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

Introduction

What is unique about the Sonoran Desert region? What kinds of organisms, ecological interactions and landscapes can be found here and nowhere else? Which of these have we safeguarded, and which are we letting slip away? What stresses and forces, both from within and from beyond the region, are threatening various elements of our region's biodiversity - the variety of distinctive populations, species, guilds, communities and habitats found here? How can we shift our ways of thinking, behaving and consuming to better protect what remains of our region's diversity? Where should our shared priorities lie?

Although many communities across the American continent have been asking these and similar questions for some time, such inquiries have been spoken aloud only relatively recently in the Sonoran bioregion. Perhaps language barriers and an international boundary have prevented us from asking such questions as they relate to a realm of arid land, water and life found across two nation states. For most of this century, perhaps national conservation organizations have assumed that so few people lived in this "big empty" that the protection of natural areas remaining in more heavily-populated regions deserved higher priority.

And yet since World War II, the "Sunbelt'- including the Sonoran Desert and adjacent biomes - has suffered from the greatest in-migration and most massive land conversion occurring within any fifty-year period in human history. Within the last half century, the number of human inhabitants in the region has increased sevenfold, but few of these residents are aware of how profoundly their collective presence is changing the desert. Unless we begin to understand how our own lives interact with those of other species in this desert biome, they are likely to "go away" before we know it. The Sonoran Desert biome extends beyond the boundaries of any single nation-state, tribe, or economy; a "state of the nation" will not tell us how it is faring. As defined here, the Sonoran bioregion or biotic province includes subtropical forest, thornscrub, semi-desert grassland and other biotic communities within and adjacent to the Sonoran Desert proper, and aquatic habitats as well. Although its name is derived from the geopolitical state of Sonora, Mexico, this bioregion also covers parts of Arizona and California, USA, and Chihuahua, Baja California Norte, and Baja California Sur in the Republic of Mexico.

At least twenty indigenous nations have a long tenure within this region. Despite the widespread view that "deserts" are impoverished places for humans to live, the Sonoran bioregion retains as much biological and cultural diversity as any region on the North American continent.

As portrayed here (Figure 1), the Sonoran bioregion is delineated much the same way that Dice (1943) and Dasmann (1974) demarcated the Sonoran biotic province. Its terrestrial habitats cover between 310,000 and 330,000 square kilometers, depending on how one deals with intermittently flooded wetlands, playas, deltas and riverine corridors. Within this particular bioregional assessment, nine of the twenty-seven biotic communities displayed on the Brown and Lowe map of the Biotic Communities of the Southwest are considered (Brown 1982): Sinaloan Deciduous Forest at its northern limits; Foothills of Sonora and Coastal Thomscrub; Semidesert Grassland at its western limits and Sonoran Subtropical Grassland; "Cape" Thomscrub of Baja California; Sonoran Desert/Arizona Uplands; Sonoran Desert/Lower Colorado Lower River Valley; Sonoran Desert/Central Gulf Coast; Sonoran Desert/Viscaino-Magdalena Plain; and Sonoran Desert/Plains of Sonora. In addition, we consider here a number of riparian, coastal, wetland and oceanic communities which are difficult to map but nevertheless critical to maintaining the region's biodiversity (Minckley and Brown 1982).

This report aims to set conservation priorities for the establishment of a bioregional conservation plan sponsored by The Wildlands Project. To establish such priorities on a firm scientific basis, it has been necessary to first characterize the spatial patterns of biodiversity within the region, then to assess what protective measures already exist, and finally to determine where current conservation and management practices fall short of ensuring the long-term survival and health of the region's remaining biological riches.

To our knowledge, no one has ever assessed this entire region from the perspective of conservation biology. A region-by-region survey of Mexico (Villela and Gerez Fernandez 1989) included a preliminary biodiversity assessment of the Sonoran Desert in relation to other regions, but in it, statistics for the region as a whole were sometimes confused with those for the state of Sonora. Nevertheless, this report raised international concern regarding the rapidity of environmental change occurring within the region, reporting that at least 60 percent of its native vegetation had already been converted or destroyed.

As an initial phase in the development of a more comprehensive bioregional conservation strategy, we wish to draw attention to the threatened biota, disrupted ecological processes and diminished integrity of habitat mosaics in the region as they exist today. We also hope to acknowledge currently-effective conservation measures, but without naively assuming that they will continue to function well even if certain threats and pressures continue to increase in magnitude and severity. Finally, we wish to compile scientists' initial suggestions for better protecting certain species, landscapes and processes, topics which will be discussed more intensively during later phases of conservation planning.

Because the quality of published information on biodiversity and natural resource use is seldom collected on the same scale and with equal intensity on both side of the U.S. - Mexico border, we chose to supplement published literature with questionnaires directed to over 120 of the region's most active field biologists and protected area managers.

Fifty-four scientists responded to our surveys in whole or in part, an extremely high return for any survey tool. The average number of years that these scientists have been actively conducting field studies in this bioregion is twenty, perhaps making them the most seasoned body of experts ever asked to speak on behalf of the desert. Although they have collectively accomplished nearly 1,000 years-worth of field studies in this bioregion, this is but a crude indicator of their depth of understanding of the kinds of environmental change which is occurring. Some of them have literally worked at hundreds of field sites on both sides of the border, and have been responsible for publishing the bulk of the ecological monographs and articles on the region's biodiversity over the last quarter century.

To more specifically address issues regarding the pace and extent of environmental change that they have personally witnessed, we asked these field experts to assess trends since 1975, in the nine (mappable) biotic communities mentioned above, as well as in aquatic and island habitats. Although their responses are not necessarily site-specific, they do offer a very tangible assessment of conservation issues at the level of specific biotic communities.

We are extremely grateful to the scientists who found time in their busy schedules to respond to our surveys; their contributions form the cornerstones of this report. We also thank the Arizona Sonora Desert Museum for providing information that contributed to this assessment, Rob Marshall and Peter Wan-en of the Arizona Chapter of The Nature Conservancy for reviewing this report, Jennifer Dastrup for skillfully formatting the report and Anne Gondor for creating the color maps. Finally, we thank The Town Creek Foundation who supported The Wildlands Project in the compilation of this report.

"...the world drifts and our maps don't work anymore, our paradigms and stories fail, and we have to reinvent our understandings, our reasons for doing things ... What we need most urgently, in both the West and all over America, is a fresh dream of who we are, [and stories] which can tell us how we should act... They will be stories in which our home is sacred, stories about making sense of a place without ruining it ... Wreck it and we will have lost ourselves, and that is craziness."

- William Kittredge
Who Owns the West? (1996)