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The
Chihuahuan Desert Region
A desert region can be defined many ways. To
a physical scientist such as a meteorologist, a desert can be defined as an
area receiving an average annual rainfall of 10" or less. On the other
hand, a biologist will consider how moisture moves through an ecosystem and
the effect it will have on life forms. His definition of a desert region is
an area in which the evaporation rate exceeds the annual rainfall. At the
Fort Davis Headquarters of the CDRI, the annual evaporation rate is
36-48" while the annual rainfall averages 17". In addition, the
evaporation rate on an open body of water such as a pond, river, or stock
tank can be as high as 72" annually. Pretty dry! As a result, a desert
region is usually defined by a combination of factors such as climate,
topography, and plant and animal communities.
The Chihuahuan
Desert region stretches from the Rio Grande Valley
in southern New Mexico and the San Simon  Valley of southeastern Arizona
to an area just north of Mexico City.
It is approximately 800 miles long
and 250 miles wide. Included in the Chihuahuan Desert region
are parts of the states of Arizona, New Mexico, and Texas and the cities of
El Paso and Las Cruces in the United States, as well as parts of the states
of Chihuahua, Coahuila, Nuevo Leon, Durango, Zacatecas, and San Luis Potosi
and the cities of Juarez, Cd. Chihuahua, Torreon, and Saltillo in Mexico.
Elevation
and Climate: Although the Chihuahuan Desert
region extends farther south than other North American deserts, extensive
portions lie above 4,000 ft. in elevation. The lowest portions of the Chihuahuan Desert
region are about 1000 ft. in elevation along the Rio
Grande while the highest portions, in Mexico, approach 10,000 ft. in
elevation.
Winters are characteristically cool,
especially in the northern reaches where nighttime temperatures drop below
freezing on average over 100 times per year. In the summer, daytime high
temperatures in the Bolson de Mapimi have reached a reported 122 degrees F.
The dry early-summer months of May and June are typically the hottest part of
the year in the Chihuahuan
Desert.
The Chihuahuan
Desert is dry because it is
surrounded by Mexico's two
great mountain ranges: the Sierra Madre Oriental and the Sierra
Madre Occidental. As warm, moist air rises to move across these
mountains, it is cooled rapidly causing rainfall. The result is that most of
the moisture has been removed from these air masses before they ever reach
the Chihuahuan Desert basins. While the other North
American deserts each have a summer and winter rainy season (because of their
location further to the west), rain typically comes to the Chihuahuan Desert
only from July to October. As much as 90% of the annual rainfall takes place
during this period. When rain does come, afternoon thunderstorms cool the
desert.
 Floral
Characteristics: The lower
elevations of the region is characterized by botanists as Chihuahuan Desert
Scrub. Comprising up to one half of the total vegetation in the region,
it has been suggested that Desert Scrub communities have grown to their
present extent through the invasion of eroded grasslands. In fact, it has
been suggested that the Chihuahuan
Desert region may have
grown by as much as a third of its size in the last few hundred years due to
man's activities including poor agricultural practices and water use
management. Creosotebush, Larrea tridentata, is a prominant element of
Desert Scrub often covering large expanses. Stem succulents such as Lechuguilla,
sotol, and yucca are dominant features of the Desert Scrub landscape. Other
common shrubs include mimosa, acacia, mesquite, Mariola, Fourwing Saltbush,
Tarbush, Javelinabush, Goldeneye, Allthorn, and Ocotillo.
Desert Grasslands are best developed on plateaus, rolling hills, and
basin floors where the soils are relatively deep. Blue Grama, Bouteloua
gracilis, is the dominant species here. Netleaf Hackberry, Little Walnut,
and a number of oaks are common woody components. Extensive Tobosa Grass flats
occur in low elevations where water run-off tends to accumulate while
beargrass and sotol commonly occur on hillsides.
Oak-Juniper-Pinyon Woodlands characterize the slopes and valleys at
mid-elevations. Common woodland trees include oaks, junipers, pines, Texas
Madrone, and Bigtooth Maple. Woodland shrubs include Texas Mountain Laurel,
Scarlet Bouvardia, and Mountain Sage.
Coniferous Forests can be found at the highest elevations. Typical
tree species found here include Ponderosa Pine, Douglas Fir, Alligator and
Weeping Juniper, Arizona Cypress and a number of oaks. Other characteristic
plants include Birchleaf Buckthorn and Needlegrass.
Riparian Vegetation occurs along the major  rivers in the Chihuahuan Desert
region. Common riparian trees and shrubs include the introduced Salt Cedar,
the Desert Willow, Cottonwoods, and the cane grass, Giant Reed.
There is more diversity of cacti in
the Chihuahuan Desert than in any other region. Many
botanists believe that these plants originated here or to the south, and
radiated out through the New World. The
common cacti include the Prickly Pears, Hedgehogs, Living Rocks, Nipple
Cacti, and Corys.
Within each vegetation zone are specific
plant associations that are often differentiated by associated soil
conditions. Examples include the characteristic vegetation of stabilized
gypsum, gypsum sand, quartz sand, and saline habitats.
Faunal
Characteristics: The Chihuahuan
Desert region is home
to a multitude of specialized invertebrates. Among the most
conspicuous are the Desert Tarantula, Whip Scorpions or Vinegaroon, Desert
Millipede, a number of scorpions, and Giant Centipede. A rich butterfly and
moth fauna is also to be found here.
As with most desert regions, fish
species have evolved through ages of isolation. Spring systems have become
home for a plethora of endemic (unique) fish. More species of pupfish occur
here than in any other desert region.
Common Chihuahuan
Desert amphibians include the Barred Tiger
Salamander, spadefoot toads, and the Rio Grande Frog. A recent introduction,
the Bull Frog, has found suitable habitat along portions of the Rio Grande and Pecos River.
A great number of lizards inhabit
the region. Among these are the whiptails, spinys, horned, collared, and
geckos. Snakes have evolved to take advantage of a wide range of
conditions. While a few are diurnal such as the garter and coachwhips, most
avoid the heat of the sun by prowling after dark. Chihuahuan snake species
include the beautiful Mexican kingsnakes, Trans-Pecos Ratsnake, and many species
of rattlesnakes. The venomous coral snakes and Gila Monster are only known
from the fringes of the Chihuahuan
Desert region where
they are rarely encountered.
 More species of birds have been recorded in Big Bend National Park alone than any other
North American national park. Despite the high numbers, most are non-resident
or seasonal. In addition, population densities are usually small.
Mammals are diverse and tend to be
nocturnal. The northern Chihuahuan
Desert region has one
of the richest bat faunas to be found anywhere. Eighteen species have been
documented in Presidio County,
Texas, alone. Most are small
and insectivorous. Notable representatives include the Pallid Bat, Western
Pipistrelle, and Brazilian Freetailed Bat.
The larger predators have struggled for
survival and while the Mountain Lion has been successful, Gray Wolves and
Black Bear still hang on in northern Mexico. Grizzly Bears were known
to inhabit at least one Chihuahuan mountain range until the late 1960s.
Mule Deer, White-tailed Deer, and Pronghorn
are common residents while attempts to reintroduce the Desert Bighorn
have met with mixed results. Other common mammals include the Coyote,
Collared Peccary, Ringtail, Black-tailed Jackrabbit, Spotted Ground Squirrel,
a number of skunk species, woodrats, and many nocturnal mice.
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CDRI, Box 905, Ft. Davis, TX 79734
Office: 432-364- 2499; Fax: 432-364-2686
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e-mail: choyt@cdri.org
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