Mesozoic Granite
Granite Mountains
About 180 million years ago during the mesozoic era, the Farallon Plate and North American plates converged causing the denser oceanic Farallon Plate to subduct beneath the western edge of the lighter continental North American Plate. As the Farallon plate subducted down into the upper mantle, the oceanic plate melted into molten rock. The buoyant molten rock then rose into the crust to formed magma chambers. About 25 million years ago, in regions where the Farallon plate completely subducted and subduction ceased, these magma chambers cooled and solidified below the surface of the earth. When rock solidifies below the surface of the earth it is called intrusive igneous, or plutonic rock. One of the most common plutonic rocks is granite, which is made primarily of feldspar and quartz. The mountain building processes of faulting, folding and isostatic equilibrium, uplifted the granite plutons.
Some of the mountains and land features of the Mojave Desert are formed of granite including the New York Mountains, the appropriately named Granite Mountains, and the Cima Dome batholith. These plutons formed in the same time period as the granite that formed the batholiths of the Sierra Nevadas. They are all known as mesozoic granite.
Weathering and erosion have since shaped the large granite plutons into the mountains we see today. The granite boulders at Granite Mountain are a result of cracks forming in the plutonic rock when it cooled deep below the surface of the earth. Before uplifting, acidic water seeped into the cracks of the rock changing the feldspar into clay. After uplifting the rocks broke apart at the cracks. Wind and water eroded the soft clay corners of the rock leaving the round boulders we see today.
Plate Tectonics
San Andreas Fault Line
About 25 million years ago during the Tertiary period of the Cenozoic era, portions of the Farallon Plate completely subducted beneath the North American Plate. When this happened the Pacific Plate collided with the North American Plate creating a new type of plate boundary. Because the Pacific Plate moves northwest in relation to the North American Plate moving west southwest, this new plate boundary formed a transform boundary, known as the San Andreas Fault Line. As more of the Farallon Plate subducted beneath the North American Plate, the length of the San Andreas Fault Line increased.
Today the San Andreas Fault extends north of San Francisco to south of La Paz in Baja California. As the two plates move alongside one another, small blocks of continental crust occasionally break free from the North American plate and rotate between the two plates. One of the more known crustal blocks is the Transverse Mountain Range. As this block rotated it formed the big bend in the San Andreas Fault, resulting in a lock zone, that is an area where friction between the two plates is increased. As the Pacific plate continues its northwest movement, this lock zone pulls, stretches and rotates the Mojave Desert in a process called extension.
Paleomagnetism
Mojave Desert Crustal Block
The portion of the San Andreas Fault that connects the town of Gorman to the Cajon Pass marks the west southwestern boundary of the Mojave Desert, which is easily identifiable by the northern foothills of the San Gabriel Mountains. The San Andreas Fault is a right-lateral fault, but this doesn’t mean the Mojave Desert is moving to the left as shown in most diagrams, it is actually being pulled to the right, just at a slower rate then the Pacific Plate.
The northwestern boundary of the Mojave Desert is marked by the Garlock Fault, which is located at the southern slopes of the Tehachapi Mountains. The orientation of the Garlock fault has rotated clockwise since its formation and forms a left-lateral fault due to the clockwise rotation of the Mojave block.
This rotation is proven by measuring the magnetic orientation of magnetite in rocks which allows geophysicists to determine the directional orientation of rocks when they were formed. In theory, rocks should have an internal magnetic compass that points to magnetic north. However, the magnetism of volcanic rocks formed during the Miocene epoch in the Mojave Desert don’t point north, they point north northeast at 025 degrees. This proves that the Mojave block has rotated 25 degrees clockwise in the last 15 to 20 million years.
Basaltic Volcanoes
Amboy Crater
Volcanic rocks are also known as extrusive igneous. These are rocks that are cooled and solidified rapidly, usually during a volcanic eruption. The most common type of extrusive igneous rock is basalt.
Basaltic volcanoes are generally associated with oceanic rift zones and hot spots like we see in Hawaii. However, the basaltic volcanoes of the Mojave Desert are a result of extension, fracturing the crust forming faults and blocks. The areas along the fractures are weakened and when pressure is built up from present magma chambers magma is released through these fractures.
Established as a national landmark in 1973 is Amboy Crater. Amboy Crater is a basaltic volcano located near Bristol Dry Lake and sits in the Barstow-Bristal Trough. The crater stands 250 feet tall with a ridge circumference of 1.5 miles. Surrounding the Amboy crater is a 27 square mile lava field of basaltic rock. Amboy Crater is one of the youngest volcanoes in the United States, estimated to have first erupted a mere 80,000 years ago. Amboy Crater has had four eruption cycles by evidence of four smaller cones within the main crater, the most recent of the four eruptions occurred just 5,000 years ago.
Cinder Cones
Cinder Cone National Landmark
Also established as a National Landmark in 1973 is the Cinder Cone National Landmark. The Cinder Cone National Landmark identifies the 32 cinder cones encompassing 41 square miles of a single volcanic field west of Cima Dome.
Cinder Cones are formed from basaltic volcanoes when erupted cinder falls and accumulates near a volcanoes vent. As the pile of cinder grows and become too steep to remain stable. Avalanches of cinder will spread the fallen cinder outward away from the volcanoes vent resulting in roughly 30 degree conical shaped volcanoes. Eruption can last for hours to months at a time and rarely do cinder cones grow higher than 1,000 feet in height.
A basaltic volcanic field is made up of one or more cinder cones that are part of a single tectonic structure or fracture. It is likely that each cone hosted a single eruption. The Cinder Cone National Landmark volcanic field first erupted 7.6 million years ago and last erupted as recently as 10,000 years ago. This volcanic field is not considered extinct and could erupt in the future as the Mojave Desert continues to expand.
Lava Tubes
Cinder Cone National Landmark
Another geological feature associated with basaltic volcanoes are lava tubes. Dependent on the makeup of the magma associated with the volcano, magma flows can vary in viscosity and fluidity. Rhyolitic magma has a high viscosity making it thick and sticky-like, preventing it from flowing very well. In contrast to basaltic magma which has a low viscosity, making it thin and more runny in comparison to rhyolitic magma.
As magma flows to the surface of the earth, it will eventually cool and solidify into rock. Generally the top layer of a lava flow which is first exposed to the atmosphere will cool and solidify while the underlying layers of magma avoid cooling and remain in liquid form. The underlying layer of lava will continue to flow through the solidified rock, forming rivers of lava that flow through these tubes and tunnels.
Once the magma chamber feeding the volcano ceases, the remaining magma will either cool and solidify in place plugging the tunnel or it will continued to flow and drain the tunnel dry, leaving behind these beautiful tubes for us to explore.
Sedimentary Rock Deformation
Rainbow Basin Syncline
Sedimentary rock has distinct layers of sediment that may differ in color. The rock forms from the compaction and cementation of sediments that have eroded from other rocks and organic materials that are deposited as marine sediment in horizontal layers, with the oldest layers on the bottom and the youngest layers at the top. This cementation process is called lithification. Of the three types of rock, sedimentary is usually the softest, making it the most susceptible to deformations.
The Barstow-Bristol trough is a basin that stretches 50 miles long and is 10 miles wide. The sedimentary rock within the basin is from sediments that lithified about 23 to 13 million years ago. The folded rock at Rainbow Basin forms a syncline, meaning the rock is pushed downward, and is a result of compressional forces generated by lateral faulting. This type of deformation is called ductile deformation and occurs when sedimentary rock has a taffy or clay like consistency. Rock deformations are dependent on time, rock composition, temperature and confined pressure.
Alluvial Fans
Amboy
In mountainous terrain, rainwater runoff is one of the leading factors to cause erosion. However, because the Mojave Desert only receives about 4 inches of rain per year, erosion in the desert occurs at a much slower rate than it does in comparison to a region like the Sierra Nevadas, where as much as a foot and a half of earth erodes every 1,000 years. With the exception of the occasional monsoonal thunderstorm, most erosion in the Mojave Desert occurred over 10,000 years ago during the ice ages of the pleistocene epoch. During this period the desert experienced more rainfall than it does today.
The eroded sediment of the mountains contain rocks varying in size from boulders to gravel and sand. The eroded sediment flows from the mountains to the desert basin via streams and canyons. As the sediment exits the mouth of a stream or canyon, the sediment loses its momentum and fans out to fill in the basin. This spreading of sediment is what’s called an Alluvial Fan. The water that carried the sediment through the canyons and streams doesn’t cut into the alluvial fan because it perculates and seeps into the deposited sediment.
The surface area of the mountains that appear darker in color than the surrounding rock is due to desert varnish. Desert varnish is a thin layer of rust on rock that is due to chemical weathering, that is the oxidation of iron and manganese.
Sand Dunes
Kelso Dunes
When wind blows sand sediments from dry lakes and river basins in a single general direction over a long period of time, the sand may accumulate in a general location creating a sand dune.
The Kelso Dunes are located in the Devil’s Playground region of the Mojave Desert and are estimated to be about 25,000 years old. Most of the sand accumulated 24,000 to 9,000 years ago with significant accumulation once again occurring 1,500 to 400 years ago. The dune stand some 550 to 650 feet tall and stretches 45 square miles of accumulated sediments that come from the Mojave River. The Mojave River flows intermittently from Silverwood Lake and Deep Creek of the San Bernardino Mountains down through Barstow and out to Soda Lake. A feature of the Kelso Sand Dunes that are uncommon to other sand dunes is the addition of a darker color sand. This is mostly magnetite originating from iron ore from Cave Mountain and Afton Canyon.
Kelso Dunes is home to hundreds of species including 7 endemic insects. For this reason these dunes are closed to off-road vehicles. The damaged that is caused by off-road vehicles is evident by the limited amount of organisms found at Dumont Dunes.
Rain-Shadow Effect
The Mojave Desert
The rain-shadow effect is caused by orographic precipitation. Orographic precipitation is when moisture-rich air moves inland from the ocean and interacts with a mountain. As a mountain forces air to rise in elevation, air pressure is decreased along with the air's ability to hold water resulting in the formation of clouds and the possibility of precipitation on the windward side of the mountain. On the leeward side of the mountain, air moves down slope decreasing in elevation and increasing in pressure. As air pressure increases so does the airs temperature and its ability to hold water, limiting condensation and increasing evaporation, leading to a hot and arid region.
The region we know today as the Mojave desert hasn’t always been a desert and once received a significant amount of rainfall. Death Valley was once home to a 600 foot deep lake known as Lake Manly. Subalpine trees like Limber and Bristlecone Pines were present at higher elevations and mammals like bison and mammoths grazed the region. It wasn’t until about 11 million years ago with the uplift of the Sierra Nevadas, Transverse Ranges and Peninsular Ranges that rain-shadow started to effect the Mojave Desert. With the last ice age peaking out about 17,000 years ago, the Mojave’s current drying trend has been in effect for about 11,000 years.
Allopatric Speciation
Shoshone Desert Pupfish (Cyprinodon nevadensis shoshone)
Following the last Ice age of the Pleistocene epoch, there were many interconnected lakes and streams in our desert region. With the onset of warmer temperatures and drought, many of the lakes and streams dried up, forming geographical boundaries that cut off populations of different species that once inhabited the larger wetland as a single population. One of the species that got divided was the pupfish. When the new geographical boundaries were formed and populations of pupfish became separated, breeding between these populations was no longer possible. Regardless of the traits that existed in the larger population as a whole, the new populations only maintained the traits that existed in each of the smaller populations. These traits can be related to size, color, health, etc. Because each smaller population’s new habitat has different ecological requirements, some of the maintained traits were lost and others were emphasized due to natural selection. Over time, speciation had occurred. This type of geographically induced speciation is called allopatric speciation.
In the Death Valley region of the Mojave Desert there are now five different species of Desert Pupfish, including six subspecies of Cyprinodon nevadensis. Each endemic and characterized by their now restrictive geological locations. Each one of these species originated from a single species of pupfish. Pictured is the Shoshone Pupfish which is endemic to Shoshone Springs, a small spring along the Amargosa River.
Ecosystems
Salt Creek
Ecosystems are made up of organisms that mix and interact with the living and non-living environment. The living environment includes plants and animals, which are referred to as producers and consumers. The non-living environment includes energy and matter. Energy comes from the sun in the form of light and heat. Matter refers to air (gases), soil (minerals) and water. A healthy ecosystem is efficient at recycling matter and does not use up all its resources. However, when a resource is in limited supply, all living organisms must adapt to the limited availability of the resource. Justus von Liebig's Law of the Minimum states that yield is proportional to the amount of the most limiting nutrient. Because water is the most limited nutrient in the desert, the amount of living organisms that can survive in the desert is directly dependent on access to water.
Salt Creek is a small oasis between Baker and Shoshone and is fed water from small desert springs and rain runoff before draining water back into the Amargosa River. This oasis provides water and shelter for many different desert animals, including coyotes, bobcats, badgers and 150 different species of birds.
Of the 270 million acres of land that the Bureau of Land Management manages, only 9% are wetlands. Wetlands are the most productive ecosystems and more than half in the United States have been lost in the last 200 years. Half a million continue to be lost every year, if nothing changes and the current trends continue, all wetlands and riparian zones may disappear in the next 200 years.
Exotic & Invasive Species
Salt Cedar (Tamarix ramosissima)
One of the most destructive plants threatening the Mojave Desert’s ecosystem is the Salt Cedar (Tamarix ramosissima). The Salt Cedar is an exotic and invasive species native to Afghanistan and Eurasia. When the plant was first introduced to the United States in the early 1800’s, it was brought over purposely for its aesthetics as an ornamental tree and used functionally to create windbreaks. Since then, the Salt Cedar has invaded nearly every wetland in the southwestern United States.
The Salt Cedar negatively affects the native ecosystem by outcompeting native species for water. The Salt Cedar also displaces native species as the salt cedar changes the natural composition of the soil. Salt cedars have deep roots that allow them to absorb 200 gallons of water a day per plant, essentially drinking wetlands dry. After absorption, water is released by the plant via evapotranspiration and salt is concentrated in leaves. When the plant sheds its leaves its increases the salinity of the soil to a point that nothing else can grow around it. The Bureau of Land Management has been working to remove this invasive plant from Salt Creek Since 1992.
Producers
Creosote Bush (Larrea tridentata)
The trophic structure, or food pyramid, determines the path food energy flows through the food chain. At the first level of the trophic structure are plants and algae, which are called primary producers. While all organisms need food energy to drive cellular respiration, only primary producers can photosynthesize and produce their own food energy from light energy generated by the sun. Because primary producers can produce their own food energy, they are called autotrophs, which means self-nourishment.
While there is an abundance of sunlight in the desert with about 90% of available light reaching the deserts surface, as compared to about 40% in more humid regions, water and carbon dioxide are also needed to drive photosynthesis. With water in short supply in the desert, the number of plants that can grow in the desert is limited. In fact, the Mojave Desert averages only 890 lbs of edible vegetation per acre for the entire year.
The Creosote Bush (Larrea tridentata) is one of the most abundant primary producers in the Mojave Desert and is a common food source for herbivorous desert reptiles, including the Desert Iguana (Dipsosaurus dorsalis) and the Common Chuckwalla (Sauromalus ater). A few animals like the desert woodrat (Neotoma lepida) and the Black-tailed Jack Rabbits (Lepus californicus) also feed on the Creosote Bush.
Consumers
Desert Iguana (Dipsosaurus dorsalis)
Animals and insects cannot produce the energy required to drive cellular respiration on their own. They must therefore get their energy from other organisms by consuming them. This is why they are referred to as consumers, or heterotrophs. Unfortunately, as energy flows from one organism to another, approximately 90% of the consumed energy is lost as heat, and only 10% of the energy is stored. This is referred to as the 10 Percent Rule.
Some consumers eat plants and others consume insects and animals. The consumers that eat plants are called primary consumers, or herbivores. The consumers the consume herbivores are called secondary consumers, or carnivores. The consumers that eat carnivores are called tertiary consumers, quaternary consumers and so on.
To put the 10 percent rule in practice, a herbivore, like the Desert Iguana (Dipsosaurus dorsalis) is a primary consumer and gets its energy directly from consuming vegetation. Because the desert can only sustain about 890 lbs of edible vegetation per acre per year, the desert can only sustain 89 pounds of all herbivorous organisms and 8.9 pounds of all carnivorous organisms per acre per year.
In any ecosystem, there will always be more plant mass than insect and animal mass and there will always be more herbivores than there are carnivores. Because a desert biome has less water than other biomes, vegetation will always be limited. Other biomes may have the ability to home larger animals further up the food chain like tertiary and quaternary consumers, but organisms this high on the food chain are rare in the desert.
Resource Partitioning
Common Side-Blotched Lizard (Uta stansburiana)
Lizards and snakes are the most abundant carnivores of the Mojave Desert. There are 30 different species of lizards and 21 different species of snakes found in california deserts. The competition to obtain the same resource between one species and another species is called interspecific competition. The Competitive Exclusion Principle, also known as Gause’s Principle, states that two or more species cannot occupy the same ecological niche while competing for the same limited resource. Because one species will be more successful at obtaining its required resources, the second species will need to change its behavior and/or physiology to limit or all together avoid competition with the more successful species. These differences can be related to prey preferences, size and habitat. This will allow the second species to access the resource in a different manner. This change is referred to as Resource Partitioning.
Some examples of these differences are the Chuckwalla and Desert Iguana, who are primary consumers and only feed on plants. The Desert Horned lizard is a secondary consumer and feeds in sandy areas and feeds primarily on ants. Some larger lizards feed on larger insects found in plants and trees while the smaller Common Side-Blotched Lizard (Uta stansburiana) feeds on smaller insects out in open areas.
Camouflage
Desert Horned Lizard (Phrynosoma platyrhinos)
Camouflage, which is also called cryptic coloration, allows an organism to use their colors, markings and shape to hide and blend in with their environment to avoid detection. One of the most common forms of cryptic coloration is called background matching. This allows the organism to hide themselves from both their predators and their prey in plain sight. Organisms that blend in with background matching usually have earth tone colors.
The Desert Horned Lizard (Phrynosoma platyrhinos) can only run a short distance. In the presence of a predator like the Roadrunner (Geococcyx californianus), it may try to flee and hide under low lying plants, run into the borrowing hole belonging to another animal or even bury itself in the sand with only its head exposed. However, because running isn’t its best defence mechanism, the Desert Horned Lizards primary defense relies on remaining motionless and its ability to go unseen by blending in with its background. The Desert Horned Lizards colors vary, matching the local soil and rock. While they are often found in open sandy areas showcasing tan, gray and beige colors, they can also be found around darker rocks like basalts showcasing red, brown and black colors.
Flora: Mojave Vegetation
Joshua Trees and Creosote Bush
Plants of the Mojave Desert have adapted to survive in the harsh desert environment. Challenges for desert plants include exposure to extreme temperatures ranging from 30 to 130 degrees, receiving less than 4 inches of rain per year, exposure to high levels of ultraviolet light and rooting in alkaline soils.
Two of the most well adapted and dominant plants of the Mojave Desert are the Joshua Tree (Yucca brevifolia) and the Creosote Bush. In fact, the presence of these two organisms in many of the valleys branching away from the Mojave Desert constitutes those valleys as part of the Mojave Desert.
Yuccas like the Joshua Tree are leaf succulent plants meaning they have the ability to store water in their leaves allowing them to survive in regions of limited water. Joshua Trees are also found in slightly higher elevations of the desert where temperatures are a bit more forgiving.
Creosote Bush are drought enduring plants. They have large root systems that are capable of absorbing water from all soil levels and small leaves with a waxy coating to prevent water loss through evapotranspiration. Creosote Bush has also adapted a uniform dispersion pattern that is key to its survival.
Uniform Dispersion
Creosote Bush (Larrea tridentata)
There are three different types of species dispersal patterns. A dispersal pattern is the placement of individuals of a population within a given geographical boundary. These patterns include clumped dispersion, random dispersion and uniform dispersion. A uniform dispersion pattern is when individual organisms of a population are spaced evenly from one another, like the squares of a checkerboard. This occurs when the individual organisms of a population interact with one another in some form.
The most common plant of the Mojave Desert covering 70% of the Mojave Desert is the Creosote Bush. Creosote Bush is found in uniform dispersal patterns on flat and gentle sloping alluvial fans and bajadas that form the desert basin. It was once believed that the uniform dispersal pattern between Creosote Bush was due to allelopathic chemicals released by the Creosote Bush that prevented the germination and the growth of other Creosote Bush and plant species within a certain proximity. While the Creosote Bush does release allelopathic chemicals, it is not certain it effects other Creosote Bush, but it may affect a couple other species of plants from germinating or growing too large. It is likely the real reason for the plants uniform distribution pattern is likely do to the equal distribution of rainfall across desert basins and the established root systems of more mature plants that prevents the germination of seeds.
Communities
Joshua Tree Woodland
A community is composed of a variety of different organisms that live and interact with one another in a particular habitat. The organisms that make up a community are not just a random selection of organisms, but organisms that have a interdependence on one another for food, reproduction and protection.
Joshua Tree Woodlands are home to different species and support numerous organisms in an otherwise nutrient-deficient ecosystem. One of the organisms that live in the Joshua Tree is the Yucca Weevil (Scyphophorus yuccae). Forming a mutualistic relationship with the Joshua Tree, the Yucca Weevils larvae feeds on the growing stems of Joshua Trees which allows the Joshua Tree to produce new shoots to branch out and grow. There are at least 25 different birds that nest in Joshua Trees. One of these birds is the Ladder-backed Woodpecker (Picoides scalaris). This woodpecker also forms a mutualistic relationship with the Joshua Tree by feeding on termites that infect the tree. In return the Joshua Tree provides the woodpecker protection from its predators. Fallen trees and branches also provide food and shelter to Desert Night Lizards (Xantusia vigilis) and Desert Night Snakes (Hypsiglena chlorophaea).
Coevolution
Joshua Tree & Yucca Moth (Tegeticula sp.)
Some species go beyond having just mutualistic relationships. The Joshua Tree and the Yucca Moth (Tegeticula sp.) are two species that have coevolved alongside one another and literally could not survive without the existence of the other.
Unlike most flowering plants, Joshua Trees do not produce nectar and therefore do not attract common pollinators. The only single organism that pollinates the Joshua Tree is the Yucca Moth. The Yucca Moth does not utilize the Joshua Trees flowers for food but as a place to lay their eggs. The moth pollinates the Joshua Tree by collecting balls of pollen from one flower and deposits the pollen on the stigma and her eggs into the ovary of another flower. When the flower develops into a fruit and produces seeds, caterpillars hatch from the eggs and eat some of the seeds before crawling into the ground to cocoon.
For every species and subspecies of yucca plant, there is a single species or subspecies of Yucca Moth to pollinate that yucca. The species with matching physiological characteristics are the species that are successful at reproducing.
Parasitism & Mutualism
Desert Mistletoe (Phoradendron californicum)
A symbiotic relationship that hasn’t been discussed yet is parasitism. Parasitism is when one organism benefits from a relationship at the unfortunate expense of the second organism. Native to the Mojave Desert is the Desert Mistletoe (Phoradendron californicum) which has both parasitic and mutualistic relationships with a variety of different desert species. The Desert Mistletoe forms a parasitic relationship with Honey Mesquite (Prosopis glandulosa) and Catclaw Acacia (Senegalia greggii), both of which can be found in clumped dispersal patterns near creeks and streams. The desert mistletoe harms the Mesquite and Acacia by stealing nutrients for itself from its host plants. Mistletoe is also known as a hemiparasite. Meaning in addition to obtaining nutrients from its host plant, it can also obtain nutrients via its own ability to photosynthesize.
The desert mistletoe isn’t parasitic to all organisms as is has a mutualistic relationship with the Northern Phainopepla (Phainopepla nitens). In this relationship the bird benefits from the mistletoe by feeding on the mistletoes seeds and the mistletoe benefits from the bird spreading its seeds from plant to plant via the birds droppings.
Mutualism
Lichen
In North America alone, there are more than 3,600 different types of lichen. Lichen isn’t a single organism, but two or more organisms that have formed a mutualistic relationship that benefits all organisms involved to survive as a single unit. The organisms that make up a lichen unit are most commonly an ascomycete fungus and a green or blue-green algae known as cyanobacteria. In recent years it has been discovered that there is a third organism present in many lichen as well, a basidiomycete yeast.
In this relationship the fungus forms the body of the unit protecting the algae and giving the lichen its physical characteristics like shape, size and color. The yeast that makes up the cortex of the fungus also adds to its physical characteristics. Algae benefits because on its own it can only survive in water, but when paired with a fungus to form a lichen, algae gains the ability to survive anywhere in the world. A fungus is generally a decomposer and gets its food energy from the organic matter of dead organisms. However, in this relationship, the fungus benefits because the algae provides food energy for both itself and the fungus via photosynthesis.
Fauna: Mojave Animals
Common Chuckwalla (Sauromalus ater)
Like desert vegetation, desert animals must also adapt to survive and live with drought, heat and a poor food environment. An organism’s ability to survive in severe drought conditions is key to its survival in the desert. While animals utilize a form of water evaporation to keep cool, too much water evaporation is extremely dangerous for the organism and could lead to the animal dehydrating and eventually die. This makes it essential for desert animals to avoid the extreme heat of the desert as much as possible so they can maintain the storage of water in their body. For example, the Common Chuckwalla stores water in lymph spaces under its skin on the side of its body. The lymph spaces of the chuckwallas we found at Amboy Crater were sunken in, indicating that these organisms were dehydrated and certain to die if they didn’t have access to water soon. The best way for desert animals to avoid the heat is to hide in the shade of plants and/or borrow themselves in a hole. A side effect of drought is that not only is water limited to animals, but plants as well which limits plants growth and the amount of shade available to animals. Fortunately for reptiles, as ectotherms, they don’t need to eat much to drive there metabolism like endotherms do, which is a benefit in a food poor environment like the desert.
Threatened Species
Desert Tortoise (Gopherus agassizii)
Another desert reptile that has evolved to adapt to survive the harsh weather, lack of water and lack of vegetation is the Desert Tortoise (Gopherus agassizii). The Desert Tortoise may only have access to water for 2 to 3 weeks out of the year, yet has the ability to stay hydrated by storing a quart of water in it urinary tract for the remainder of the year. Despite the desert tortoises adaptability, the tortoise is listed on the states list of threatened species. It is estimated that 90% of the total population has been lost in the last 50 years. Reasons vary from interspecific competition from introduced grazing animals like cattle, sheep and donkeys, to threats from the Common Raven (Corvus corax). Ravens feed on young tortoises whose shells have not hardend enough to offer them protection from the ravens beek.
Additional threats to the desert tortoise is land being used by humans for electrical stations, military bases and both off-road and highway vehicles. Perhaps the worst crime committed towards the desert tortoise are people who hunt them. It is obviously illegal to hunt the desert tortoise but it is also a crime to interfere with a desert tortoise in anyway. As mentioned earlier, the desert tortoise can hold a quart of water in its urinary tract. As a defense mechanism, a desert tortoise may urinate if threatened. Therefore if someone was to pick one up or frighten a Desert Tortoise, without it having the ability to rehydrate, one will essentially be killing the tortoise via dehydration.
Conservation
Desert Tortoise (Gopherus agassizii)
In recent years the Common Raven’s (Corvus corax) population has exploded in the Mojave Desert. Between 1968 and 2014 the ravens population grew by 795%. One of the leading factors to the increased number of ravens in the desert is directly correlated to increased human activity. Humans have provided ravens with homes for nesting with the introduction of electric towers. Humans also leave trash and increase roadkill in the desert providing an unlimited food source for the ravens in a place where food is limited for other organisms. As mentioned on the previous slide, ravens attack and feed on Desert Tortoises. Because it takes about 10 years for the tortoise's shell to completely harden, it is susceptible to the pecking of a raven’s beak.
In effort to save the Desert Tortoise, Tim Shields, a biologist of 35 years and founder of Hardshell Labs has developed 3D printed models of baby Desert Tortoises. He has sold 1,000 decoy shells to the U.S. Fish and Wildlife Service. Additionally, Shields has 5 shells that spray a bird repellent when activated by the presence of ravens. Because ravens are among the most intelligent of all animals, Shield’s goal is that ravens will learn the unpleasantries of attacking the decoy shells and change their habit and no longer attack real desert tortoises.
Ecological Succession
Cima Dome
In August of 2020, during a series of thunderstorms, a lightning strike started a fire that burned 43,273 Acres in the Mojave Desert. It killed an estimated 1.3 million Joshua Trees. The fire was named the Dome Fire after originating near Deer Spring on Cima Dome. The fire was accelerated by the presence of cheatgrass, which is an exotic and invasive species that is native to Europe, northern Africa and southwestern Asia. A study suggest that if a third of the Joshua Trees were to have survived, the Joshua Trees would have had a chance to recover and repopulate the area. With the near total loss of Joshua Trees on Cima Dome, it is unlikely they will ever repopulate the area to what it once was.
Before the fire, the Joshua Trees were the dominant species on Cima Dome and were considered a climax community. That is a community of organisms that had remained stable and relatively unchanged for a long period of time. With the death of the established Joshua Trees and the lack of competition came the rebirth and reintroduction of a wide variety of new opportunistic (r-selected) species. This change in species is referred to as an ecological succession. In the case of natural disasters like a fire, flooding, drought or even human induced disasters, this type of ecological succession is referred to as a secondary succession. This is opposed to a primary succession which only occurs when plants grow on newly formed or exposed earth and rock.
Human History
Mojave Road
Archaeologist found that there have been five periods of native occupation in the eastern Mojave Desert dating back some 7,000 years. The most recent tribes to live within the eastern Mojave Desert are the Paiute, Chemehuevi and the Mohave. These tribes moved and traveled across the desert hunting and camping, but lived closer to the Colorado River. The tribes had at least two established routes they used to navigate across the desert.
In 1776, with the help of the Mohave people, a spanish born priest named Father Francisco Garces, who was traveling west to the future site of the San Gabriel Mission was the first european born individual to travel across the Mojave Desert. He traveled via the routes established by the Native Americans, today that route is known as Mojave Road.
In 1826 Jedediah Smith crossed the Mojave on his way to become the first European-American to reach the California coast line from mid-America. In 1844 John Fremont, who was a mapmaker with the Army Topographical Corps of Engineers, crossed the Mojave from the west. He noted the absence of natives which was likely do to their departure and purposeful avoidance of settlers due to the the Santa Fe traders who abused them. In the 1860’s, in conjunction with the discovery of silver, the United States Government established outposts in the Mojave Desert to secure mail routes.
Mining
Bristol Dry Lake
In 1863, silver was discovered near Rock Springs in the Providence Mountains. After this discovery the first european-american settlers moved to the Mojave Desert. Over the next 100 years, gold, copper, iron, borax, tungsten, lead, zinc, and manganese were all mined to varying degrees of success. Towns and cattle ranches were established to support the miners. Cattle ranching became a secondary industry in of itself behind mining. In 1894 as many as 10,000 cattle were grazing in the Mojave Desert. Mining peaked during World War I and slowed at the conclusion of the war. There was a slight resurgence of mining during World War II.
Since 1909 and still active today is salt (halite) mining at Bristol Dry Lake, just 3 miles southeast of Amboy Crater. The salt extracted at Bristol Dry Lake is used for both industrial and food products. Bristol Dry Lake is estimated to be holding 60 million tons of salt. The surface bed of the dry lake contains about 3 to 7 feet of sand, clay and gypsum that must be removed to access the 5 foot thick layer of salt below. Underneath this first salt layer is an 8 to 10 foot layer of clay with a second 5 foot layer of salt below.
National Parks
Mojave National Preserve
In 1994, as part of the California Desert Protection Act, the Mojave National Preserve was established. The Mojave National Preserve boundary lies primarily between Interstate 15 to the north, Soda Lake and Devils Playground to the west, Interstate 40 to the south and the Nevada border to the east. Outside this boundary but also within the preserve are the Clark Mountains. The Mojave National Preserve encompasses 1,589,165 acres, making it the 3rd largest national park in the contiguous United States. However, only about 50% of the area is designated as wilderness.
Within the boundary are many private lands. The California Department of Fish and Game owns 139 acres near Piute Spring. The State of California operates 5,200 acres of the Providence Mountain recreation area, including Mitchell Caverns. The University of California owns 2,200 acres of the Granite Mountains Natural Reserve in the Granite Mountains. State schools own 35,398 acres. Union Pacific owns 1,366 acres and about 75,000 acres remain privately owned by established ranchers. Of the 17,000 mining claims that were active prior to 1994, only 87 remain active today totaling 1,850 acres. That number will no longer increase as no new mining stakes can be claimed within the Mojave National Preserve.
Quarrying
Pisgah Crater
In the Mojave Desert, but not within the boundary of the Mojave National Preserve is Pisgah Crater. Pisgah Crater is another basaltic cinder cone of the Mojave Desert. It is associated with Amboy Crater as they are both part of the Lavic Lake Volcanic Field. Like Amboy Crater, Pisgah Crater is one of the youngest volcanoes in the Mojave Desert and has had 3 eruptions, with the last eruption being about 20,000 years ago.
The natural shape of Pisgah Crater has been destroyed as the crater’s 120 acres has been quarried for its cinder rock. An old faded sign at the entrance of the volcano says the quarry is operated by the Twin Mountain Rock Company, but I can’t find any information on them ever owning or operating Pisgah Crater. However, at one point the quarry was owned and operated by Santa Fe Railroad. Santa Fe Railroad used the cinder rock primarily for their railroad ballast. Today the quarry is owned by Can-Cal Resources Limited, a company incorporated in Nevada.
Cinders are also used by Caltrans who scatter the cinder rock on icy and snow covered roads to increase traction for vehicles. Cinder rock is also sold at plant nurseries to be used aesthetically for landscaping.