|Life at the Extremes |
Sensitive Fish Species
Fire and Weeds
Rising to the Occasion
|Shaping the "Classic American Desert" |
Despite its archetypal status, the Mojave does not really fit the classic image of a desert. For example, it contains sand dunes only in limited areas, and is characterized by parched mountains that rise abruptly from alternating plains, or basins. The area of most dramatic relief is around Death Valley, California, where the elevation drops from 3,400 m above sea level at Telescope Peak to 88 m below sea level at Badwater--a surface distance of only 23 km.
The Mojave's mountains are in fact burying themselves in their own debris as storm-driven rocks and boulders pour from canyons, spreading out to build gigantic alluvial slopes. Where several deposits join, they form bajadas, long, sedimentary slopes sprinkled with cacti and creosote bush.
Bajadas are capped with desert pavement, composed of gravel and small
rocks, overlaid with delicate desert varnish--thin clay and other particles cemented to rock surfaces by manganese and iron that have been oxidized by bacteria over thousands of years. The large expanses of the Mojave's bajadas make them the dominant geomorphologic feature of the Mojave Desert ecosystem.
|The sands of Kelso Dunes, within the Mojave National Preserve, are of a rare type known as "booming" sands because of their tendency to hum or boom when in motion. About 200 m high, these dunes are some of the tallest in North America.|
DORAN SANCHEZ, BLM
Since basins have no outlets, when rainfall collects and evaporates, it leaves behind flat alkaline deposits on which almost nothing will grow. The white surfaces of these playas, or dry lakes, are startling against the duller, brown background. Playas, such as Soda Lake at the end of the Mojave River in California, are among the flattest landforms in the world. Dry riverbeds, or arroyos, are also present in the Mojave, as are volcanic remnants.
The Mojave Desert is a subsection of the Basin and Range Physiographic Province, which is characterized by long, north-south-trending mountain ranges separated by broad valleys. Igneous, metamorphic, and sedimentary rocks of the Mojave Desert reveal prehistoric landscapes as old as 2.7 billion years--over half the age of the Earth.
During the Paleozoic era the Mojave Desert was covered by shallow seas, as evidenced by fossil marine creatures in limestone and dolomite. These fluctuating seas deposited thousands of meters of sediment that can be seen in banded mountains throughout the Mojave. Limestone has played an important role in the history of the Mojave as a host rock for metallic mineral veins, which were produced when magma intruded.
During the Mesozoic era the Mojave region was uplifted. Salt-laden water bodies evaporated, leaving behind puffy crusts of evaporite minerals, such as halite and gypsum. Rivers and ephemeral streams carried large amounts of eroded materials into the lowlands.
The Mojave's climate became even more arid and windy than it is today, causing the formation of belts of sand dunes and creating the cross-bedding patterns that are visible in the colorful sandstone of Nevada's Red Rock Canyon and elsewhere.
At the beginning of the Cenozoic era tectonic activity resumed. Movement along faults deformed the Mojave's layers of sediment and underlying metamorphic rock. Shifts along thrust faults forced older layers on top of younger ones, as can be seen at Red Rock Canyon National Conservation Area.
Faulting was accompanied by an increase in volcanic activity. At present-day Death Valley, these processes pushed up mountains and caused the valley floor to drop, forming a below-sea-level trough. Hot springs near Nevada's Lake Mead and cinder cones and lava flows in the eastern Mojave are reminders of this active period. About 17.8 million years ago, a powerful volcanic eruption in the Woods Mountains dispersed a low cloud of ash and rock fragments across the countryside, burying shallow lakes and stands of trees. The fossils of ancient life forms are preserved in sediments below this ash layer.
The Mojave is a geological "accommodation zone" between the surrounding Pacific and North American plates, which causes the region to experience relatively strong earthquake activity along numerous faults. For example, a 7.6 magnitude earthquake in the western Mojave shook a wide area in June 1992--the largest quake to strike California in 40 years. In October 1999, a 7.1 magnitude earthquake jolted Barstow, California, and surrounding areas. Twenty-five aftershocks of magnitude 4.0 or greater occurred within 36 hours of the main shock.
Life at the Extremes
The Mojave Desert, the driest and smallest of the North American deserts, occupies only 7 percent of the total arid lands of North America, yet offers a wide diversity of habitats, from salt pans to mesa tops, high sand dunes to alkaline springs. It is home to over 2,500 species of plants and animals, more than 100 of which are considered to be in some degree of peril.
The Mojave receives an average of less than 15
cm of precipitation per year, distributed unevenly. The California portion of the desert, for example, often receives as little as 3 cm of rain. In fact, the Mojave is considered a rain shadow desert because the mountains surrounding it block precipitation. The former community of Bagdad, California, once went an incredible 767 days without a drop of rain. Summers are long and hot in the Mojave; in fact, the highest temperature ever recorded in North America was 57ºC in 1913 at Death Valley.
|The towering, multihued spires of Red Rock Canyon evidence the strong carving power of water. The area's rugged cliffs have been used as a scenic back-drop for countless motion pictures and television programs. |
The Mojave Desert is dominated by low, widely spaced shrubs, including the creosote bush (Larrea tridentata) and Mojave sage (Salvia mohavensis). The Mojave flora includes few trees other than the signature Joshua tree (Yucca brevifolia), but features a number of cacti, several parasitic plants, and a vast variety of wildflowers. About 25 percent of the plant species in the Mojave are endemics--found nowhere else in the world. The spiny-armed Joshua tree lives only within the Mojave Desert; in fact, the boundaries of the species' range define the areal limits of the ecosystem. And some remnant environments within the Mojave host plant species that are found nowhere else within even this ecosystem.
The Mojave's diversity of vegetation results from its extensive variety of rock types, wide range of elevations, and the existence of microclimates. Some of the higher mountain ranges, such as the Spring Mountains, even host coniferous forests.
The fauna of the Mojave Desert is composed of both wide-ranging species, such as the desert bighorn sheep (Ovis canadensis) and jackrabbit, and highly specialized species that live only in small relic habitats, including the Inyo Mountain's slender salamander (Batrachoseps campi) and several species of pupfish (Cyprinodon sp.).
Because of the harsh nature of the Mojave Desert ecosystem and the rarity of free water, riparian systems provide oases of species diversity not found elsewhere in the region. Important riparian systems within the Mojave include the Colorado, Mojave, and Amargosa Rivers. Desert springs also offer some of the Mojave's most unique riparian environments. Virtually all Mojave riparian systems are extremely deteriorated because of the introduction of exotic flora and fauna and the diversion of water resources by humans.
The Mojave Desert encompasses large tracts of publicly owned lands. The Bureau of Land Management (BLM), the National Park Service, and the Department of Defense manage most of these lands. But because ecosystems do not recognize such boundaries, federal land managers work in cooperation with one another, local governments, and private landowners.
Rapid urban expansion--coupled with scarce water resources--creates special management challenges for the entire region. Increased human use of the desert is the direct cause of the majority of resource issues. Livestock grazing, utility corridors, military training, and recreational activities all impact fragile native plants and animals.
In addition, pressure is mounting as cities and counties grow up to adjacent public land boundaries with no room to expand. In some cases, federal lands near cities are being sold or exchanged for more environmentally sensitive lands further from urban areas. For example, Congress is considering legislation that would sell public lands to Clark County, Nevada, for construction of a new airport to serve Las Vegas. The money from this sale could be used to buy environmentally important land elsewhere.
|California's Afton Canyon, one of few places where the|
Mojave River flows aboveground, is a prime location for
camping, bird watching, and wildlife viewing.
While some see the desert as a barren landscape suitable for garbage dumping, many citizens understand the beauty and fragility of the landscape and seek to restore it. A recent volunteer cleanup of public lands near Barstow, for example, attracted more than 500 people from nearby communities, who collected 5.5 metric tons of trash, including discarded cars and washing machines.
The Mojave Desert attracts millions of tourists each year to such sites as Death Valley, Lake Mead, Joshua Tree National Park, the Mojave National Preserve, and the millions of acres of public lands containing canyons, sand dunes, and dry lakes managed by the BLM. Therefore, managers not only must protect the fragile desert environment, but they also must plan to accommodate an increasing variety of uses and demands for desert lands.
Groundwater and the Mojave and Colorado Rivers are the primary water sources for the Mojave Desert's plants and animals, and for the booming populations of desert cities. Las Vegas is the nation's fastest growing city; its population is expected to double in the next 40 years. In southern California, fast-growing desert communities in Victor and Antelope Valleys will nearly double in size over the next 25 years. Providing water to sustain metropolitan growth in desert areas is a constant challenge, and often has extreme consequences. For example, so much water is drawn out of the Colorado River that it often runs dry before it reaches the Pacific Ocean.
|Afton Canyon is designated as an Area of Critical Environmental Concern to protect plant and wildlife habitat and to preserve the scenic values of the canyon's riparian area. |
Las Vegas--"the meadows" in Spanish--was once watered by underground springs, creating a natural oasis. Today this groundwater is pumped faster than it is replenished, so much of the city's water is pumped from the Colorado River at nearby Lake Mead, one of several reservoirs for Mojave communities. (Reservoirs hold vast amounts of water, but their huge surface areas result in considerable evaporation in desert environments.)
Because local water demand is fast approaching capacity, large-scale engineering projects to draw more water from Lake Mead are underway. Complex water laws control the amount of water that can be taken by each state. With a finite amount of water available locally, new water sources will eventually be needed if growth is to be sustained. Proposals to pipe water from distant aquifers will cost billions of dollars.
Critics fear that the ecological costs of such plans could also be great. Groundwater pumping to serve urban and agricultural areas lowers the water table, which can have devastating impacts on desert habitats such as mesquite woodlands and fragile riparian complexes. Some interested parties are calling for more water conservation and restrictions on urban growth to ensure a healthy water supply for communities as well as for the Mojave's plants and animals.
One of the more popular--and controversial--recreational uses of the desert is riding motorcycles, all-terrain vehicles, dune buggies, and four-wheel-drive cars and trucks. Such off-highway vehicles (OHVs) can seriously damage fragile desert ecosystems.
Compacted soils severely inhibit the growth of plant roots. They also keep water from seeping into the ground, increasing water runoff and altering the water-holding capacity of the soil. This is particularly significant because disturbed desert soils can take thousands of years to be restored given the desert's limited supply of biodegradable materials (or biomass), intense solar exposure, and low moisture.
|Off-highway vehicle use can damage fragile desert soils, leaving scars that may persist for many years. |
Vehicles can also cause direct damage by running over and destroying plants and even animals such as lizards and tortoises, or crushing the underground burrows where tortoises often spend much of the day. As a result, OHV use is managed by federal agencies to keep vehicles away from the most fragile areas, while allowing OHV activities on more durable surfaces, such as barren sand dunes or clay flats. In most cases OHV use is confined to designated roads and trails; some areas are fenced to keep vehicles out of sensitive habitat. Other areas, such as the Desert Tortoise Research Natural Area near Ridgecrest, California, and designated wilderness areas within the Mojave, are closed to OHV use altogether. In many areas of the Mojave the BLM has implemented OHV policies that require active management of sensitive areas.
|The Stoddard Valley Off-Highway Vehicle Area near Barstow, California, offers diverse landscapes for off-highway vehicle recreation. The area is characterized by steep, rocky mountains; rolling hills; open valleys; and winding sandy washes. |
Balanced management helps prevent OHVs from stirring up and loosening thin desert topsoil layers--which accelerates erosion--or destroying microbiotic crusts, thin soil surface layers bound together by living microorganisms, including lichens and algae. At the same time, management of OHV access avoids soil compaction and the creation of hardened "roadways."
Extensive military exercises that entail the movement of heavy equipment also can have serious impacts on desert soils similar to, but much more intensive than, those caused by OHVs. The U.S. Army, Air Force, Marine Corps, and Navy all operate major training or testing facilities in the Mojave Desert. The Department of Defense is working with the Department of the Interior to reduce the impact of its activities.
Because the Mojave Desert is sandwiched between high-tech, urban areas, there is an increasing need to connect these areas with new fiber-optic telecommunications lines and other utility services. Often these utility rights-of-way bisect wildlife habitat, causing weeds to spread and reducing species diversity. Land managers are working with utility companies to reduce these impacts by establishing right-of-way corridors along already established routes and by limiting new routes.
|Constructed in the 1960s, the Los Angeles Aqueduct carries water from Owens Valley, Nevada, to Los Angeles, California. The land's recovery from the aqueduct's burial is still incomplete as evidenced by the nearly barren surface strips extending from either end of this exposed portion of the pipeline where it crosses a canyon. |
Sensitive Fish Species
With water in diminishing supply, desert fish species face great peril. In fact, more than half of the native desert fish species in the Mojave are either federally listed as threatened or endangered or are species of concern. An estimated eight percent are already extinct. Native desert fish often are specifically adapted to the heat and elevated salinity of the water, particularly those species that evolved in small, isolated springs
The Devils Hole pupfish (Cyprinodon diabolis), whose entire population lives in one small spring-fed pool in Nevada, has developed a physiology specifically adapted to the high temperature and salinity of this water. The Ash Meadows National Wildlife Refuge was created in 1984 to preserve the largest concentration of endemic plants and animals in one location in the United States. Twenty-four species, including the Devils Hole pupfish, have adapted to this unique salty desert environment and are found nowhere else in the world. Habitat restoration efforts include converting concrete irrigation ditches back to their natural courses and removing nonnative tamarisk trees and crayfish.
River fish in desert areas are also in decline. For example, native fish of the Colorado River, such as the Colorado squawfish (Ptychocheilus lucius) and razorback sucker (Xyrauchen texanus), have suffered population declines due to dams that have altered the temperature and flow of the river. Introduced sport fish also outcompete these native species.
The desert tortoise (Gopherus agassizii) is a widespread desert species of the southwestern United States. Since its federal listing as a threatened species in large parts of its range, it has become a primary consideration in almost every activity that occurs in the Mojave Desert. Population
declines are due primarily to disease, collection, raven predation, and loss of habitat as a result of urbanization. Other contributing factors include the spread of invasive species and human activities such as off-road driving, military maneuvers, and livestock grazing. These activities not only crush the tortoises and their eggs, they also destroy native plants that provide food, water, shade, and shelter. Urbanization causes habitat fragmentation, a serious problem as each tortoise may require access to up to 3.9 km 2 of habitat. Some desert tortoises also suffer from an upper respiratory disease, and ravens prey on hatchlings. Raven populations have increased in the Mojave as people have brought new food sources, such as landfills and garbage dumps, to the desert.
|As a result of the desert tortoise's listing as threatened over much of its range, the welfare of the species is a primary consideration in almost all Mojave Desert management decisions. |
BLM RED ROCK CANYON NATIONAL CONSERVATION AREA
The BLM administers about 75 percent of the remaining high-quality desert tortoise habitat in the Mojave Desert. The agency is taking a lead role in tortoise habitat management, and has eliminated livestock grazing on almost a half-million hectares in Nevada alone. The BLM has designated these lands as Areas of Critical Environmental Concern, according them special protections.To both accommodate urban growth and protect the tortoise, a team of federal, state, county, city, and private partners has developed a special "habitat conservation plan" to help streamline compliance with federal and state endangered species laws. This has allowed continued development around Las Vegas and other areas in Clark County, Nevada, while raising millions of dollars in mitigation fees to support broad conservation and research efforts. One result is the Desert Tortoise Conservation Center, located on public lands south of Las Vegas, which supports critical research on the species.
Under BLM leadership, an interagency habitat conservation plan is also being prepared for a 3.8 million hectare region within the western Mojave. The plan addresses the conservation needs of over 100 sensitive species in the face of rapid development of suburban Los Angeles.
The desert bighorn sheep will also benefit from desert tortoise habitat recovery efforts. Like the tortoise, its population has suffered as a result of habitat fragmentation. It also could not compete with domestic livestock and wild burros for food and water. Its numbers declined precipitously in the Mojave Desert until about 20 years ago, when steps were taken to reverse this trend. BLM and the State of Nevada have now reintroduced the desert bighorn sheep to most of its former range in Nevada, and managers are cautiously optimistic about its future.
Fire and Weeds
Fires historically were not common in the Mojave Desert, and when they did occur, they did not spread rapidly or very far. The natural spaces left between native desert plants usually kept fires under control. Today these spaces are increasingly filled by nonnative grasses, such as cheat-grass (Bromus tectorum) and red brome (Bromus rubens), which remain rooted after they die, creating an uninterrupted fuel source for fires. (By contrast, native desert annuals crumble and blow away after they die.) Nonnative grasses also are the first to emerge after a fire, giving them a competitive edge. The result is weed-infested landscapes. In the Mojave, land managers now suppress fires when possible to inhibit weed growth..
|Prescribed burns (purposely set, controlled fires) are sometimes used in the Mojave Desert to remove tamarisk overgrowth and improve riparian habitat. |
Tamarisk (Tamarix chinensis), another desert alien, is one of the most invasive shrub-trees in the southwestern United States. Also known as saltcedar, it destroys riparian habitats, taking in great amounts of water and depositing salt into the soil. The plant also fuels fires with its large masses of dry branches and leaves. After a fire it quickly re-sprouts and easily out-competes native plants because of the salt it has deposited into the soil. Tamarisk also has an advantage because the Mojave's native cottonwood and willow trees need periodic floods to survive, a natural process that has been virtually eliminated by modern water control projects. Most riparian areas in the desert are therefore dominated by tamarisk.
Complicating removal of tamarisk is the situation of the endangered southwestern willow flycatcher (Empidonax traillii extimus), a bird that has turned to the tamarisk for shelter because of the destruction of its native cottonwood and willow habitat. This poses a special challenge for managers, who must take care not to disturb this endangered species as tamarisk is removed.
Rising to the Occasion
The Mojave is the smallest North American desert, yet it has captured the public's
imagination far out of proportion to its geographical extent. And thanks to its wide exposure as the setting for countless films and advertisements, the Mojave's allure has only grown in recent decades.
|The endangered southwestern willow flycatcher uses nonnative tamarisk for shelter. Population declines are due in part to cowbird parasitism. Female brown-headed cowbirds (Molothrus ater) lay their eggs in the flycatchers' nests, and the stronger and more aggressive cowbird hatchlings often kill the young songbirds. Because cowbirds are typically associated with and follow cattle, land managers are altering grazing regimes during flycatcher-breeding seasons. |
S. AND D. MASLOWSKI, U.S. FWS
|Desert bighorn sheep are usually found near "escape terrain" (steep, rocky slopes free of dense brush) where they are capable of evading almost any predator. This tendency sometimes results in the isolation of groups of individuals when human activities encroach. |
GARY M STOLTZ, U.S. FISH AND WILDLIFE SERVICE
As more people seek escape from the congestion of nearby urban centers, the Mojave Desert, paradoxically, has become an oasis for many diverse groups, from hikers and ranchers, to biologists and OHV riders. Each type of user prizes this unique ecosystem for different--and sometimes conflicting--reasons.
Fortunately, Mojave users have begun to agree that this harsh landscape is at the same time a deceptively fragile one, harboring ecological riches found nowhere else on Earth. Federal agencies and other government groups are making inroads into reversing environmental damage inflicted over generations of past use, and private citizens are volunteering in growing numbers to help conserve the Mojave's treasures. The encouraging results of many such efforts offer lessons about what can be accomplished when people and organizations work collaboratively for common conservation goals.
The one-of-a-kind world of the Mojave undoubtedly has countless hidden treasures to reveal and important lessons still to offer. As its guardians, citizens and governments must work to ensure that the Mojave will survive intact to impart its wisdom for millennia to come.
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