State of the Sonoran Desert Biome: Uniqueness, Biodiversity, Threats and the Adequacy of Protection in the Sonoran Bioregion

Executive Summary

This report highlights (1) what is unique about the Sonoran Desert bioregion with respect to its organisms, ecological interactions and landscapes and (2), what threatens the future of this region's biological diversity. It is based on the compilation of surveys of 54 field scientists who average twenty years of field experience in this region of the southwestern United States and northwestern Mexico.

Regional Overview

The most tropical of the North American deserts, the Sonoran bioregion has distinctive biotas in each of its 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. Such extremely high levels of endemism can not only be found for plants, but for reptiles and small mammals as well. In general, these regional patterns of endemism should serve as one of the key guides to prioritizing the location of future protected areas. With regard to indicators of terrestrial biodiversity among bioregions, the Sonoran Desert and adjacent biotic communities should rank higher than is commonly assumed. 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. Reptile and riparian breeding bird diversity are notable. The overall pollinator diversity of the Sonoran region's bees, butterflies and bats is remarkably high compared to other areas of North America. The extant cultural diversity of indigenous communities is as high as any region north of the tropics.

Stressors: Threats to Biodiversity

Thirty-three of the field scientists responded to the portion of our written questionnaire which asked them to rank the ten most significant threats to the biodiversity of the Sonoran bioregion on the basis of their observations since 1975. The top ten threats, according to the tally of their responses, are as follows:

1. Urbanization's aggravation of habitat conversion and fragmentation;
2. The high rate of in-migration of newcomers to reside, work and recreate in the region, and their contribution to population growth and resource consumption;
3. Surface water impoundment and diversion from places where native vegetation and wildlife have access to it;
4. Inappropriate grazing of vegetation by livestock, especially when combined with conversion of plant cover to exotic pasture grasses;
5. Aquifer mining and salinization, the drop in water table, and their long-term effects on riparian vegetation and wildlife;
6. lack of planning for growth
7. Exotic grass planting;
8. Conversion to farmlands;
9. Recreational impacts;
10. Biological Invasions.

Since World War II, the Sunbelt of the U.S. Southwest and Northwest Mexico has been the setting for the largest in-migration in human history. A century and a half ago, indigenous communities still outnumbered European colonial communities, both in number and in the amount of land and water they managed. Today, the economic activities of the region are dominated by individuals who have lived in the region for less than a decade. The region's population nearly doubled (+98%) between 1970 and 1990 to a total population of 6.9 million. The greatest increases in population occurred in coastal resort areas, state capitals, and along the border. Currently, there is no sign that human population growth rates in the region will taper off during the next few decades.

Between 1940 and 1990, the populations of Arizona, Baja California Norte, and Sonora shifted from being one half to two-thirds rural, to over three-quarters urban. The present inhabitants' unfamiliarity with desert land and water management poses profound threats for most land, water, vegetation and wildlife resources within a half-hour's drive of the region's largest metropolitan areas. The actual effects of this urbanization on biodiversity are many and mutually reinforcing, including the aggravation of the "urban heat island effect"; the channelization or disruption of riparian corridors; the proliferation of exotic species; the killing of wildlife by automobiles, by toxics, and by pets; and the fragmentation of remaining patches of natural vegetation into smaller and smaller pieces that are unable to support viable populations of native plants or animals.

Hydrological engineers in the Sonoran Desert have impounded and diverted water flows from virtually all of the region's major rivers by constructing 41 major dams and associated irrigation canals. Among U.S. Federal Register notices listing plants and animals as endangered species, water impoundment and diversion are among the most frequently cited threats mentioned. Inundating vegetation in reservoirs behind dams and changes in river flow are among the most severe pressures on threatened plants and nesting birds in the U.S./Mexico borderlands. The regional decline of 36 of the 82 breeding bird species which formerly used riparian woodlands is a case in point. In combination with water diversion, groundwater pumping has affected nearly all river valleys in Arizona's portion of the Sonoran Desert. In the heart of agricultural areas, groundwater overuse has been most precipitous, leading to ground subsidence, salinization and the demise of riparian forests.

With regard to grazing, overstocking still continues on public and private lands in Arizona and Mexico's CODECOCA statistics confirm that 2 to 5 times the recommended stocking rates occur with regularity on the Sonoran side of the border. The cattle-related introduction and intentional sowing of African grasses in the Sonoran bioregion has not only affected the biotic composition of semidesert grasslands, but has profoundly changed vegetation structure, fire intensity and frequencies and migratory wildlife corridors within several subregions of the Sonoran Desert proper. The "grasslandification" of the Sonoran Desert, adjacent thornscrub and subtropical savanna by buffelgrass has already occurred on some 600,000 hectares of Sonora.

Adequacy of Current Measures to Protect Biodiversity

Although there are many stresses on the region's biodiversity, we have witnessed more areas decreed as protected (as international, national or state biosphere reserves) in the last decade than any other decade in the history of the Sonoran bioregion. In addition, there are now more resource managers working on both sides of the border than there were a decade ago, although many more need training to better manage their areas for biodiversity instead of for single species or for recreation. For each Sonoran Desert subregion, vulnerable species and areas, and areas that merit protection are listed.

When asked if protected area managers still allow activities which deplete biodiversity, twenty-five of the surveyed scientists answered yes, nine answered no, and seventeen answered that such harmful activities now occur less than before. However, it is a hopeful sign that over one quarter of the respondents see fewer harmful activities occurring within protected areas today than "before" -- meaning either before the decree of these areas, or for early-established parks and wildlife refuges, before 1975. A notable portion of the scientists felt that grazing was finally being addressed sufficiently in discussions between resource managers, ranchers and scientists. Others felt that the impacts of ecotourism (e.g. whale watching) and outdoor recreation were being sufficiently dealt with at the local level. However, a majority of the scientists felt that virtually no threat is truly being adequately addressed anywhere in the Sonoran biome where they have worked.

Emerging Conservation Needs and Priorities

When field experts conversant with the Sonoran bioregion were asked what they felt should be the number one priority for conservation, they responded in a variety of ways, noting policy issues, research and education needs, action strategies, as well as earmarking species, habitats or landscapes in critical need of conservation. The extensive list includes the need to shift away from social and economic systems that reward consumptive behaviors and short-term gain while damaging natural systems, manage irrigation tailwaters and sewage effluent to restore the wetlands of the Colorado River delta, and many other recommendations.

What's Next?

It is clear that there is much reported by the field scientists surveyed here that bears reflection, discussion, debate and action. It is also abundantly evident that scientists' attention is not spread evenly across the biotic communities of the bioregion -- some habitats such as mangrove swamps, riparian gallery forests and semidesert grasslands south of the U.S. - Mexico border are irregularly visited by biologists and poorly monitored relative to their significance.

There are four problems identified as the emerging issues which still require considerable discussion if they are to be resolved for the region:

1. The need for urban planning and agricultural lands restoration to allow for continuous corridors for wildlife passage through urban areas where their movements are currently blocked.
2. The need for guaranteeing river flow into coastal lagoons and estuaries of the Gulf of California (including the Colorado River delta) to ensure nutrient and fresh water flow essential to nursery rounds for invertebrates, fish, and waterfowl.
3. The need to redirect the management of critical habitats in state parks, wildlife refuges and national monuments away from recreation or protection of single species or features; focus needs to shift to overall biodiversity and the integrity of habitats, so that the interactions between species and natural communities persist.
4. The need for planning that reduces impacts of coastal and island development in the Gulf of California region where endemism is the highest.