Desert biomes <ul><li>Deserts are arid (dry) </li></ul><ul><li>environments where there </li></ul><ul><li>is less than 250mm of </li></ul><ul><li>rainfall annually. </li></ul>N.B not all deserts are hot for example the Gobi desert in Asia is a cold desert
Global distribution of desert biomes equator 30° South 30° North Desert biomes are generally around a belt of 30° north and south latitude .
Desert types <ul><li>Subtropical </li></ul><ul><li>The climate is warm or hot and dry year-round. These regions receive an annual rainfall between 30 to 65 cm and occurs mostly during the summer monsoon season. Temperatures during summer may rise to 40°C. The climate is pleasant between October and March, hotter between April and June. Monsoon rains occur in most regions in summer between June and September. </li></ul><ul><li>E.g. Sahara </li></ul><ul><li>Coastal </li></ul><ul><li>These have cool winters and warm summers. A coastal desert, the Atacama of South America, is the Earth's driest desert </li></ul><ul><li>Cold deserts </li></ul><ul><li>These deserts have long, dry summers and low rainfall in winter, generally found in the Antarctic regions. They have short, moist, and moderately warm summers with fairly long, cold winters. </li></ul><ul><li>E.g. Gobi desert </li></ul><ul><li>Polar deserts </li></ul><ul><li>Climate is cold year-round. In polar deserts, snow dunes occur commonly in areas where precipitation is more abundant. </li></ul><ul><li>E.g. Antarctica </li></ul>
Climate- what causes deserts? air masses found 30° north and south of the equator move in circular patterns ( Hadley Cells ) In a Hadley cell air rises at the equator . As it rises it cools causing the moisture to condense and precipitate , leaving the air dry . This dry air descends around 30° north and south of the equator. In areas where the air descends a high pressure zone is created. Winds blow outwards from high pressure areas. (So no moisture can be brought in by the wind ) Therefore there are very low precipitation levels Hence the location of many deserts are found there
Climate- continued <ul><li>Seasonal variations: </li></ul><ul><li>During the summer months temperature ranges are between 30- 49°C . Very little or no precipitation occurs during the summer months and evaporation rates often exceed precipitation </li></ul><ul><li>During the winter months temperatures are lower around 10-20°C . Most of the annual precipitation occurs during these months. </li></ul><ul><li>Diurnal (daily) variations: </li></ul><ul><li>Temperatures exhibit daily extremes because the atmosphere contains little humidity to block the Sun's rays . So during the day the sun’s energy is absorbed on the ground surface which heats the surrounding air. </li></ul><ul><li>At night the reverse happens the hot ground (and air) radiates the heat energy gained during the day back out to space causing the rapid temperature drop . Night temperatures can reach below 0°C! </li></ul>Hot deserts have extreme seasonal and daily temperature variations
Soil <ul><li>Occasionally when a lot of rainfall occurs in a short space of time very little of the water is infiltrated into the soil as it is very hard and dry. Therefore there is high surface runoff. </li></ul>coarse dry alkaline Little humus Little moisture
Plant adaptations Phreatophytes Are plants that have adapted to arid environments by growing extremely long roots, allowing them to acquire moisture at or near the water table . e.g. Ocotillo bush Xerophytes Are plants that have adapted to arid environments by storing as much of the little water made available and reducing evapotranspiration rates. e.g. Cacti
Other common adaptations <ul><li>Reduction of leaves , often into spines for protection against animals trying to consume the stored water. (The flat surface of normal leaves provides a huge area for water loss.) </li></ul><ul><li>Development of the stem as a major photosynthetic structure . With leaves reduced, photosynthesis has to occur somewhere. </li></ul><ul><li>Water storage in the stem water is needed for many things; if the plant is to have it available it must be stored. </li></ul><ul><li>Development of defence structures- spines , but chemical defences are also very common. With less photosynthesis taking place, growth of desert plants is often very slow, and under these conditions it is advantageous for the plants to make greater investments in defence. </li></ul>Coating the plant with a thick waxy cuticle- this helps reduce water loss. A dense coating of hairs (trichomes) . This slows air moving over the surface of the plant; since air in the desert is very dry any air movement tends to increase evaporation. The trichomes create a microlayer of humid air around the pant, particularly in the vicinity of the stomata. Extensive underground root systems. These roots can either grow straight down to groundwater, if it is available, or spread out extensively under the surface of the desert. The latter growth form allows the plant to take advantage of short, intermittent rains. Key in this strategy is elimination of competitors' roots; many desert plants inject toxic chemicals into the soil to kill their competitors roots.
Animal adaptations -Camels Two rows of long eyelashes Adaptation Function Protect against blowing sand and the sun Fat stored in hump(s) Help it to survive long periods without food and water Keep out blowing sand Nostrils can be closed Provide warmth during cold desert nights and insulation against daytime heat Thick fur and under wool Pads spread out when the camel places its feet on the ground thus creating a "snowshoe effect" and preventing the camel from sinking into the sand Broad, flat, leathery pads at the bottom of their hooves
Animal adaptations- Fennec fox Adaptation Function large ears (about 15cm) help dissipate excess body heat on hot days in the desert. burrowing and nocturnal lifestyle Thick, sandy fur helps insulate them from the cold desert nights but also reflects heat, As well as providing excellent camouflage. Cooler temperatures at night mean less water loss while carrying out activities.
Other common animal adaptations <ul><li>Panting to reduce body heat </li></ul><ul><li>Secreting highly concentrated urine (contains less water) </li></ul><ul><li>Seasonal migration </li></ul><ul><li>Long- term dormancy that ends only when triggered by moisture and temperature conditions </li></ul>
Nutrient cycling- Net primary productivity (NPP) NPP is the amount of organic material available for consumption in a given area minus the losses plant respiration . Normally expressed as g/m²/yr Deserts have one of the lowest mean NPP of 0.003 g/m²/yr compared to 2.2 g/m²/yr for a tropical rainforest! B L S Fall- out as litter decomposes Uptake by plants Degradation & mineralisation Loss in run- off Input from rainfall Nutrient cycle for a desert biome = biomass, = soil, =litter Pyramid of numbers
Human impacts Oil drilling Agriculture Irrigation Development of cities pollution Settlements Interference of natural ecosystems
Human impacts Deserts are also being destroyed by development . As populations in desert cities rise , so does the demand for water . Many desert streams, and ground water sources, once used by animals have been drained for industries , agriculture and people .