Natural History of the
Southwest Fieldcourse

Overview of Topics

I. GEOGRAPHY OF THE DESERT SW (Be able to locate these features on a map.)
  1. Rocky Mountains and Sierra Madre
  2. Sierra Nevada
  3. Colorado Plateau, including Grand Canyon and San Francisco Volcanic Field
  4. Basin and Range -- Desert Regions
    1. Intermountain Desert (not visited on this trip)
    2. Mojave Desert (not visited on this trip)
    3. Sonoran Desert, including Santa Catalina Mountains (Mount Lemmon)
    4. Chihuahuan Desert, including Tularosa Basin (White Sands) and Guadalupe Mountains
II. GEOLOGY OF WESTERN NORTH AMERICA (CORDILLERA)
Plate Tectonics Overview
The surface of the Earth is broken into several plates. Convection currents in the molten mantle slowly move the plates. Mid-oceanic ridges form where convection currents diverge at surface; basalt is deposited at ridge, which pushes the adjoining plates apart.
Trenches (subduction zones) form where the heavier oceanic plate moves under the lighter continental plate (e.g., subduction of the East Pacific Plate under North America west of the Cascade Mountains). As the subducting plate is deflected under the continental plate, it melts at depth of about 100 km. Molten material from the subducting plate rises to the surface, generating volcanic activity (such as the Cascade Volcanoes).
Where the continent is separated from a mid-oceanic ridge by oceanic crust, it is known as a passive margin because erosion dominates. Where the continent is near the leading edge of the plate, it is known as an active margin because volcanic activity and accretion of materials along the coast occur. The cordillera of the western United States reflects this activity along the western margin of the continental plate of North America.
Volcanism
Lava flows, cinders, and ash are examples of extrusive deposits, in which the molten material reaches the surface before it solidifies. Most southwestern volcanoes are either composite volcanoes, which are built of layers of flowing lava that alternate with cinders and other airborne debris, or they are cinder cones, which are conical deposits of cinders with flows of basalt emanating from their base.
Batholiths (large) and plutons (small) are examples of intrusive deposits, in which the molten material solidifies before reaching the surface (e.g., granite peaks in Sierra Nevada, which were later exposed by uplift and erosion).
Extension and Compression (Faulting and Folding)
In addition to volcanic activity, the movement of the plates and the underlying convection currents produce strains on the surface that can also create mountain ranges. Extension can create faults and move elongated blocks of land that rise (mountain ranges) or fall (basins). These mountain ranges tend to run parallel to each other, as seen in the linear ranges of the Basin and Range region of the Great Basin and Desert Southwest. Compression can fold layers of rock into upward bulges (anticlines). The Tularosa Basin (Valley of Fires and White Sands) was an anticline when the central portion collapsed, creating the San Andres (west) and Sacramento (east) mountains on either side of the basin.

Large rifts also can form under continental plates where currents in the underlying magma causes strains in the overlying rock. The Rio Grande valley occupies a rift. Volcanic activity is possible in rift valleys.

Reef Formation (Lime Deposition)
Lime (calcium carbonate) was deposited by algae and sponges (not coral) and by precipitation from water near the shore of an ocean embayment that extended over southern New Mexico and West Texas 250 million years ago. Later buried, heat and pressure turned the deposits into the limestone now uplifted and exposed as the Guadalupe Mountains.
Erosional Processes
Surface erosion and deposition have occurred in the Southwest as a result of water (e.g., Grand Canyon and bajadas), glaciers (e.g., San Francisco Peaks of northern Arizona), and wind (e.g., White Sands). Subsurface erosion and deposition have occurred in limestone deposits, where water and acids have reshaped the rocks underground (e.g., Carlsbad Caverns).
Basic Sequence of Selected Geological Events
    1. 320-245 mybp: Supercontinent of Pangea formed
      1. Ancestral Rocky Mountains rise
      2. Limestone reefs develop in Delaware Basin, an arm of the sea extending into southern New Mexico and western Texas
    2. 200-80 mybp: Pangea begins to break apart into northern and southern continental masses
      1. Sierra Nevada volcanic activity occurs as the East Pacific Plate subducts under the North American Plate
      2. At the close of this time, waters from the Arctic Ocean and Gulf of Mexico flood present Great Plains and create Western Interior Seaway
    3. 80-40 mybp: North America separates from Eurasia and rotates clockwise, centered on Colorado Plateau
      1. Subduction rate increases (causes shallower angle of descent)
      2. Volcanic activity shifts west to east across present Great Basin region
      3. Present Rocky Mountains uplifted
      4. Western Interior Seaway retreats to Hudson Bay and Gulf of Mexico
    4. 40-10 mybp: northern Mexico and southern California override the East Pacific Ridge
      1. Rate of subduction slows again (causes angle of descent to steepen)
      2. Volcanic activity largely shifts east to west across present Great Basin region
      3. Uplift and extension cause faulting in present Basin and Range Province (west and south of the Colorado Plateau)
      4. Initiation of the Rio Grande Rift
    5. < 10 mybp: Miocene-Pliocene Uplift elevates western USA and Rocky Mountains (general uplift of western North America)
      1. San Francisco Peak volcano grows and collapses
      2. Colorado River downcuts through Colorado Plateau
    6. < 2 mybp: Glacial advances and retreats, including mountain glaciers
      1. Southwestern forests extend into basins during glacial advances; retreat into mountains during periods of warming
      2. Cinder cones and basalt flows continue, primarily west of Rio Grande Rift
III. CLIMATE
Solar Radiation (also see Sowell, 2001:11-12)
Heat is transferred from objects of higher temperature to objects of lower temperature. Solar radiation differentially warms objects in a narrow layer at the Earth's surface (e.g., rocks, soil, vegetation). Most warming of the air takes place near these surfaces (infrared radiation). The amount of heat retained by the atmosphere increases as levels of water or dust increase. Thus, clear, dry air in the deserts allows "maximum" heating near surface during the day and allows "maximum" loss of heat at night.

Heating at different latitudes is most intense when the sun is directly overhead (seasonal variation):
    0° (March) -- Equator (equinox)
    23.5°N (June) -- Tropic of Cancer (solstice)
    0° (September) -- Equator (equinox)
    23.5°S (December) -- Tropic of Capricorn (solstice)

Heating at different elevations also varies. Heat is lost as air pressure decreases (as the distance between molecules expands), and air pressure decreases as the distance from the Earth's center increases. Thus, heat is lost as air rises and expands. The lower density of air at high elevations allows more radiant energy to escape from surface objects, so temperatures are cooler at higher elevations of mountains than on the desert floor.

Air Movements (also see Sowell, 2001:3-8)
In the tropics (about 0° latitude), air is heated, becomes less dense, rises, cools, and releases precipitation (tropical rains). At about 10 km in altitude, this rising air diverges to the north and south and continues to cool. It is pushed from below by air moving into the low pressure area created at the surface as the air rises, but it can rise only so far. Denser surface air to the north and south flows into the area of reduced atmospheric pressure near the equator.

In the subtropics (about 30° latitude), the cool air descends, warms, and dries, creating an area of high pressure near the surface; because this warmer air holds less water, subtropical deserts form at this latitude. The air reaching the surface diverges away from the area of high pressure back to the south (back to the tropics) and to the north. This circulation causes a second area of rising air at about 60° latitude, which creates a northern area of lower pressure.

Rain Shadows and Slope Effect (also see Sowell, 2001:5-7)
Moisture-laden air from the Pacific Ocean moves west to east, rises over mountains, and cools, in which case, the precipitation is greatest on the western slopes. This now drier air warms as it descends the eastern slope, so conditions are more arid on eastern slopes. Also, in the Northern Hemisphere, radiant energy from the sun is greatest on south-facing slopes, which increases evaporation and produces drier conditions here than on north-facing slopes.
Climate in the Basin & Range Region
As previously stated, daily temperature extremes in deserts are associated with low atmospheric humidity. Patterns in seasonal temperatures are due to the tilt of the Earth on its axis, which means that the area receiving the most solar radiation moves north during the summer and south during the winter (i.e., hotter in summer; cooler in winter).

The source of moisture for precipitation varies with the seasons. Winter storms from the Pacific Ocean bring precipitation over the Cascade-Sierra spine into the Intermountain, Mojave, and western Sonoran deserts (typically gentle, soaking, widespread rains). Summer storms from the Gulf of Mexico bring precipitation up into the Chihuahuan Desert (typically local, heavy, convection thunderstorms). The Sonoran Desert Upland (south-central Arizona) can receive winter and summer storms.

Mountain ranges in the deserts can receive winter snowfall and summer showers. The greater precipitation supports coniferous forests at middle and upper elevations (only a few southwestern mountains have tundra). Some of this runoff flows into desert basins, where it is referred to as exotic water. As a stream flows from a canyon in the mountains onto the surrounding desert it carries material of various sizes (i.e., boulders, cobble, pebble, sand, slit, and clay). Leaving the mountains, the ability of the flowing water to transport these materials gradually diminishes, and the materials are deposited along the flanks of mountains as alluvial fans, with larger materials deposited first and finer particles deposited at the base of the alluvial fan. This gradation of particle sizes influences the vegetation that grows on them. As erosion continues to expand alluvial fans, they merge with those of adjacent streams into bajadas.

IV. DESERT ECOSYSTEMS
Soils (also see Sowell, 2001:14-17, 120-123)
The types of soils in the desert differ from those of the woodlands and forests of the mountains. Desert soils are typically calcareous in areas where calcium carbonate was deposited by shallow tropical seas (as once covered portions of the Southwest). Minerals are carried down only as far as the water percolates, so they are not leached from the soil. In some areas, a generally impenetrable layer of calcium carbonate (caliche) is deposited at the depth that precipitation infiltrates the soil. Salts (e.g., sodium chloride, gypsum) also can accumulate where water evaporates from the soil surface (e.g., playa lakes). Semidesert grasslands, on the other hand, receive greater precipitation than deserts, which means that water can infiltrate deeper into the soil profile. In addition, the leaves and fibrous roots of the grasses add considerable organic material to the soil compared to that of deserts, and the greater availability of water contributes to more decomposition. Coniferous forests of desert mountains have acidic soils. The cool temperatures inhibit microbial activity (limit the rate of decomposition), and organic acids from humus leach through the soil, replacing cations with hydrogen ions (lowering the soil pH).

In addition to constraints imposed on desert plant communities by the limited availability of water, limits on the availability of nutrients, especially nitrogen, also constrain the growth of vegetation. Although enough nutrients might be present in the soil to meet the demands of desert plants, the availability of these nutrients can be limited because they occur only near the surface, which often is dry. Roots cannot effectively obtain nutrients in dry soil. The lack of soil moisture also limits microbial decomposition of organic materials near the surface. In addition, the distribution of nutrients across the surface of the desert is patchy, with more organic material (nutrients) accumulating under productive plants, around obstructions that block the wind, and in subsurface deposits of animals.

An often overlooked feature of desert soils is the cryptogamic crust, comprised of cyanobacteria, lichens, fungi, or mosses near the soil surface that are metabolically most active when moisture is available. Fibrous components of these organisms help to hold soil particles in place. Some of the cyanobacteria, as well as symbiotic bacteria associated with species in the vascular plant family Fabaceae (e.g., acacias, palo verde, mesquite), have the ability to fix nitrogen, helping to make this essential element available to the wider plant community.

Net Primary Productivity (also see Sowell, 2001:118-123)
Net primary productivity (NPP) is the gross photosynthetic productivity minus metabolic use by the plants (i.e., NPP is the amount of energy available to other organisms in the ecosystem). NPP in deserts is relatively low compared to other terrestrial ecosystems, primarily as a result of limited moisture, which is both temporally and spatially variable. NPP also is limited by variability in the availability of nitrogen as nitrate (NO3-) or ammonium (NH4+) ions. Nitrogen, which is essential for the production of amino acids and nucleotides, can be limited for 4 general reasons: 1) pulses of rapid plant growth when water is available can cause nitrogen to be used faster than it is replaced through decomposition; 2) some desert soils (e.g., sand) are nutrient poor or do not retain nutrients well; 3) nutrients seldom leach deeply into the soil where they can be absorbed by roots and the surface layer is often dry, which impedes decomposition and the ability of plants to absorb nutrients; and 4) detritus is deposited unevenly across the desert, accumulating under widely spaced plants, near surface obstructions, or in underground burrows or tunnels. Water enhances decomposition and helps plant roots to absorb nitrogen from the soil, so the 2 nutrients are best considered together as limiting factors.
Food Webs (also see Sowell, 2001:123-130)
Most of the plant material in a desert enters the food web as detritus, which is available through much of the year. Across much of the desert, fresh plant material generally is available for only limited times during the year, so detritus is a more reliable food source. Because foods are seasonally limited, many desert animals are omnivores, and their diets shift to what is available at a given time. Despite the relatively low productivity of desert ecosystems, they still support extensive food chains. Arthropods are an important component of many of the longer food chains because they are relatively efficient at assimilating energy as new biomass -- roughly 40% production efficiency compared to 10% for reptiles and 1-2% for birds and mammals -- which means more energy remains available for other organisms.
Community Succession (also see Sowell, 2001:135-137)
Primary succession of communities follows the formation of soil through weathering of rock and accumulation of organic material. These soils are formed through erosion (e.g., wind, flowing water, and freezing & thawing), accumulation of wind-borne or other allochthonous materials, and biological activity (e.g., lichens, growth of roots). During this extended process, a sequence of communities of microbes, plants, and animals develop. These organisms and continued soil development alter the microclimate, making it possible for additional organisms to become established and continue the process, perhaps excluding some of the organisms characteristic of the earlier stages in the sequence of communities.
Secondary succession of communities follows the disturbance of a community (e.g., following a fire). A different community is established in the altered microclimate, and a sequence of communities develops, with the microclimates altered by successive communities. Large-scale, non-human disturbances are rare in deserts, but two generalizations about secondary succession in deserts can be made: 1) recovery is slow and 2) colonizing species are often the same that were present prior to the disturbance, possibly because many desert plants do not substantially modify their microclimate and create new habitats for other species.
Human Impacts in Southwestern Deserts (also see Sowell, 2001:163-166)
Human impacts on desert ecosystems include alteration of streamflows (e.g., impoundments, diversions), dewatering (diversions, groundwater mining), grazing, introduction of nonnative species (e.g., competition, predation), suburban sprawl, and strip mining. These impacts affect other ecosystems, but their potential negative impacts on desert ecosystems are arguably more serious because the low productivity of desert communities means that recovery (if it occurs) takes decades or centuries. Damage to extensive areas of legumes (Fabaceae) and soil cyanobacteria is particularly serious because of the ability of these organisms to fix nitrogen, which often is a limiting nutrient.

Another human impact is desertification: the conversion of semidesert grasslands into deserts. Several factors contribute to desertification. Irrigation of crops can concentrate salts in the soil, limiting the plants that can grow in an area. Overgrazing can reduce grassland species of plants and enhance encroachment of desert plants. Fire suppression allows shrubs to spread across grasslands.

"Reverse desertification" is occurring in many areas of the Desert SW of the USA and Mexico through the introduction of buffelgrass from East Africa and other non-native species of grasses to convert deserts to grasslands for grazing. In addition to the clearing desert vegetation from extensive areas to plant buffelgrass, it is an aggressive perennial that displaces native vegetation, partly through the promotion of extensive fires that kill native desert plants, which are not adapted to fire.

V. DESERT ADAPTATIONS: PLANTS (see Sowell, 2001:24-58 and Desert Plants)
Dealing with Limited Water
Some desert plants deal with aridity through their life history. Annual (or ephemeral) plants escape drought conditions by surviving as seeds. They take advantage of moisture during the rainy season to quickly germinate, grow, and reproduce. Perennial species of plants in the desert must survive dry conditions through adaptations of their anatomy and physiology that limit water loss and enhance their ability to obtain water. Drought-deciduous leaves, thick waxy cuticles or resins covering the surface of leaves or stems, closed and shielded stomata, small leaves (reduced surface-area-to-volume ratio), leaf-curling, modifications in photosynthesis, and succulence (storing water) are among the adaptations used by desert perennials to conserve water. However, these adaptations generally reduce photosynthetic efficiency because they require additional energy expenditure by plants, reduce photosynthetic surface area, and reduce the efficiency of CO2 uptake. To obtain water, some plants have extensive, shallow root systems that quickly absorb water following precipitation, and others have roots that tap into groundwater as deep as 50 m.
Dealing with Heat
Perennial plants reduce leaf heating by producing small leaves or leaflets, which have a smaller boundary layer of calm air, so they can be cooled more efficiently than large leaves at a given wind speed. Some perennial plants also increase their albedo with light-colored trichomes, spines (e.g., cacti), or salt excretions, which cause more light to be reflected. Orienting leaves to get maximum sunlight in the cooler morning or evening but less direct sunlight during the heat of the day also keeps some perennials cooler. Ephemeral (annual) plants need to maximize productivity during their short period of growth, so they tend to have larger leaves oriented toward the sun.
Dealing with Herbivores
In addition to protecting themselves from water loss and heat, desert plants also must protect themselves from herbivores. Herbivory defenses include spines, bad tastes, pungent odors, nutrient-poor tissues, and poisonous compounds. All of these efforts are designed to discourage animals from consuming the energy-expensive structures and precious water resources in a desert plant.
Reproduction
Semelparous ("once-bearing") plants (e.g., annual plants and agaves) often produce large numbers of flowers to increase the likelihood of attracting animal pollinators. Wind pollination is relatively rare among desert plants compared to those in grasslands and forests because the low density of plants of the same species reduces the likelihood that wind-blown pollen would reach the flower of another individual of the same species.

Seed dormancy is particularly important in deserts because ideal conditions for growth are brief and unpredictable. Many semelparous plants and iteroparous ("repeat-bearing") plants produce large numbers of seeds (called mast years for iteroparous plants). If many of the seeds are eaten, the plant is benefited by "saturating" the area with seeds to help ensure that some will survive predation.

Seedlings of some plants are established under a nurse plant (e.g., saguaro under a palo verde or Christmas tree cholla under a creosote bush). The young plant benefits from shading, reduced herbivory, enhanced moisture (hydraulic lift by shrub), and protection from cold temperatures. These plants eventually replace the nurse plant, perhaps through more efficient use of water and nutrients.

VI. DESERT ADAPTATIONS: ANIMALS (see Sowell, 2001:59-115 & Desert Animals)
Dealing with Heat
Ectothermic animals (e.g., reptiles) have a metabolic rate that increases as body their temperature increases. Many endotherms, on the other hand, generally maintain relatively high metabolic rates and body temperatures. The cactus mouse, roadrunners, and some other endothermic animals also can lower their body temperature and lower their metabolic demand during periods of inactivity to conserve energy.

Desert animals use various microclimates to avoid overheating. For example, they move to shade, underground, or into a plant (off the soil or rock surface). Birds can reach cooler temperatures at higher altitudes or elevations. Animals also alter the timing of their activities to help maintain optimal body temperature. Some animals are nocturnal, some are diurnal, and some are crepuscular.

Physiological adaptations help animals to maintain their body temperature within an optimal range. Animals use conductive cooling through vasodilatation of blood vessels in the skin to dissipate heat. Animals also use evaporative cooling from their respiratory systems (mammals, reptiles) and gular flutter (throat movement to draw air into moist mouth of birds). This lowers the temperature of the blood on these surfaces as water evaporates. Of course, evaporative cooling places a greater demand on the animal to obtain water.

Dealing with Limited Water
Birds and larger mammals generally are "obligatory drinkers", and they have the ability to travel great distances to find water. Most desert animals are "facultative drinkers" (they drink when water is available), and some animals do not drink at all. They obtain some of their water from their food, and they obtain a portion as a by-product of metabolism. During cellular respiration (specifically the electron transport system in mitochondria), O2 serves as an electron acceptor and combines with H+ to form water. Kangaroo rats are noted for their ability to survive on the limited water in seeds (generally 10-20%). Thus, they obtain much of their water by metabolizing carbohydrates, fats, and proteins. However, to breakdown fats and proteins, more O2 is required than for the breakdown of carbohydrates, which means more water is lost through respiration. The metabolism of proteins also means additional loss of water through urine to excrete the nitrogenous wastes created.
To limit water loss through the excretion of nitrogenous wastes, terrestrial animals use energy to convert ammonia into less toxic molecules that can be more concentrated (i.e., less water). Mammals convert ammonia to urea; arthropods, reptiles, and birds use additional energy to produce uric acid or guanine, which can be excreted as moist solids. Arthropods run their nitrogenous wastes from the Malpighian tubules through their hindgut. Reptiles concentrate their urine in their cloaca, where water is reabsorbed from the both feces and urine. Kidneys of birds and mammals can concentrate urine (return water to the circulatory system) through the loop of Henle and collecting ducts, from which water moves by osmosis out of the tubules into the medulla, where solutes are concentrated. However, birds have relatively shorter loops of Henle, so they further concentrate their urine in their cloaca.

The waxy exoskeleton of an arthropod or the integument of a reptile limit water loss through evaporation. However, the exchange of O2 and CO2 occurs on the surface of wet membranes, which means that water is lost during respiration. Some of this moisture can be retained in the nasal passages of some terrestrial vertebrates. Dry air drawn into the nasal passages causes evaporative cooling. After the air picks up moisture in the lungs, it passes back through the somewhat cooler nasal passages and moisture condenses.

VII. DESERT MOUNTAINS (also see Sowell, 2001:140-149)
Montane Life Zones Island Biogeography
The theory of island biogeography predicts a relationship between the number of species and the area of an "island": the number of species tends to increase as area increases. For example, "mainlands" in the Desert SW, such as the Rocky Mountains and Sierra Nevada, have more boreal species than the "sky islands" of isolated ranges and high plateaus located between the Rockies and Sierras. For some organisms, the desert surrounding the isolated mountains serves as a barrier to dispersal from one range to another. In these instances, the sky islands might represent Pleistocene refugia. During the cooler, wetter conditions of the Pleistocene, coniferous forests and woodlands grew across portions of the basins between the mountain ranges. The distributions of organisms that were widespread during this time were later constricted to the isolated mountains as the climate became warmer and drier and the forests retreated up the mountain slopes. However, the desert does not serve as an effective barrier to all species. Birds, bats, and flying insects, for example, can move more readily than small, ground-dwelling animals. Riparian corridors also might provide avenues of dispersal for some animals in some areas.
Return to Course Homepage