Coastal deserts

Coastal deserts are sometimes called mild deserts because the presence of the nearby ocean is a moderating influence on temperature and narrows the distance between the experienced extremes. They are most frequently found along the western coasts near the tropics. Major coastal deserts include the Atacama Desert, the Namib Desert, the western part of the Sahara, and the Baja California portion of the Sonora Desert. In biological terms, deserts are areas with sparse vegetation and wildlife as a result of prevailing aridity, and in which native life has adapted to survive in arid conditions.

Those arid conditions may result from temperatures hot enough to evaporate precipitation too quickly for it to accumulate, or from naturally low levels of rainfall: In either case, evaporation exceeds precipitation. In some cases, cold ocean currents can dessicate coastal terrestrial ecosystems because they contribute little atmospheric moisture through evaporation, and therefore limit coastal precipitation. This is the case with the Namib Desert (along southern Africa's Atlantic coast) and the Atacama Desert (along South America's Pacific coast). Many of the world's deserts are caused by the subtropical high pressure belts at roughly 30 degrees north and south. There may be multiple causes for a desert's aridity—the Atacama, in addition to being on a cold coast with a polar current and falling within one of those subtropical belts, is in the rain shadow of the Andes Mountains, which block the flow of humid air from the Amazon River.

Variations in Aridity

Deserts are generally arid or hyperarid, and usually semi-arid or dry semi-humid areas with similar characteristics are called semi-desert or desert fringe (and they usually appear in the transitions between deserts and less-arid ecosystems). There are exceptions: Most of the Kalahari Desert is semi-arid, for instance, and coastal deserts may be semi-arid or dry semi-humid for certain parts of the year, or in certain geographic portions. Climatically, coastal deserts are one of the three fundamental divisions of deserts. Though cold deserts (e.g., those of central Asia) and hot deserts (e.g., those of Australia) are so categorized because of their prevailing temperatures, coastal deserts are those which, because of their location on or proximity to a coastline, experience frequent fogs. These fogs, which deliver moisture to an otherwise arid environment, are as characteristic a feature of the coastal desert ecosystem as the summer highs and winter lows of hot and cold deserts. In some years, many coastal deserts experience more foggy days than rainy days, and this fog may be the main source of moisture. Coastal deserts also experience fewer temperature extremes because of the moderating effect of the ocean. While hot and cold deserts often experience a diurnal range—the difference between cold nighttime and hot daytime temperatures—of 95 degrees F (35 degrees C), coastal deserts typically have a diurnal range of only 50 degrees F (10 degrees C), comparable to less-arid regions.

Though it is easier to describe deserts in terms of their vegetation and soils, because of the scarcity of climate data as a result of the deficiency of permanent human settlements in deserts, soil and vegetation are in turn impacted by, if not outright determined by, climate. Desert soils are predominantly mineral soils with low organic content. They're generally fine-textured and porous, with good drainage. Coastal desert plants tend toward extensive root systems to take advantage of rainstorms, fleshy leaves and stems for water storage, and in some cases longitudinal ridges and grooves, which allow stems to swell when full of water. Salt bush, buckwheat bush, rice grass, and black sage are all common coastal desert flora. Rainfall is variable in deserts, enough to make annual average statistics less than elucidating. A single heavy storm in the Namib Desert in 2006 brought six times more rainfall than its annual average. Desert rainfall also tends to be “spotty” because desert storms are often convective; rainfall may strike heavily, but affect a very small space, perhaps only a mile or two across.

The role of moisture in the desert is complex because there is so little of it, and what little there is plays an extraordinarily large role in characterizing the ecosystem and the landscape. For instance, outside of a few subareas—such as along a riverbank winding through the desert—deserts are devoid of plant life, which is dependent on regular water access. Even the life in coastal desert fog zones is specially adapted to that fog, and differs from the plant life that depends on periodic rainfall. Because that plant life is not present, there are fewer grasses and root systems to hold soil in place, and less microbial activity and decomposition of organic material impacting the character of that soil. When rain occurs, therefore, it has a much greater erosive effect because the landscape has fewer protections against erosion; the 12 inches (30 centimeters) of annual rain in a desert may have much greater effect than four or five times that much rain in another environment, and a rainstorm can completely change the shape of the land and wipe out fragile ecosystems.

Similarly, the presence of even trace moisture increases the efficiency of temperature extremes, contributing to the fragmentation and weathering of rocks; and in coastal deserts where such extremes are less common, the more saline precipitation leaves salt crystal deposits in the soil, which can pry rocks apart as they grow. Even in noncoastal deserts, salt is more common in the soil than in nondesert environments because there is so little rainfall to wash it away. Coastal desert soil salts can include common or “table” salt (sodium chloride); gypsum [CaSO₄∙2(H₂O)], which is more likely to appear in fog zones or semi-arid areas; and calcrete, a duricrust rich in calcium carbonate. A famous form of desert salt is the sand rose, an assemblage of gypsum crystals that are flat and blade-shaped, resembling rose petals. In the soil of the coastal Atacama Desert, not only common salt and gypsum are found, but also deposits of perchlorates, iodates, and nitrate deposits so concentrated that since the 1830s they have been mined for use in fertilizer.

Inhabitants of Coastal Deserts

There is more life in a desert than is at first apparent. A fundamental part of the coastal desert ecosystem is the biofilm, the thin layer of organisms that forms on the surfaces of rocks, in the cracks that form in them, and in their pore spaces. These organisms include cryptogams—organisms that reproduce by spores, most commonly lichen, algae, and mosses in deserts—as well as fungi and bacteria. The biofilm not only makes a living space out of the pores and cracks in rocks, but in fact contributes to their formation through the acids it excretes; biofilm and weathering are major contributors to the desertification of arid spaces and the breakdown of rock into the characteristic desert soils.

Microbes also play a role in the formation of “desert varnish,” which appears in many environments, but is vividly noticeable in deserts, where a paper-thin coating of mineral clay, manganese, and iron paints the surface of exposed rocks, leaving them brown or black. Microbes also thrive in coastal deserts' fog zones, where the dominant vegetation is cryptogams, which can make better use of fog moisture than vascular plants. Desert fog water has been studied for its chemical composition and ion concentrations in order to learn more about the types of life it can and does support; studies have found that although the acidity, salinity, and levels of chemicals such as manganese, calcium, and nitrous oxide can be higher than the norm, they are all within the levels allowed by the World Health Organization for water collection for human consumption. They are safe for most animal life, in other words; the differences come in part from the way the moisture is collected (plants do best when they can take it from the air or from condensation, rather than relying on a root system underground), and the way that trace elements accrete over time, such as the accumulation of salt in soil.

Much of the fauna in coastal desert fog zones obtains its water through what it eats; while desert cultures have always had erroneous beliefs about various large mammals (such as gazelles) never needing to drink water, it holds true for many small mammals who subsist on insects, larvae, and plant matter. There are also animals that have adapted to obtain their water directly from the fog. The Namib Desert has 13 different species of Onymacris (darkling beetles, or tenebrionid beetles). They have longer legs than other beetles, resembling spiders but for the number of limbs, which allows them to escape the hot boundary layer of air clinging to the desert sand. They obtain their moisture through “fog basking.”

When the fogs appear in Namibia's coastal desert, two species in particular—Onymacris unguicularis and O. bicolor—have been observed climbing to the tops of dunes in great numbers in order to allow the fog to condense on their abdomens (cooler than the sand temperature because of those long legs) and drip into their mouths. Fog basking has been shown to have evolved independently on two separate occasions, and to constitute a true adaptation to desert conditions. There has been no conclusive proof that this is also true of stilting (the term entonomologists use for the tenebrionids' long legs).

The fauna present in a coastal desert may depend in some way other than the fog on the nearby presence of the ocean. The black widow spider (Latrodectus indistinctus) thrives in the Namib Desert, and although the principal effect of this within the desert is to reduce the herbivore population on the dunes (which protects dune vegetation, as well as the microbial ecosystem surrounding it), these high spider populations are maintained not by prey in the desert, but by the detrital-algae-feeding flies the black widow consumes, which have flown into the environment from the adjacent marine biome. In Baja, California, coastal spiders are six times more abundant than inland, and they subsist on a principally marine diet.

The Baja Desert system is also home to 3–24 times more insects, scorpions, lizards, rodents, and coyotes on the coast than inland, and studies of coastal desert mammal populations suggest a great dependence on marine life, which constitute more than half the diet of the coastal coyote. Marine-subsidized coyotes also depress coastal rodent populations; on desert islands off the coast of Baja, coyote-less islands have far more rodents than the coyote-rich islands. The proximity of the ocean also leads to changes in the types of algae present in the ecosystem. In the Atacama Desert, the marine green alga Ulva is foraged by many of the invertebrates, who in turn are preyed upon by scorpions (Brachistosternus ehrenbergii), solifuges (Chinchippus peruvianus), and geckos (Phyllodactus angustidigitus). Coastal deserts play a key role in nutrient transfer from marine biomes to terrestrial biomes, because some of these desert fauna will in turn be preyed upon by predators in the fringe, and so on as those nutrients move further and further inland as they pass through the food chain. It has even been demonstrated that the productivity of Amazon rainforests depends on fertilization from phosphorous-rich dust blown in from the coastal western Sahara Desert, 3,107 miles (5,000 kilometers) away.

Desert Adaptations

The Atacama Desert is thought to be the driest in the world, receiving no rainfall from 1570 to 1971, over 400 years. While other coastal deserts often receive periodic floods, they are not always able to make use of that suddenly available moisture. Some deserts develop temporary lakes as a result of flooding, and if this happens regularly enough, an ecosystem can develop around those lakes. However, when the floodwaters are the result of overflowing rivers or other bodies of water outside the desert environment, they will bring marine life with them, and in some cases may introduce new microbial life or other fauna, or attract new migratory birds or other fauna. Because there is so little life in the desert, the introduction of a new species can have drastic consequences; in Australia, a common problem is the threat to grasses holding desert dunes together because of overgrazing by newly introduced rabbits.

Desert centers of endemism—species peculiar to a given ecosystem, in other words, deserts with a great number of unique species—are always near coasts (which includes coastal deserts), which is believed to be because of Pleistocene expansions of the central deserts. Endemic species are very often well-adapted for a particular ecosystem, but unlike species found in a wide variety of environments, their life cycle may depend heavily on the balance of that ecosystem; they may be ill-equipped to deal with a new disease, predator, or competitor for resources, where a more commonplace species might be more robust, having weathered those challenges many times in its evolutionary past.

The pulse-reserve paradigm of describing rain events in the desert considers deserts as pulse-driven ecosystems. A rain event triggers a pulse of biological activity; some of it is lost to consumption or mortality, and the remainder is committed to a reserve, such as seeds or the water storage of vegetative life like geophytes or succulents. Life in the desert has adapted to dealing with rainfall that arrives in pulses, rather than continuously, and which is sometimes too much and usually too little. For example in the Arctic, the life cycle of various microflora and microfauna may all but pause during the winter, when the sea is covered in ice and neither new nutrients nor sunlight are available, only to pulse back to life during the summer thaw. In the desert, many species conserve their energy during the dry periods in order to act on the sudden availability of rainwater to replenish their reserves.

The manner of adaptation varies considerably; even in the same desert, a kangaroo is adapted to move long distances from food source to food source in the most energy-efficient way possible, whereas smaller rodents are adapted to move very slowly in order to consume less of their energy with unnecessary movements. Some amphibians in coastal deserts have accelerated larval stages, which allow them to reach maturity faster, improving their chances of survival; on the other end of the spectrum, some burrow-dwelling toads essentially hibernate for the dry season, sealing their burrows off with gelatinous slime that is washed away by the next rainy season. While coastal deserts receive some moisture from fog, this is true only for a portion of the desert, and the pulse-reserve paradigm still holds true for the desert's response to rainfall. Pulses replenish soil water better than fog can, leading to plant growth, and perhaps triggering the mating of certain fauna.

The ecosystems that make the best use of floodwaters include shrubs with grasses at their bases instead of bare soils, because the root matrix of the grasses and shrubs creates macropores in the soil, encouraging greater infiltration of water. Otherwise, most of the water will simply run right off the dusty, hard-packed surface of the desert, compacted by weathering and wind, glazed by desert varnish, and will eventually evaporate or continue to flow into another environment at a lower elevation.

In the Namib Desert, plants with different types of C₄ photosynthesis, all of which initially fix carbon dioxide (CO₂) in the mesophyll cells to form oxaloacetate, have different responses to rainfall. Some convert the CO₂ mainly into aspartate, with an inner bundle between metaxylem elements and the Kranz sheath. Others primarily convert the CO₂ into malate, with a single chlorenchymatous or Kranz sheath and centrifugal chloroplasts formed around vascular bundles. The malate converters lack the well-developed grana and higher mitochondrial frequency of the aspartate converters, and increase in abundance as rainfall increases, while the aspartate species decrease as rainfall increases.