Several factors are involved in the notably high biodiversity of the Chihuahuan Desert. One is its great geographical range, extending at its extremes roughly 11° north and south and the same east to west (southeastern Arizona to western Nuevo León). Because of this extent, different parts of the desert vary in the number and severity of freezes, in the timing and amount of precipitation, and various other climatic permutations. Similarly, the elevational variation within the desert strongly affects local climate and weather. The effect is so dramatic in terms of flora and fauna that the higher mountain ranges often are called sky islands, recognizing the "islands" of highland vegetation surrounded by a "sea" of lowland desert. Finally, topography strongly affects microclimate and, to a degree, macroclimate. The isolation of mountain ranges by intervening lowlands and of lowlands by intervening mountains allows local populations to evolve independently of other populations of the same species and thus promotes biodiversity.
In general, the northern parts of the desert are subjected to occasional hard freezes; this restricts frost-sensitive plants to low elevations in the north (portions of Big Bend) and to areas far enough south to avoid the lowest temperatures. As mentioned earlier, rain shadows affect the desert as a whole and also have local effects.
Equally important, or perhaps even more so, numerous microhabitats favor or disfavor specific kinds of plants and animals. A microhabitat is a set of climatic, biological, or physical (or combinations of these) of limited extent that differs importantly from other sets of conditions. Some microhabitats are obvious, such as a desert pool; others may be subtle, such as a difference in the chemical composition of a soil. Meaningful microhabitats from our point of view are often taxon specific. That desert pool supplies meaningful resources to fish, insects that live in water at some stage (such as dragonfly or mosquito larvae), birds and mammals that require drinking water, plants that require constant moisture in the root zone and thus may live adjacent to the pool, etc. On the other hand, the pool microhabitat is unsuitable for cacti and superfluous for most insects.
A list of possible microhabitats would be overkill, but some conditions that commonly provide such may be instructive (and some specific microhabitats will be considered when we look at various organism later on). Availability of water and situations that tend to conserve moisture obviously are important in a desert habitat. Most obvious in terms of availability of water are rivers, which mostly consist of the Rio Grande and the Rio Conchos (draining part of northern Mexico and joining the Rio Grande at Presidio, Texas). Less prominent, but more important outside of the major river valleys, are seeps and springs. Cuatro Ciénegas, in Coahuila, is probably the most diverse area in the Chihuahuan Desert and supports more endemic species than anywhere else in North America. This is a result of perhaps up to thousands of geothermal springs, lakes, and streams in the Cuatrociénegas Valley.
Areas of topographic relief provide microhabitats of several types. Consider insolation. In the northern hemisphere north of the Tropic of Cancer, the sun's pathway always makes an arc whose high point is south of directly overhead (higher in summer, lower in winter, but never reaching directly overhead). Thus south slopes always have the sun's rays striking nearer to the perpendicular than level ground or northern slopes. Just as the angle of insolation makes a big difference in the earth's climatic system, so does the angle locally. All else being equal, a south-facing slope is hotter (and thus usually drier) than level ground or north-facing slopes. As a result, of course, north-facing slopes tend to be cooler and preserve moisture longer. These factors often tend to build up to greater differences. Because of the cooler temperatures and more available moisture, northern slopes often have more vegetation; this, in turn, may slow down rain runoff, allowing more moisture to sink into the soil. Although of more importance at high elevations and farther north, areas where there is significant snow tend to have colder temperatures in winter time on south slopes; snow tends to linger on the north slopes, acting as insulation from nighttime lows.
If you drive north on the interstate from El Paso to Albuquerque, keep an eye on the shallow arroyos near the highway when you are near Socorro, New Mexico. For a fair stretch, the south slopes and level areas between arroyos are in creosotebush, but the north slopes support grasses.
Canyons and arroyos tend to benefit from slope effects, and if deep enough, the north slopes and canyon bottom may be in shadow throughout the hot portions of the day. Additionally, these drainageways may not only carry water during and immediately after precipitation, but the sedimentary fill (sand, gravel, etc.) at the bottom may remain moist far longer than other areas or even support a permanent water table. The combination of factors is why much of plant biodiversity tends to occur in areas of considerable relief. Variations in slope angles, length of time in sun or shade, and available moisture may result in a number of microhabitats (e.g., full sun, relatively abundant moisture; full sun, dry; shaded, relatively abundant moisture; shaded, dry; and so on.
Although a number of subtle differences between soils (coarse, fine, clayey, etc.) may affect plant growth strongly, a few characteristics are more general. Limestone mountains tend to be relatively dry, so that an igneous range and a limestone range receiving the same precipitation may have very different floras. The reason is that limestone dissolves relatively rapidly as acids formed by plants (carbonic acid resulting from interaction of carbon dioxide produced by plant-root respiration and ground water) eat away the limestone, resulting in fissures; also, the layered, brittle limestone tends to fracture along joints that then may be enlarged by solution. The result is that precipitation tends to drain quickly beyond the root zone and thus is effectively lost. The effect can be seen quick clearly in the Hueco Mountains in the vicinity of Hueco Tanks east of El Paso: several relictual populations of plants are found in the tank area (intrusive igneous rocks) that are found nowhere else in the Huecos.
Lava beds and boulder fields tend to present some microhabitats not available elsewhere at the same elevation and latitude. Relatively young lava beds often have deep crevices, and boulder fields may have passageways open deep into them. This not only provides shade from the summer sun, but if deep enough, may collect cold air in the depths during winter and, the cold air being heavy, it may linger through the warmer season. Animals may take advantage. For example, the Mexican packrat (Neotoma mexicana) occurs at notably lower elevations than its normal elevational range in the Carrizozo lava beds and in boulder fields near Truth or Consequences, New Mexico.
Many organisms essentially construct their own microhabitats. Most burrowing animals have a very different habitat than if they were not burrowers. Many insects (e.g., ants, termites) burrow extensively as do many vertebrates. Various mice, for example, spend daytime in a burrow and may block the tunnel with soil. The result is relative coolness (soil is a good insulator) and high humidity (both from any moisture that might be in the soil and from moisture produced by the rodent by respiration). Thus the most extreme desert effects of high temperature and low relative humidity are avoided.
This should be enough to get a general idea of what microhabitats are and that there may be a multitude of them, particularly in areas of diverse topography.
Last Update: 16 Aug 2006