can do this. • Small organisms (less than 20 g) are evaders. • Intermediate sized organisms are evaporators. • Large organisms (mostly mammals and very large birds) are endurers.
This depends on a simple relationship between body surface area and body volume • Small organism with a surface area of 1cm X 1cm X 6 sides= 6 cm2 , has a body volume of 1cm X 1cm X1cm= 1 cm3. • Intermediate sized organism with a surface area of 2cm X 2cm X 6 sides= 24 cm2 , has a body volume of 2cm X 2cm X 2cm= 8 cm3. • Large organism with a surface area of 3cm X 3cm X 6 sides = 54 cm2 , has a body volume of 3cm X 3cm X3cm = 27 cm3. Ratios: 6:1 / 3:1 / 2:1
Thus a small organism will be able to gain heat and lose it again very quickly through its large surface area, while the larger organisms will do this more slowly.
Small animals are usually evaders such as invertebrates, desert amphibians, reptiles and small mammals (rodents, bats, rabbits, etc…….). • Evaporators must depend on sufficient water supply to cool their body by evaporative cooling. Thus few of these species can survive in the desert and mainly they live on the edges of the desert or have behavioral and physiological adaptations to reduce evaporative cooling. • Evaporators include medium sized mammals such as jack rabbits, dogs, fox and some desert birds.
Torpor: • It is a behavioral adaptation (a form of dormancy) to resist extreme temperatures in the desert. It helps the animal to reduce its water needs and energy needs. 1- Daily aestivation: less than 24 hr torpor in response to heat and/or dryness. 2- Seasonal aestivation: seasonal torpor in response to heat and/or dryness 3- Hibernation: seasonal torpor in response to cold and insufficient energy uptake. So torpor includes: 1- Aestivation (in response to high temperature during hot seasons) 2- Hibernation (in response to low temperature during cold seasons).
Life of mammals in deserts Large mammals in hot deserts: • These are endurers, their body is too large and they can not hide in burrows or shade thus they are forced to remain exposed to direct sunlight during the day but the advantage is that they can travel for long distances to search for food or water. • They use the risk of evaporative cooling for heat loss (includes panting and sweating). • They are inactive during the hottest part of the day.
The most familiar large desert mammal is the camel but there are others such as the Oryx, Antelope, Gazelle, desert sheep, Zebra, fox, etc…..
The camel • One of the best adapted animals to desert conditions. There are two species of camels: • The Arabian camel (dromedarian camel) (Camelusdromedarius). Single humped camel in the Middle East and North Africa, it is more adapted to hot conditions. • The Bactrian camel (Camelusbactrianus). Two humped camel in central Asia, it is more adapted to cold conditions.
Bactrian camel Camelus bactrianus Dromedarian camel Camelus dromedarius
Camels can live for more than 17 days in summer without drinking, and some of its adaptations are: • At rest, the normal body temperature can range from 34˚C to over 40.5˚C. That is a rise of 6.5˚C in the same day and this decreases the difference between the body temperature and air temperature thus decreases the water loss through sweating. • Heat regulation in camels occurs by evaporative cooling (sweating) with no increase in respiratory rate (without panting). • Camels do not store free water in the hump (they only store fat). They can use metabolic water produced from metabolism of this fat. The oxidation of 1g fat gives 1.07g water, thus 40kg hump gives 43 liters of water.
4- Their hair (fur) plays an important rule in insulation both from heat during the day and cold during the night. During hot days the fur surface may reach 70 to 80˚C while the skin temperature is only 40˚C. 5- Water loss in urine and faeces is very low. 6- Tolerance to dehydration: in summer it can lose water equivalent to 25% of its body weight. Under the same conditions, most animals die when they lose half that amount of water (12%). This is related to the camel’s ability to maintain its blood volume near normal levels during dehydration (water loss happens mainly from the tissues and not from the blood). While in most mammals water loss happens from the blood and affects the blood volume and increases the blood viscosity leading to death.
