14 Desert Ecosystem

Sharda R. Gupta

epgp books

 

 

Objectives:

  • Controlling Factors Forming Deserts
  • The distribution and climate of deserts
  • Adaptation of Plants and Animals to Dry conditions
  • Biodiversity and Community Composition

• Desert ecosystem functions

• Human impact on Desert Ecosystems

26.1. Introduction

Dry areas as created by global circulation patterns contain most of the deserts on the Earth. Mostly, deserts occur in specific latitudes (25 to 35° north and south of the equator). Deserts are arid where less than 250 mm of rain falls in a year resulting in low soil water availability, sparse growth of vegetation, and low productivity. However, evaporation exceeds rainfall in these ecosystems. The deserts in the world are not restricted by latitude, longitude, or elevation and show extreme variability in abiotic and biotic components. The causes of formation of deserts are varied; they show disjunct distribution and have independent floral histories. Mostly, deserts are often regions of extreme temperatures where living conditions are harsh; temperature variability is extremely variable. For example, many deserts such as the Sahara in Africa are hot all year-round but others, such as Asia’s Gobi desert, become quite cold in winter. It is interesting that deserts are having high biodiversity in spite of their low productivity. The world’s deserts occupy almost 17 percent of the earth’s land surface. The deserts and desert margins are home to about eight per cent of the global human population; some of the poorest and most marginalized people in the world live in deserts. The poor people depend on deserts for sustainable ecosystem services under changing climate.

26.2. Controlling Factors Forming Deserts

26.2.1. Earth’s air circulation patterns: Subtropical deserts are formed because of the circulation patterns of air masses. They are found along the Tropic of Cancer, between 15o and 30 o north of the Equator, or along the Tropic of Capricorn, between 15o and 30o south of the Equator. Included in this group are the Sahara Desert and the Kalahari Desert in Africa. In these regions, hot, moist air rises into the atmosphere near the Equator. As the air rises, it cools and drops its moisture as heavy tropical rains. The resulting cooler, drier air mass moves away from the Equator. As it approaches the tropics, the air descends and warms up again. The descending air hinders the formation of clouds, so very little rain falls on the land below.

26.2.2. Rain shadow effect: A rain shadow is a patch of land that becomes a desert because mountain ranges blocked all rainy weather. On one side of the mountain, wet weather systems drop rain and snow. On the other side of the mountain, the rain shadow side, it’s warm and dry (Fig.26.1). When an air mass moves from a low elevation to a high elevation, it expands and cools. Cool air forms clouds, causing rain and snow, as it rises up a mountain. After the air mass crosses over the peak of the mountain and starts down the other side, the air warms up and the clouds dissipate resulting in less rainfall. Rain shadow deserts exist near the leeward slopes of some mountain ranges. For example, the Gobi Desert, The Tibetan Plateau, the deserts of Nevada east of the Sierra Nevada Mountains. Major rain shadow effects occur in Atacama Desert, located in Chile.

26.2.3. Distance from the ocean. Some land is so far from oceans (where air absorbs most of its water) that deserts form. Interior deserts, which are found in the heart of continents, exist because no moisture-laden winds reach them. The Gobi Desert, in China and Mongolia, lies hundreds of kilometers from the ocean. Winds that reach the Gobi have very low moisture. The Gobi is also in the rain shadow of the Himalayan Mountains to the south.

26.2.4. Near cold ocean currents: Cold ocean currents contribute to the formation of coastal deserts. The cold ocean air warms as it passes over continents. A coastal desert may be almost totally rainless, yet damp with fog. The Atacama Desert, on the Pacific shores of Chile, is a coastal desert.

Fig. 1. The rain shadow effect, windward and leeward sides of a mountain range, the windward side receives heavy precipitation while the Leeward side is dry ( adapted from Singh et al., 2015).

26.3. The main Characteristics of Deserts

  • Deserts are dry and arid, have an aridity index (P/PET) lower than 0.20.
  • Humidity (water vapor in the air) is near zero in most deserts.
  • Light rains often evaporate in the dry air, never reaching the ground.
  • Temperature extremes are a characteristic of most deserts.
  • Annual precipitation less than the potential evaporation.
  • Low soil moisture to support primary productivity.
  • Animals and plants that live in deserts have adapted to survive in harsh conditions.
  • Many desert dwellers rely on groundwater, stored in aquifers below the surface.

26.4. The Desert Ecosystems: A Global Perspective

Aridity is an important criterion for defining a desert. One of the most common approaches to measure aridity is through an estimator called the Aridity Index, which is the ratio between mean annual precipitation (P) and mean annual potential evapotranspiration (PET) (Thornthwaite 1948).

The hyperarid and arid regions of the world are defined as those areas with an aridity index (P/PET) lower than 0.20 (Table26.1). Potential evapotranspiration (PET) is calculated from Thornthwaite’s (1948) equations as a function of mean monthly temperatures and mean monthly number of daylight hours, while precipitation (P) is measured directly from weather stations. Arid and hyper-arid regions have a P/PET ratio of less than 0.20; rainfall supplies less than 20 per cent of the amount of water needed to support optimum plant growth (UNEP 1997). Based on this criterion, aridity is highest in the Saharan and Chilean-Peruvian deserts, followed by the Arabian, East African, Gobi, Australian, and South African Deserts, and it is generally lower in the Thar and North American deserts.

Table 26.1. Area of hyper-arid and arid regions of the world (based on UNEP 1997; Safriel et al. 2005)

* The ratio of precipitation (P) to potential evapotranspiration (PET)

The global distribution of deserts as Hyperarid and arid regions is shown in Fig. 26.2. In the northern hemisphere, the deserts are found in three regions (UNEP, 2006, http://www.unep.org/geo/gdoutlook/112.asp):

i. The Mojave, Sonoran, and Chihuahuan Deserts in North America,

ii. The Sahara desert forms an immense swathe in Northern Africa and the Somali-Ethiopian deserts in the Horn of Africa.

iii. The deserts of Asia, including the Arabian, Mesopotamian, Persian, and Thar deserts that stretch from West Asia into Pakistan and India, as well as the Central Asian deserts in Uzbekistan, Turkmenistan, and the Taklimakan and Gobi deserts in China and Mongolia.

The world’s largest desert, the Sahara of North Africa, which experiences temperatures as high as 57°C, is a trade wind desert. Mid latitude deserts occur between 30° and 50° N. and S, poleward of the subtropical high pressure zones. These deserts are in interior drainage basins far from oceans and have a wide range of annual temperatures. The Sonoran Desert of southwestern North America- is a typical mid latitude desert. The Rajasthan Desert of India and the Thar Desert of Pakistan are parts of a monsoon desert region west of the range.

 

In the southern hemisphere, the desert chainis formed by (i) the Atacama, Puna, and Monte Deserts in South America,

(ii ) the Namib and the Karoo in southern Africa,

and (iii) the vast expanse of the Australian Deserts,

( UNEP 2006 and references there in) ( http://www.unep.org/geo/gdoutlook/112.asp).

Parts of the Arctic and the Antarctic are classified as deserts. These polar deserts contain great quantities of water, but most of it is locked in glaciers and ice sheets year-round. The largest desert in the world is also the coldest. Almost the entire continent of Antarctica is a polar desert, experiencing little precipitation. Few organisms can withstand the freezing, dry climate of Antarctica

Fig. 26.2. The hyperarid and arid regions representing deserts cover about 17 percent of the Earth’s land surface and are found on every continent. The largest hot desert in the world is North Africa’s Sahara (adapted from FAO 2016).

26.5. The Abiotic Environment

26.5.1. Climate

Deserts do not have similar climate, show high spatial and temporal variability of the abiotic environment. The differences in moisture, temperature, soil drainage, topography, alkalinity, and salinity create variations in vegetation cover, dominant plants, and associated species. Resources such as nitrogen are also low in deserts and have been found to limit productivity. On the basis of combination of low rainfall and different average temperatures, three types of deserts could be recognized, i.e., hot and dry desert, cold/temperate desert, and coastal desert (Table 26.2).

Table 26.2. Some characteristics of the three different types of deserts along with their location in different regions of the world (based on UNEP, 2006)

As discussed in Table 26.2, most deserts receive less than 250 mm of rainfall per year. The daytime temperature averages 38°C while in some deserts it can get down to -4°C at night. The temperature varies greatly depending on the location of the desert. To show variations in temperature and rainfall in different months, the climate graph for Sahara Desert is shown in Fig.26.3.

Fig. 26.3 ; Climate graph for Sahara desert. Hot conditions dominate (though often seasonal). Little precipitation (dominated by long dry periods sometimes with brief wet periods). (http://www.bbc.co.uk/)

In cold deserts, Temperature show extreme fluctuations from hot summers to cold winters. Precipitation is low and reasonably consistent growing season is short during the hot months

26.5.2. Desert Soils

Most desert soils are called Aridisols (very dry soil). These soils are CaCO3-containing soils of arid regions that exhibit at least some subsurface horizon development. They are characterized by being dry most of the year with limited leaching. The climate in which Aridisols occur also restricts soil weathering processes.

Aridisols contain subsurface horizons in which clays, calcium carbonate, silica, salts, and/or gypsum have accumulated (Fig. 26.4). Materials such as soluble salts, gypsum, and CaCO3 tend to be leached from soils of moister climates. In hot deserts, soils are course-textured, shallow, rocky or gravely with good drainage and have no subsurface water. They are coarse because there is less chemical weathering. The finer dust and sand particles are blown elsewhere, leaving heavier pieces behind. In dry regions of the Sahara and Australian desert, the soil orders are called Entisols. Entisols are new soils, like sand dunes, which are too dry for any major soil horizon development.

Figure 26.4. Aridisol soil profile, showing a low-humus surface layer above a clay and calcium carbonate horizon. (adapted from https://www.britannica.com/science/Aridisol )

The soils of the Thar desert in India consist of several main groups,i.e., desert soils, red desertic soils, sierozems (brownish gray soils), the red and yellow soils of the foothills, the saline soils of the depressions, and the lithosols (shallow weathered soils) and regosols (soft loose soils) found in the hills. All those soils are predominantly coarse-textured, well-drained, and calcareous (calcium-bearing). A thick accumulation of lime often occurs at varying depths. The soils are generally infertile and, suffer from severe wind erosion (http://media.web.britannica.com/eb-media/81/89881-050-DE3F1A3C.gif)

26.6. Adaptation of Plants and Animals to Dry conditions

In desert ecosystems, both plants and animals adapt to scarcity of water either through evasion or resistance to drought. Some animals have the opportunity to move in response to water availability and long-range migrations are a common feature of drylands. reproduce in the severe climates of the deserts requires that they adopt particular strategies.

Some strategies of plants and animals to adapt to dry conditions in deserts based on Davies et al.(2012),

(https://www.iucn.org/sites/dev/files/content/documents/conserving_drylands_biodiversity_iucn_unccd _book_0.pd) are discussed as follows:

26.6.1. Drought escapers

Many desert plants live for one season. Their seeds may lie dormant for years during long dry conditions. With the onset of rainfall, these plants quickly pass through various growth phases, germination, flowering, fruiting and seed dispersal in only a few days. The plants evading the dry conditions are known as ephemerals. Seeds of most desert annuals have temperature or moisture controlled dormancy which may prevent germination, but seed viability is initially high.

Some animals escape the heat in cool burrows they dig in the ground. The fennec fox, for example, is native to the Sahara Desert. Fennec fox communities work together to dig large burrows, dew can collect in these burrows, providing the foxes with fresh water. However, fennec foxes have adapted to retain enough water in their kidneys from the food they eat. During dry conditions many desert animals remain underground in holes or burrows in which the air is relatively cool and humid; more than half of desert animals are subterranean in their habits.

26.6.2. Drought evaders

The roots of Prosopis and Tamarix can reach up to water table more than 30 meters underground. The plants like Larrea and Atriplex are deep rooted shrubs with a large lateral expanse from the stems. One of the processes of soil water redistribution is called ‘hydraulic lift’, where deep soil moisture is lifted to shallow dry layers through root systems, especially during the night, which improves nutrient cycling and water balance. The hydraulic redistribution of deep-rooted plants markedly improves the survival of shallow-rooted shrubs and herbs in arid deserts, which subsequently maintain species diversity.

Drought evading animals adopt an annual cycle of activities or go into aestivation or some other dormant stage during the dry season. Animals such as certain reptiles avoid the heat by burying themselves underground.

26.6.3 Drought resistors

Desert succulents, such as cacti or rock plants (Lithops) survive dry spells by accumulating moisture in their fleshy tissues. For example, Opuntia (family Cactaceae) and many desert CAM plants can tolerate high temperatures. The succulent plants like Opuntia and cacti develop fleshy organs, accumulating large amounts of water during the short wet season of the year. Succulents have generally shallow roots (e.g., Opuntia, Euphorbia, Agave and Cacti, Fig. 26.5) as they can store water in aerial parts. Leaves are often reduced to spines, and this increases the volume to surface ratio. Spines help to reduce heat load, and dissipate heat. The plants such as Opuntia (cactus) and many desert CAM plants native to deserts can tolerate much higher temperatures.

Animals that have adapted to a desert environment are called xerocoles. Xerocoles include species of insects (beetles, ants, and locust), reptiles, birds, and mammals (Davies et al., 2012). Xerocoles contain a built-in mechanism which lowers the moisture loss all through excretion and evaporation. Camels are very efficient water users. The camels have developed the most remarkable adjustments to desert conditions as they minimize water loss; can tolerate wide fluctuations in body temperature.

26.6.4 Drought Endurers

Shrubs and trees that go dormant, or animals such as frogs that aestivate during dry seasons.

Most of the desert animals are nocturnal. They sleep through the hot days and do their hunting and foraging at night. For example, a desert tortoise’s thick shell insulates the animal and reduces water loss. Using its tail like a parasol, the African ground squirrel (Xerus inauris) protects itself from the sun in the Namib Desert. Some seed-eating rodents do not need water to survive and depend on their burrows to regulate their metabolism, that constitutes an extended part of their body.

In the case of trees, some mixed strategies to adapt to dry conditions are summarized in Box 26.1.

 

26.7. Biodiversity and Community Composition

Deserts typically have a plant cover that is sparse but enormously diverse. Water scarcity plays a major role in influencing biological diversity in the drylands, but variations in topography, geology, soil type and quality and variation in other resources have also been important factors (Davies et al., 2012). The deserts in different parts of the world show large variations in rainfall patterns, temperature regime, and evolutionary history that contributes to their unique biodiversity, diverse life-forms and adaptations. Perennial shrubs dominate most of the deserts, but in a single habitat the plants range from some of the shortest-lived ephemeral plants, to some of the longest-lived giant cacti. In the hot deserts, giant cacti and trees with large fleshy stems coexist with some of the toughest hardwoods, ground-creeping succulents, and fog harvesting rosettes. Importance of desert biodiversity is summarized in Box 26.2.

The Sonoran Desert of the American Southwest has the most complex desert vegetation. One of the unique and keystone species of the Sonoran Desert is the saguaro cactus, characterized by giant, spiny, green arms reaching upward. The giant saguaro cacti provide nests for desert birds and serve as “trees” of the desert. Saguaro grows slowly but may live 200 years. Although cacti are often thought of as characteristic desert plants, other types of plants have adapted well to the arid environment. They include the plants belonging to Fabaceae and Asteraceae.

Cold deserts have grasses and shrubs as dominant vegetation. Some facts about desert ecosystems and their biodiversity are given in Box 26.3. Bactrian camel (Camelus ferus) is critically endangered and highly adapted to the harsh conditions of Gobi Desert (Box 26.3).

Many desert plants have very specific requirement of pollinators and seed dispersers; e.g., giant cacti and agaves of the new world produce sugar-rich nocturnal flowers facilitating the pollinating services of nectar-feeding bats, while the sweet pulp of their fruits attracts birds to disperse. The symbiotic relationship is represented by mutualism (Bees pollinate the cacti and depend on it for food). Desert rodents feed largely on seeds and play an important role in the dynamics of desert ecosystems. Reptiles make up one of the dominant animals in the desert biome and the second group is represented by burrowing animals. Snakes are important because they are predators and they control the rodent populations.

The Namib is a cool coastal desert in southern Africa, extends along the coast of Namibia, merging with the Kaokoveld Desert into Angola in the north and south with the Karoo Desert in South Africa(http://wwf.panda.org/what_we_do/where_we_work/namib_desert/). There is an extraordinary diversity of succulent plants, as well as the shrub-like plant Welwitschia mirabilis, which has only two leaves seen aboveground and can live for over 1,000 years. This plant provides moisture and nutrients for desert mammals, shelter for snakes, lizards and arthropods, and is an attraction for eco-tourists. A number of animals and plants have adapted to life here, including the mountain zebra, gemsbok (Oryx Gazella), short-eared elephant shrew, Grant’s golden mole ,and Karoo bustard (Eupodotis vigorsii)

 

The Thar Desert, extends over about 0.32 million km2 forming about 10% of the total geographic area of India; ~ 60% of the desert lies in the State of Rajasthan and 20% in Gujarat (Krishnan 1977; Sharma and Mehra 2009). It is one of the smallest deserts of the world, has a wide variety of habitats and a high biodiversity. It is the most densely populated desert of the world. During summer, the maximum temperature generally varies between 40 and 45◦C, occasional reaching 51◦C. During winter, minimum temperatures may fall to −2◦C at night. The true desert or marusthali, consisting of Jaisalmer in its entirety, northern Barmer, and the western parts of the Jodhpur, Bikaner and Churu districts (Sharma and Mehra, 2009). Some aspects of the biodiversity of the Thar Desert based on Sharma and Mehra (2009) are summarized in Box 26.4.

Termites and Ants in Deserts

In most arid regions, termites are numerous in species and number. These insects eat and provide food for many other animals; especially important in accelerating decomposition and nutrient-cycling rates (Orgiazzi et al., 2016). African and Australian termites are the most diverse, whereas North American termites are poor in diversity. Many species clear vegetation around their nests, thus affecting plant distribution; their mounds, which are up to five meters wide and one meter high, affect local water patterns. It has been reported that harvester ants eat more than 100 species of seeds, but different species often show narrow seed preferences found in the soil surface. Ants are important seed dispersers because they drop many of the seeds they collect; their nests also provide shelter for several other animal species such as beetles, collembolan.

(http://esdac.jrc.ec.europa.eu/public_path/JRC_global_soilbio_atlas_online.pdf)

26.8. Desert Ecosystem Functions

26.8.1. Net Primary Productivity

In the highly stressful desert environment, net primary productivity is generally very low; however, it is also highly variable from time to time and from place to place. Temporal variations are caused by the occasional input of moisture; this allows the vegetation to grow for only a short period before arid conditions resume. Spatial variations are due in part to the structural patchiness of the vegetation itself, as surface soil beneath shrubs is several times more fertile than it is between shrubs. However, deserts are regions of low productivity in general.

Primary production in desert ecosystems is limited by precipitation, nutrient avail-ability (especially nitrogen), and the species’ production potential (Hadley and Szarek, 1981). The annual above-ground net primary production varies from 30 to 300 g dry wt . m-2 yr-1in arid zones( Hadley and Szarek, 1981). According to these workers, the lowest observed production (2.6 g dry wt. m-2 yr-1) has been reported for a dune community during a dry year. The average production over a four year period was 180   g dry wt m-2 yr-1 (bajada) and 137 g dry wt m-2 yr-1 (playa). The largest year-to-year variation in above-ground production occurs for annual species in the Mojave and Sonoran deserts, and perennial grasses in the northern Chihuahua Desert.

26.8.2 Foodwebs

A     Schematic representation of various components and interactions in a desert food web is shown in Fig. 26.6. For example, in Namib Desert dune ecosystem, the grazing food chain and the detritus food chain parallel one another, with the energy ultimately consumed by the higher trophic level organisms being derived principally from the detritus. For the most part, the system lacks decomposer microorganisms, whereas important detritivore groups include termites, millipedes, and isopods (Seely and Louw, 1980). A portion of the food web based on the grasshopper Trimerotropis pallidipennis, an important herbivore in the Sonoran Desert, is diagrammed in Fig. 26.7.

In deserts, food chains are long with several predation links up to the top, in spite of the desert’s overall low primary productivity (Ayal, 2007; Megías et al., 2011). Most of the desert’s primary productivity is not consumed by herbivores and becomes plant litter. Plant litter in deserts is not readily decomposed by soil micro-organisms, because of periods of low moisture. As a result, litter is consumed by a large number of macrodetritivores , which are preyed upon by only slightly larger small predators, such as arachnids and reptiles, which in turn are preyed upon by birds and mammals. Ayal (2007) suggested that the desert food webs have different properties that enable a high number of trophic levels with low productivity. In desert ecosystems, the body size of the primary consumers determines the length of food chains rather than the quantity of primary production (Ayal, 2007; Megías et al., 2011). Omnivory, intra-guild predation, cannibalism, and indirect effects increase the complexity of desert food webs.

Megías et al. (2011) reviewed interactions above and below ground and food web functioning in an arid environment at the Baza Basin, in the Iberian southeast. According to these workers, some herbivores (rabbits) and granivores (messor ants) create nutrient and detritus-rich patches, which have marked effects on the diversity and abundance of species both above and below ground. Trophic interactions in this semi-arid area are numerous and complex with many of the interactions involving more than two or three organisms. These habitats in which organisms deal with extreme abiotic conditions promote unusual interactions resulting in an increase of biodiversity (Megías et al., 2011). In Negev desert, plant cover has been found to mediate predation; some habitats support an effective third trophic level, indicating that energy is not the major limiting factor in determining the length of food chains (Segoli et al., 2016).

Fig. 26.7. Schematic representation of various components and Interactions of a food web in a desert ecosystem

Fig. 26.8. Partial food web for the grasshopper Trimerotropispa llidipennism in the Sonoran hot Desert, north America. Solid lines indicate preferred food items of the grasshopper and its principal predators. Dotted lines indicate presumed predators and predator-prey relationships between representatives of the higher trophic level (based on Hadley and Szarek, 1982).

26.8.3 Nutrient Cycling

The nutrient supply in arid regions is confined largely to the upper surface (0- 5 cm); lower soil layers are typically “nutrient poor” due to low decomposition and leaching rates. Large quantities of nitrogen are lost to the atmosphere via erosion and volatilization, leaving only a small percentage available to roots of higher plants. Nutrient return via litter and dead plants is also strongly localized around the plants (Garcia-Moya and McKell, 1970). The best documented spatial patterns of nutrient distributions in arid ecosystems are the “islands of fertility” associated with shrubs and trees. The concentric pattern of nutrients and microbial activity that results may be an important factor limiting the establishment and growth of other plant species in the spaces between the permanent vegetation (Muller, 1953). Mycorrhizal fungi are known to enhance nutrition of the host plant and receive benefit from the host plant in the form of usable energy. Desert plants form mycorrhizae with endomycorrhizal arbuscular fungi as well as with ectomycorrhizal fungi. Both form extensive networks of hyphae in the soil, and glomalin, a glycoprotein produced by hyphae of arbuscular mycorrhizal fungi, which plays a crucial in soil structure and carbon storage.

The Nitrogen Cycle describes the routes that nitrogen takes through the ecosystem. The nitrogen cycle relies on bacteria that make nitrogen useful to organisms and bacteria that can return it to the atmosphere (Fig.26.9). The atmosphere (nitrogen gas) goes through the fixation of lightning and the fixation of the organisms and into the soil organic matter (ammonia). Symbiotic fixation by leguminous and other naturally occurring plants may be significant in some warm-desert communities (Noy-Meir, 1974). Nitrogen fixation by soil algae and bacteria probably provides a major input of nitrogen in most desert ecosystems. Annual nitrogen fixation by algal lichen crusts in the Great Basin Desert, the largest U. S. desert, was estimated at 10 to 100 kg N ha-‘ yr-~’. Blue-green algae (Anabaena and Nostoc spp.) perform a similar function in the desert west of Alexandria, Egypt. In the Namib Desert dune ecosystem, the major source of nitrogen is possibly obtained from uric acid excreted by the tenebrionid beetles that forage on the accumulated detritus (Seely and Louw, 1980). Nitrogen return via litter and dead plants is also localized around the plants. In the soil, the organic matter is converted by soil microorganisms to ammonium then to nitrate through the process of nitrification (Fig. 26.9). In arid regions, nutrient levels may limit plant productivity besides moisture availability. Nitrogen appears to be the key limiting nutrient in most hot deserts, while in arid regions of Australia, phosphorus levels are also often insufficient (Charley and Cowling 1968).

26.9. Human impact on Desert Ecosystems

All deserts have evolved under water scarcity and drought has not destabilized them. But human induced degradation impacts the desert ecosystems through habitat conversion, overgrazing, clearance of woody vegetation, farming, irrigation-induced salinity, climate change, over-harvesting, soil and water contamination by agrochemicals, groundwater exploitation, and traffic and urbanization. The Millennium Ecosystem Assessment provides details information on human impact on desert ecosystems (MA 2005). As shown in Fig. 26.10. the impacts are excessive loss of soil, change in vegetation composition and reduction in vegetative cover, deterioration of water quality and reduction in available quantity, and climate change lead to desertification. The intensity and impact of these activities can vary from place to place and change over time depending upon the level of aridity and the varying human pressure on the ecosystem’s resources.

Desertification, one of the most severe types of land degradation in the world, is of great importance because it is occurring, to some degree, on approximately 40% of the global land area and is affecting more than 1 billion people. In areas where the vegetation is already under stress from natural or anthropogenic factors, periods of drier than average weather may cause degradation of the vegetation. If the pressures are maintained, soil loss and irreversible change in the ecosystem may begin to occur, so that areas formerly under savanna or scrubland vegetation are reduced to desert.

Desertification is defined as “land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors, including climatic variations and human activities”(UNEP 1994). Desertification is occurring in many areas of the world. Africa, Asia and Latin America are the most threatened by desertification. 2/3 of the continent is desert or drylands in Africa and is strongly linked to poverty, migration, and food security. The main regions currently at risk of desertification are the Sahel region lying to the south of the Sahara, parts of eastern, southern, and northwestern Africa, and large areas of Australia, south-central Asia, and central North America (MA 2005).

According to Davies et al. (2012), human population increase has led to steady expansion of cropping even on the most marginal lands due to irrigation especially from the Indira Gandhi Canal in the Thar Desert of India. Since the 1960s, changing land use has significantly affected biodiversity, with desert-adapted species being replaced by species that demand more water. Reptiles such as lizards, snakes, which are adapted to survive in extremely dry environments, are likely to be affected by the transformation of desert areas to irrigated crop fields.

26. 10. Resource Management For Desert Ecosystems

Sustainability is difficult to define for desert ecosystems and may not be achieved by prescribing a fixed carrying capacity (such as, the number of livestock that a particular region can sustain). Modern practices of the sedentary pastoralists and the provision of subsidized supplemental animal feed increase pressures on ecosystems due to long periods of stay (MA 2005). The various development pathways that can help avoid or reduce desertification are shown in Fig. 26.11. Land users can respond to stresses by improving their agricultural practices and livestock production, which may lead to reduced soil erosion, and salinization. Improved management practices can lead to high biological productivity and, improved human well-being, besides providing political and economic stability (MA 2005).

Mobile, extensive forms of grazing have been found to be well adapted to the highly variable resource availability in desert ecosystems (Niamir-Fuller et al., 1999). Traditional users have learned to exploit ecosystem cycles sustainably, by practicing mobile and regulated grazing as a means to moderate rangeland use. Sustainable resource management policies must respond to the pulse-reserve character of the desert ecosystem by supporting mobile or otherwise flexible systems, which can remain economically viable over long periods of time (UNEP, 2006).

Mitigating the “bust” part of the cycle is another important component of the sustainable management. This includes emergency support during drought crises, proactive management to increase societal resilience, creating diverse rural income sources, providing credit, and sustaining rural livelihoods during times of stress (UNEP, 2006).

Figure26.10.Schematic description of the various human factors that lead to desertification (based on MA 2005; www.millenniumassessment.org/documents/document.355.aspx.pdf )

Figure26.11.    Some   management practices     to avoid or reduce desertification (based on                                  MA 2005; www.millenniumassessment.org/documents/document.355.aspx.pdf)

26.10. Summary

i. Deserts are arid and hyperarid lands where less than 250 mm of rain falls in a year resulting in low soil water availability, sparse growth of vegetation, and low productivity; evaporation exceeds rainfall in these ecosystems.

ii.   The deserts in Asia are including the Arabian, Mesopotamian, Persian, and Thar deserts.

iii. The Sahara of North Africa is the largest desert in the world.

iv.  In the southern hemisphere, the main deserts are the Atacama, Puna, and Monte Deserts in South America, the Namib and the Karoo in southern Africa, and the vast expanse of the Australian Deserts.

v. Deserts show high spatial and temporal variability of the abiotic environment; the temperature varies greatly depending on the location of the desert.

vi. The deserts in spite of their low productivity have high biodiversity, diverse life-forms and adaptations. The reptiles, including snakes, tortoises and lizards are one of the more familiar species groups associated with deserts.

vii. Desert genetic biodiversity is the key to improving dryland agricultural productivity.

viii. The Thar Desert in India supports rich biodiversity because of its unique location at a biological crossroads of the Indian subcontinent.

ix. The Namib Desert has a shrub-like endemic plant Welwitschia mirabilis, which has only two leaves and can live for over 1,000 years.

x. In desert ecosystems, both plants and animals adapt to scarcity of water either through evasion or resistance to drought.

xi. In the highly stressful desert environment, net primary productivity is generally very low, variable over time and space, complex feeding and predator-prey relationships; food chains are long with several predation links up to the top.

xii. In arid regions, nutrient levels may limit plant productivity besides moisture availability. Nitrogen appears to be the key limiting nutrient in most hot deserts

xiii. Land users can respond to land degradation and stresses by improving their agricultural practices and livestock production, practicing mobile and regulated grazing, and proactive management to increase societal resilience.

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References

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