Regional Overview

A. Uniqueness of the Sonoran Bioregion

Picture it as a hyperarid horseshoe surrounding a hypersaline sea, the Gulf of California. Imagine it as a relatively frost-free landscape -- the dream of any horticulturist -- with not one, but two shots at drought aimed at crop failure each year. Consider it as a place for tropical plants to grow in the worst of all soil media: infertile sands, alkaline talc, or burning volcanic cinder heaps. View it as place where vegetative cover is not so lush and monotonous that it interferes with seeing good geology and world-class sunsets.

As these rather whimsical scenarios suggest, the Sonoran Desert is physically and climatically distinctive in several ways. It is the most tropical of the North American deserts; that is, its southerly, low elevation vegetation subtly extends the ranges of certain freezeintolerant tropical plants and animals northward, where they are ultimately limited by high temperatures and damaging solar radiation. Its gentle winter and spring rains foster a biota related to that of the Mohave Desert, whereas its more tempestuous thunderstorms and hurricane-fringe chubascos of late summer and fall foster a warm season biota related to that of the Chihuahuan Desert and the Neotropics. The classic view of biotic communities in deserts is that they are remarkably static unless perturbed by humans and their livestock, whereupon they become fragile and/or irreversibly damaged. However, recent longitudinal studies of environmental fluctuations at sites fully protected from grazing and direct human manipulation demonstrate that the Sonoran desertscrub communities are dynamic, responsive entities with considerable resilience (Turner 1990).

The Sonoran bioregion has distinctive biotas in various subregions due to the pervasive influence of geographic isolating factors. Most obvious is the Gulf of California, which has fostered high levels of endemism -- unique sets of species -- on its 21 islands and on peninsular Baja California. In addition, extensive talc playas, sand dune fields and volcanic flows serve as edaphic seas isolating mountain ranges of limestone, granite or basalt from others of their ilk. This is true to varying degrees in all North American deserts, where basin and range physiography sets up an interplay between mountain islands and desert seas. Finally, aridity itself is an isolating factor, slowing the dispersal of colonizing species, most recently, the influx of certain Eurasian weeds.

Another important geographic factor of the Sonoran bioregion as it affects migratory species is the prevalence of easy-to-navigate north/south corridors. The Gulf of California and Lower Colorado River comprise the most extensive corridor. The Upper Rio Yaqui/Rio Bavispe form another corridor with the Rio San Simon. The Rio Sonora and the Rio San Pedro form another, and the Rio Magdalena/Rio Santa Cruz form a final corridor of more than 400 kilometers. These extensive corridors remain extremely important to migratory birds, but were even more remarkable before riparian gallery forests were dramatically reduced by agriculture and groundwater overdraft.

B. Biodiversity within the Sonoran Desert Bioregion

The Sonoran Desert and adjacent biotic communities do not necessarily rank high among bioregions with regard to the most commonly-cited indicators of terrestrial biodiversity: bird, butterfly or flowering plant species richness per hectare, or per square kilometer. Scientists have typically used these indicators because there are many more collection records of these taxa than there are of wild bees, moths, reptiles or lower plants. Some biogeographers have suggested that diversity of resident species per se is not the best criterion for evaluating the importance of desert habitats; use by migrants, or levels of endemism might be more revealing criteria.

This Sonoran region as a whole does indeed have remarkable levels of endemism found within certain of its subregions. For instance, 501 of 552 endemic plant species found on the Baja California peninsula and its adjacent islands occur within its desertscrub and Cape thomscrub subregions, rather than in its Mediterranean chaparral, or in the uplands of the Sierra de San Pedro Martir and Sierra de Juarez (Villasenor and Elias 1995). Over half of these are highly restricted or "microareal" endemics (Table 1), isolated to a single island or geographic zone of the peninsula. In addition to other species at risk (Appendix 4), they form the region's most unique biological legacy.

Such extremely high levels of endemism can not only be found for plants, but for reptiles and small mammals as well. The islands of the Gulf are, of course, relatively rich in endemics: 12 plant species and 6 additional subspecies are restricted to the islands; 65 mammal subspecies, 15 species and one genus of fish-eating bats are restricted to them; and more than 30 species or subspecies of reptiles are endemic to one or more of the islands.

Although arid regions usually rank low relative to other biomes in overall species richness, this is not true across all taxonomic groups. Búrquez and Martínez-Yrízar (1988) report that current estimates of the plant species richness in the state of Sonora alone may be as high as 4,500 species, or 20% of Mexico's total flora in an area of less than 10% of the country. Rozenzweig and Winakur (1969) claimed that the Upper Rio San Pedro watershed harbored a higher diversity of small mammals than any other area in North America known at that time. The species richness of mammals recorded in what is now the San Pedro Riparian National Conservation Area - some 86 species, including 12 at risk -- remains unsurpassed for any single landscape of comparable size in the U.S.. Overall, the region harbors perhaps 130 species of mammals, extrapolating from Hoffmeister's (1986)'s inventory of those found in Sonoran desertscrub and semi-desert grassland in Arizona, in addition to those found on the Gulf Islands. With at least 146 species found on the desert mainland, peninsula and adjacent islands. The region's reptile diversity is also high, with as many as 96 endemic taxa being found among the Gulf Islands, Sonora and Baja California (Flores-Villela and Navarro 1993). Certainly, with only 20 species, amphibian diversity is low: the islands and the peninsula have no endemics, and the mainland harbors only 11 endemic amphibians (Flores-Villela 1993). Freshwater native fish diversity is rather low for a region of this size - perhaps 25 to 30 species (Minckley 1973; Minckley and Deacon 1968), but there are at least 250 marine species of rocky shore and reef fish in the northern and central Gulf of California (Thomson, Findley and Kerstitch 1979). The eleven endemic fishes of freshwater springs and creeks is perhaps a more revealing measure of the Sonoran Desert's biological value: desert pupfish, Yaqui suckers, Sonora chub, Colorado River squawfish, razorback suckers and other very narrowly-restricted species demonstrate that water in the desert is a limiting factor for evolution as it is for productivity.

Buchmann and Nabhan (1996) have projected that with an estimated 1200 species, there is greater species richness of native bees within an hour's drive of Tucson than anywhere else in the Americas, and perhaps anywhere else in the world. The overall pollinator diversity of the Sonoran region is remarkably high, with upwards of 150 butterfly species, perhaps as many as 1,200 moth species, 17 hummingbird species, and at least 5 nectar-feeding bats servicing the region's flowering plants.

Finally, we must consider bird diversity from a variety of perspectives. There have been at least 500 bird species reported in the Sonoran bioregion, roughly half the known number of birds present in the continental U.S. or in all of Mexico. Remarkably, northsouth corridors such as the Rio San Pedro or the Rio Colorado may each harbor as many as 400 species for breeding, overwintering and migrating; that includes 75 percent of all the bird species which migrate between the U.S. and Mexico (Stevens et al. 1987; Anderson et al. 1987). There are roughly fifty species found in the Arizona portion of the Sonoran bioregion that are seen nowhere else in the U.S., and 15 species endemic to Mexican portions of the region (Flores-Villela and Navarro 1993).

In desertscrub and semidesert grassland habitats, the per unit area diversity of breeding birds is not particularly remarkable -- 30 150 pairs per 40 hectares (Johnson et al. 1987). However, the deciduous riparian gallery forests of the Sonoran biome may have the highest breeding bird densities on the continent, harboring 304 to 847 breeding pairs per 40 hectares (MacArthur and MacArthur 1961; Carothers, Johnson and Aitchison 1974; Johnson et al. 1987). It is fair to say that in terms of breeding bird diversity and productivity, the Sonoran biome's riparian habitats are among the richest in all of North America.
The Sonoran biome is peculiar in another kind of diversity -extant cultural diversity. Although indigenous cultures in Baja Califomia were so devastated by European-introduced diseases that only the Paipai, Kiliwa, and Cucupa have persisted on the entire peninsula, the rest of the region has most of its native cultures still alive and thriving. The Guarijio, Yaqui, Mayo, Seri, Pima Bajo, Tohono O'odham, Hia c-ed O'odham, Gila River Pima, Cucupa, Maricopa, Mohave, Quechan, Cahuilla, Chemehuevi, Walapai, Havasupai, Westem Yavapai, and Westem Apache are among the indigenous cultures with long tenure in this bioregion. While the Yaqui reservation in Sonora is among the largest and most secure in all of Mexico, there are also five other reserves in Sonora and Baja Califomia where indigenous people live today, as well as poorly-enforced Seri tribal rights to Islas Tiburon and San Esteban.

In the United States, a significant portion of all Arizona and Califomia desert lands fall within the reservations of the Tohono O'odham, Ak-Chin, Gila and Salt River Indian Communities; of the Cahuilla and Chemehuevi; of the San Carlos, Camp Verde and Fort McDowell Apache and Yavapai; of the Cocopah, Ft. Yuma, Ft. Mohave and Colorado River Indian Tribal communities, and of the Walapai, Havasupai, Clarkdale and Prescott Yavapai. In the ten states running along both sides of the U.S./Mexico border from the Pacific to the Caribbean, indigenous communities manage as much as 17.8 million hectares - which is more than all the private land reserves of The Nature Conservancy and other non-profit conservation groups in North America (Nabhan et al. 1991).

It has only been recently recognized that this diversity of human occupants historically fostered a mix of desert land and water management strategies, which no doubt kept habitats more heterogeneous than they are today (Rea 1997; Nabhan et al. 1982). Although the U.S. Bureau of Indian Affairs and Mexico's Instituto Nacional Indigenista have attempted to homogenize land use practices surrounding some indigenous communities, tribal responses to these pressures have varied, and the degree of habitat protection on tribal lands is not at all uniform.

In general, the very presence of the border has set up some "natural experiments" where it becomes easy to compare different land management "treatments" on either side of the boundary line. Juxtaposing ecosystem health in Anglo-, Hispanic- and Native-American communities adjacent to one another in the same habitat type has inadvertently allowed for ecologists to clearly see how different cultural management practices affect the same biota (Minnich 1981; Balling 1988; Nabhan and Suzan 1994). At another scale, the same principle holds true in habitat types where private, Forest Service, Park Service, Bureau of Land Management and state or reservation land managers all protect or manage wildlife and vegetation to different degrees.

We are only beginning to objectively compare the long-term effects of such a diversity of management strategies on the region's biodiversity. Nevertheless, conservation biologists working in the Sonoran Desert biome can at least be relieved that the entire landscape does not currently fall under the custody of a single management style, i.e., that of the Army Corps of Engineers, the Savory System, or shrubland chaining by range managers intent on "Ecosystem Improvement."

From:
Nabhan, Gary Paul and Andrew R. Holdsworth. 1998. State of the Sonoran Desert Biome: Uniqueness, Biodiversity, Threats and the Adequacy of Protection in the Sonoran Bioregion. p.34-36. Tucson, Ariz.: The Wildlands Project.

Sources

• Anderson et al. 1987
• Balling 1988
• Buchmann and Nabhan 1996
• Búrquez and Martínez-Yrízar 1988
• Carothers, Johnson and Aitchison 1974
• Rodolfo Dirzo. Mexican Diversity of Flora (1994)
• Flores-Villela 1993.
• Flores-Villela and Navarro 1993.
• Hoffmeister's 1986's
• Johnson et al. 1987.
• Kenn Kaufman. Kingbird Highway (1997)
• MacArthur and MacArthur 1961;
• Minckley 1973
• Minckley and Deacon 1968
• Minnich 1981;
• Nabhan and Suzan 1994.
• Nabhan et al. 1982.
• Nabhan et al. 1991.
• Rea 1997
• Rozenzweig and Winakur 1969
• Stevens et al. 1987
• John Steinbeck. The Log from the Sea of Cortez (1941)
• Raymond M. Turner and David E. Brown. "Sonoran Desertscrub", Biotic Communities of the American Southwest (1982)
• Thomson, Findley and Kerstitch 1979.
• Turner 1990
• Villasenor and Elias 1995.