Trade wind deserts

The desert ecosystems of the world are often considered barren places where conditions change slowly and that support little life. In fact, these ecosystems, which cover about 20 percent of the Earth's surface and are broadly characterized by low annual precipitation rates and a high proportion of bare soil, support an incredible array of highly specialized plants and animals. They are also home to many diverse cultures and livelihoods and play a significant role in the global environment and economy.

However, desert ecosystems are coming under increasing human and environmental pressures that present serious challenges for both the ecosystems and the human populations they support. This article provides a description of biophysical and biodiversity features as well as the challenges and opportunities presented by deserts, particularly as they apply to a specific type: the trade wind deserts of the world. Deserts occur on virtually every continent, including Antarctica. They are classified into several types using different criteria, such as aridity or bioecological productivity.

The U.S. Geological Survey's (USGS) classification of deserts, based on geographical location and predominant weather patterns, includes trade wind, mid-latitude, rain shadow, coastal, monsoon, and polar deserts, as well as paleodeserts—former deserts that are now located in nonarid environments. The most important defining feature of deserts is their aridity, or the ratio between mean annual precipitation and mean annual evapotranspiration, and thus water is the main limiting factor in biological processes in deserts. Desert soils have little or no organic matter, are rich in nutrients, and yet they are unproductive in the absence of water and only become productive after rainfall events.

The trade wind deserts form in two fairly distinct belts on either side of the equator in the region known as the horse latitudes, where the northeasterly and southeasterly trade winds typically form. These winds are the result of warm air rising at the equator and then moving toward the poles as part of the Hadley cell circulation. As this warm air rises, it cools, compresses, and descends near 30 degrees latitude on both sides of the equator, creating a band of high atmospheric pressures.

Winds blowing from the northeast toward the equator in the Northern Hemisphere are deflected to the right due to the Coriolis force and are known as the trade winds. The trade winds blowing from the horse latitudes around 30 degrees north and south meet at the Intertropical Convergence Zone (ITCZ) in an area of calm winds known as the doldrums. The trade winds are dry winds that dissipate cloud cover, allowing more sunlight to heat the land and, in combination with the predominant high pressures caused by the descending air at the horse latitudes, have led to the formation of most of the major deserts of the world.

The world's largest desert, the Sahara of north Africa, also known as the Great Desert, is a typical trade wind desert even though parts of it to the north fall outside the direct influence of the trade winds. Other deserts that fall within this category include parts of the Namib and Kalahari Deserts in southern Africa, the Atacama Desert in South America, as well as large swaths of the Arabian and Middle Eastern deserts. Given the wide range of climatic conditions among these different regions, the temperature regimes and humidity characteristics of trade wind deserts in relatively close proximity to oceans tend to differ greatly from those located in continental interiors.

Diversity of Deserts

Based on climatic conditions, approximately one-third of the surface of the planet is classified as desert. As a result, the diversity of desert landscapes from a topographic and geomorphological point of view is quite astounding, ranging from variously shaped dunes, plains, and mountains to lakes, rivers, oases, and deltas. Even though typically one thinks of dunes and seas of sand when thinking of deserts, the actual proportion of the different trade wind deserts covered by eolian sand dunes varies greatly.

It is as high as 30 percent in the Sahara, southern Africa, and Arabian Peninsula and as low as less than 1 percent in South America. The landscape of most trade wind deserts is the result of millennia of geologic and geomorphologic evolution and sculpting by winds, water, and weathering processes and one of the features under protection as world heritage sites around the world.

Their landscape is thus unique as it continues to be actively shaped and modeled primarily by wind processes that are responsible for creating a wide variety of desert dunes, yardangs, pans, deflation surfaces, and the material for dust storms. Episodic rainfall events can have a rather major landscape-sculpting influence, especially during episodes of high intensity, and lead to the formation of a variety of features such as badlands, ephemeral stream channels locally known as wadis, alluvial fans and debris flows, pediments, and natural arches. Third, different forms of physical and chemical breakdown of rocks and sediments (known as weathering) also create a variety of physical landforms such as desert varnishes and karst, caverns and crusts that in turn support different plant and animal forms and create environmental niches for unique adaptations and endemic characteristics.

Most of the present-day trade wind deserts became established as deserts during the mid-Holocene geologic time period, when changes in incoming solar radiation associated with shifts in Earth's orbit led to changes in global atmospheric circulation and shifts in precipitation regimes. Trade wind desert ecosystems are defined by their overall low levels and high spatial and temporal variability of water inputs that are thus intermittent.

Because water inputs pulsate throughout the year (rainfall is sporadic and mostly unpredictable and mainly occurs in “flashes”), one of the defining features of these ecosystems for both plant and animal life is their ability to create storages and reserves of water for later use. The larger these reserves, the more water that can be accessed in the soils and possibly the more abundant the life forms supported. If there is increased water availability, then there exists increased potential for more stable and potentially less sensitive characteristics of a system, especially in the absence of human-induced disturbances. Another defining feature of trade wind deserts is the spatial stratification in response to soil water availability. For instance, if water is only available in the soil at a depth less than 2 inches (5 centimeters), algae are the most likely life forms to colonize and/or survive.

Plant Life

Ephemeral plants have deeper roots that can access water in the soil at depths between 2 and 20 inches (5 and 50 centimeters), while any water deeper than 20 inches (50 centimeters) can potentially only be accessed by shrub species that develop deep rooting systems. Despite the harsh environmental conditions, deserts are home to complex ecosystems with unique, diverse, and fragile plant and animal associations. As a result of the vertical soil water and nutrient stratification and availability and harsh climatic and environmental conditions (wind, heat, and temperature), most plant species of trade wind deserts have developed a series of adaptations to aridity, climate variability, unpredictable rainfall pulses, and scant summer and winter patterns of precipitation that allow them to survive and sometimes even thrive. Plants escape, retreat short term, or learn how to tolerate desert conditions, or they may only tolerate them to very limited degrees by colonizing oases or wetland locations with perennial water sources.

The plant tolerance adaptations to arid conditions are of a morphological or physiological nature. Usually they are represented by exploitation of favorable microclimates and ephemeral life cycles accompanied by the capacity to leave behind drought-resistant forms of propagation and reproduction (bulbs, seeds, or dormant shrubs). Morphologically, plant adaptations to desert conditions are exemplified by cacti. These plants develop thick cuticles, store and harvest water from dew and fog, and present varied coloration or different shapes and sizes. Finally, among the physiological adaptations of plants, the most common are tissue tolerance to high temperatures, tolerance to cold, nighttime photosynthesizing to avoid water loss, columnar growth to maximize exposure to sunlight in the morning and late day and avoid the excessive heat during midday, or the loss of leaves during the extent dry seasons for woody species. Plants are generally represented by perennial grasses, some succulent species (cacti), and dwarf species of shrubs, as well as many species of colorful lichens depending on the geographic location and the degree of local endemism. As deserts transition toward less arid ecosystems at their margins, for instance, or are characterized by less arid conditions, annual species of grasses begin to establish themselves. They tend to lie dormant in the form of seeds during periods of extended droughts and, with any rainfall above about 0.8 inch (20 millimeters), they sprout rapidly, transforming the landscape into a sea of grass.

Animal Adaptation

Animal species have also learned to adapt in various ways to life in the trade wind deserts. Some of the most common adaptations, as is the case with plants, are centered around increased tolerance and specialization, and animals add behavioral adaptations to the morphological and physiological adaptations displayed by plants. For example, many vertebrates go into prolonged dormancy to survive long periods without water, while others migrate long distances to take advantage of resources available at different times in different locations. Other animals take refuge in burrows beneath the sand, while others sidewind to traverse hot surfaces.

Morphologically, ears, tails, feathers, and differently colored pelages are used to cool or warm animals at different times of day, and other adaptations include water uptake from food, tolerating dehydration, optional hypothermia, or dormancy. Overall, the trade wind deserts are inhabited by relatively few large mammals, mainly because they lack the ability to access and store sufficient amounts of water and withstand the heat and huge temperature fluctuations between day and night. Thus, trade wind deserts are mainly populated by nonmammalian animals such as reptiles, some with high rates of endemism such as desert lizards, several types of sand lizards, and the day gecko and barking gecko. Second are rodent species, such as the gerbil, bats, or an eyeless mole that can swim through the loose, dry sands. Few ungulates such as the gemsbok or springbok in the Namib Desert can be present, and relatively small predators such as cheetahs, foxes, jackals, or hyenas are also present depending on location. Finally, biotic species interactions are also an important component of adapting to and living with the harsh environmental conditions afforded by the world's trade wind deserts.

Most of these interactions are of a positive nature; for example, some plants may serve as “nurse plants” to other germinating seedlings or may provide shelter or shade for animal species. Animals, on the other hand, can assist flora species with seed dispersal or function as effective pollinators. As with the flora and fauna, the peoples of the world's trade wind deserts have not only inhabited these restrictive environments since times immemorial, but they have also uniquely adapted their livelihood, lifestyles, culture, technology, and behavior and have innovated superbly through time in order to call them home.

Human Desert Survival

Unlike plants and animals, people have been able to thrive in deserts, despite meager physiological adaptations, by actively modifying their microenvironments. Examples include the use of cooling shelters and livelihood strategies that fully maximize the available resources, such as hunting and gathering, nomadic pastoralism, and, on conditions, irrigated agriculture. Humans have also adapted their clothing, diets, and housing and have been involved in traditional ways of resource use and management that allow for continued sustainability in otherwise adverse conditions. The hunters and gatherers of the world's largest deserts have an impressive repertoire of plant and animal use and knowledge, even though in recent years their livelihoods are increasingly becoming more mixed.

The San tribes of the Kalahari and Topnaar people of the Namib Desert in southern Africa are the emblem representatives of this ancient lifestyle in today's world, and their traditional way of living is surviving in increasingly isolated spatial pockets. The nomads of the Sahara such as the Tuareg, Tibbu, and Moor tribes are among the best-known pastoralists making a livelihood in these ecosystems, and they often combine hunting and gathering with domesticated-animal grazing and crop growing.

Humans have also been the most active factor shaping and transforming these desert ecosystems, and their impact is becoming more important as population numbers grow and natural resources, especially relatively large animal species in some regions, are dwindling. The trade wind desert landscape covers relatively vast areas of largely undisturbed habitat, represented primarily by sand and rock, with occasional small areas of permanent vegetation. As a result, the most ecosystem and vegetation degradation is found where water (oases, wetlands, or fringe ecosystems and ecotones) is present and these habitats may be therefore heavily altered by human activities, mainly through vegetation removal.

One of the most widespread and potentially least understood human impacts in dry regions is desert expansion, a phenomenon appropriately called desertification. Additionally, especially in regions where agriculture and/or agro-pastoralism are commonly practiced, the continual loss of fertile land on the outskirts of arid areas is also becoming a troubling development, as is overgrazing, which is the loss of protective cover of plant life, leading to increased wind and water erosion. Another major issue affecting these ecosystems is the continued loss of biodiversity, and especially the declines in the remaining populations of large mammals that are adapted to desert conditions, due to hunting for food or recreation.

A prime example is the addax of the Sahara Desert, which has been hunted to near extinction, as well as many other desert-adapted antelopes that are seriously threatened or endangered due to overhunting. A more recent development, desert groundwater pumping from ancient aquifers with extremely slow recharge rates for irrigated agriculture, is becoming increasingly necessary and desirable in countries such as Tunisia and Algeria. Such developments not only deplete aquifers that formed thousands of years ago and may lead to subsidence, but they also create a suite of problems such as soil salinization and degradation because of deficient drainage. From a conservation perspective, very small areas of the trade wind deserts are under official protection status. This may be partially because of the low population and impracticality of defining borders over this vast area as, for instance, fewer than 2 million inhabitants reside throughout the entire Sahara Desert.