Biodiversity: The Variety of Life that Sustains Our Own

Gary Paul Nabhan

An excerpt from A Natural History of the Sonoran Desert

There is a place in the Sonoran Desert borderlands which, more than any other I know, capsulizes what the term diversity has come to mean to both natural and social scientists alike. The place is a desert oasis known as Quitobaquito, centered on a spring-fed wetland at the base of some cactus-stippled hills that lie smack dab on the U.S.-Mexico border. Whenever I walk around there, I am astounded by the curious juxtapositions of water- loving and drought-tolerating plants, of micro-moths wedded to single senita cacti, and hummingbirds that have traveled hundreds of miles to visit ocotillos, of prehistoric potsherds of ancient Patayan and Hohokam cultures side by side with broken glass fragments left by O'odham, Anglo- and Hispanic-American cultures.

Walk down from its ridges of granite, schist, and gneiss, and you will see organpipe cactus growing within a few yards of arrowweed, cattails, and bulrushes immersed in silty, saline sediments. The oasis has its own peculiar population of desert pupfish in artesian springs just a stone's throw from the spot where a native caper tree makes its only appearance in the United States. The tree itself is the only known food source for the pierid butterfly that is restricted in range to the Sonoran Desert proper.

More than 270 plant species, over a hundred bird species, and innumerable insects find Quitobaquito to be a moist harbor on the edge of a sea of sand and cinder. Not far to the west of this oasis, there are volcanic ridges that have frequently suffered consecutive years without measurable rainfall, and their impoverished plant and animal communities reflect that.

Quitobaquito is naturally diverse, but its diversity has also been enhanced rather than permanently harmed by centuries of human occupation. Prehistoric Hohokam and Patayan, historic Tohono O'odham, Hia c-ed O'odham, Apache, Cucupa, and Pai Pai visited Quitobaquito for food and drink long before European missionaries first arrived there in 1698. Since that time, a stream of residents from O'odham, exican, Jewish, and Mormon families have excavated ponds and irrigation ditches, transplanting shade and fruit trees alongside them. They intentionally introduced useful plants, and accidentally brought along weedy camp-followers, adding some fifty plant species to Quitobaquito over the centuries. Native birds and mammals have also been affected by human presence there, and some increased in number during the days of O'odham farming downstream from the springs. All in all, Quitobaquito's history demonstrates that the desert's cultural diversity has not necessarily been antithetical to its biological diversity; the two are historically intertwined.

In fact, the Sonoran Desert is a showcase for understanding the curious interactions between cultural and biological diversity. There are at least seventeen extant indigenous cultures that each has its own brand of land management traditions, as well as the dominant Anglo- and Hispanic-American cultures which have brought other land ethics, technologies, and strategies for managing desert lands into the region. While some cultural communities such as the Seri were formerly considered passive recipients of whatever biodiversity occurred in their homeland, we now know that they actively dispersed and managed populations of chuckwallas, spiny-tailed iguanas, and columnar cacti. Floodwater farmers such as the Tohono O'odham and Opata dammed and diverted intermittent watercourses, planted Mesoamerican crops, and developed their own domesticated crops from devil's claw, tepary beans, and Sonoran panic grass. Anglo- and Hispanic-American farmers and ranchers initiated other plant and animal introductions, and dammed rivers on a much larger scale. Each of these cultures has interacted with native and exotic species at different levels of intensity, including them in their economies, stories, and songs. From an O'odham rainmaking song that echoes the sound of spadefoots, to the Western ballad "Tumblin' Tumbleweeds" written in Tucson over a half century ago, native and invasive species have populated our oral and written traditions as curses, cures, and resources.

Technically speaking, this stuff we call diversity eludes one single definition. For starters, however, biodiversity (short for biological diversity) can be generally thought of as the "variety of life on earth." Scientists use this term when discussing the richness of life forms and the heterogeneity of habitats found within or among particular regions. Biodiversity in this sense is often indicated by the relative richness of species in one habitat versus another. Thus it is fair to say that riparian gallery forests of cottonwoods and willows along desert rivers typically support more avian biodiversity-a greater number of bird species-than do adjacent uplands covered with desertscrub vegetation. Similarly, there is greater biodiversity in flowering vines in the moist tropical forests of southern exico than there is in the Sonoran Desert of northern Mexico.

It is worth noting, however, that ecologists such as E.O. Wilson first coined the term biodiversity to signify something far more complex than the mere number of species (termed species richness) found in any given area. Usually ecologists also consider the number of individuals within each species when they assess diversity or heterogeneity. An area where one desert wildflower such as the California poppy dominates eight other species is considered to be less diverse than an area with the same eight species where the numbers of each are more evenly distributed. As Kent Redford of The Nature Conservancy has recently explained, "A species-focused approach to biodiversity has proved limiting for a number of reasons....[The] use of just species as a measure of biodiversity has resulted in conservation efforts focusing on relatively few ecosystems while other threatened ones are highly ignored. Species do not exist in a vacuum, and any definition of biodiversity must include the ecological complexes in which organisms naturally occur and the ways they interact with each other and with their surroundings."

The integrity of biodiversity can be teased apart into the following components. Although each of them may be separated out by scientists for study, they do not truly exist "apart" from one another.

ECOSYSTEM DIVERSITY:   the variety of landscapes found together within any region, and the ways in which their biotic communities interact with a shared physical environment, such as a watershed or coastal plain. A landscape interspersed with native desert vegetation, oasis-like cienegas, riparian woodlands, and croplands is more diverse than one covered entirely by one crop such as cotton. The Colorado River Delta was once a stellar example of ecosystem diversity, displaying a breath-taking mixture of riparian gallery forests, closed-canopy mesquite bosques, saltgrass flats, backwater sloughs, rivers, ponds, and Indian fields. Much of it is now dead, except for the hypersaline wetlands known as the Cienega de Santa Clara.

BIOTIC COMMUNITY DIVERSITY:   the richness of plants, animals, and microbes found together within any single landscape mosaic; such a mosaic can range in scope from the regional to the watershed level. This richness can be shaped by a variety of factors, ranging from the age of the vegetation to land use to soil salinity and fertility. For example, the number of species on well-drained, ungrazed desert mountain slopes covered by columnar cacti, ancient desert ironwoods, and spring wildflowers is greater than that on an alkali flat grazed by goats, where only saltbush, saltgrass, and seepweed may grow. The Rincon ountains east of Tucson demonstrate a gradient of communities, each with its own diversity, as they rise from desertscrub to xeric woodlands, and coniferous forests.

INTERACTION DIVERSITY:    the complexity of interactions within any particular habitat, such as the relationships between plant and pollinator, seeds and their dispersers, and symbiotic bacteria and their legumes. A pine-oak woodland in Arizona's "sky islands"harbors more interspecific interactions than does an even-aged pine plantation. Ramsey Canyon in the Huachuca ountains showcases such interaction diversity, with over a dozen hummingbirds, as well as bats, bees, and butterflies visiting its myriad summer flowers.

SPECIES DIVERSITY:    the richness of living species found at local, ecosystem, or regional scales. A well-managed desert grassland hosts more species than can be found in a buffelgrass pasture intentionally planted to provide livestock forage without consideration of wildlife needs. Quitobaquito, discussed above, is as fine an example of localized species diversity as we have anywhere in the binational Southwest.

GENETIC DIVERSITY:   the heritable variation within and between closely-related species. A canyon with six species of wild out-crossing beans contains more genetic variation than does a field of a single highly-bred hybrid bean. Indian fields in southern Sonora demonstrate this concept, for their squashes hybridize with weedy fieldside gourds, and their cultivated chile peppers are inflamed by genetic exchange with wild chiltepines.

All of these components of biodiversity ensure some form of environmental stability to the inhabitants of a particular place. A landscape with high ecosystem diversity is not as vulnerable to property-damaging floods as a bladed landscape is, for a mix of desert grassland and wetlands serves to buffer downstream inhabitants from rapid inundations. A diverse biotic community is less likely to be ravaged by chestnut blight or spruce budworm than a tree plantation can be. A cactus forest with diverse species of native, wild bees is less vulnerable to fruit crop failure than are orchards or croplands that are exclusively dependent upon the non-native honeybees. A desert grassland with multiple species of grasses and legumes cannot be as easily depleted of its fertility and then eroded as can one with a single kind of pasture grass sucking all available nutrients out of the ground. And finally, a Pima Indian garden intercropped with many different kinds of vegetable varieties will not succumb to white flies or other pests as easily as will an expansive, irrigated lettuce field in the Imperial Valley.

In short, more of "nature's services" - the economic contributions offered by intact ecosystems-are possible when we manage these ecosystems to safeguard or restore their biodiversity, and not allow it to be depleted. Recent estimates by environmental economists suggest that the dollar value of the services such as flood protection and air purification provided by the world's intact wild ecosystems averages thirty-three trillion dollars per year, compared to the eighteen trillion dollar Gross National Product of all nations' human-made products.

The message is clear: when a mosaic of biotic communities is saved together and kept healthy within a larger landscape, few endangered species fall between the cracks and succumb to extinction processes. In contrast, a small wildlife sanctuary designed to save a single species often fails to achieve its goal, for the other organisms which that species ultimately needs in its presence have been ignored or eliminated. Not only do humans benefit from the conservation of large wildlands landscapes, but many other species do as well.

How does this play out in our Sonoran Desert region?

Ask most people to characterize life in the desert and few will think to mention the word "diversity"as part of their thumbnail sketch of this place. Most of us keep in our heads those pictures of bleak, barren, blowing sandscapes when we hear the word "desert."

The Sonoran Desert does contain one major sea of sand, as well as a long corridor of coastal dunes along the Gulf of California, but even these are seasonally lush with unique and thriving life forms. As one spends more time in a range of Sonoran Desert habitats, one is constantly surprised by how many plants and animals are harbored here.

Travel out of Sonoran Desert vegetation into the higher mountain ranges held within the region and even more astonishing levels of biodiversity can be found. In fact, the "sky islands"of southeastern Arizona and adjacent Sonora are now recognized by the Inter-national Union for the Conservation of Nature as one of the great centers of plant diversity north of the tropics.

When we compare our desert with others, the contrast is striking. Overall, the Sonoran Desert has the greatest diversity of plant growth forms- architectural strategies for dealing with heat and drought-of any desert in the world. From giant cacti to sand-loving underground root parasites, some seventeen different growth forms coexist within the region. Often, as many as ten complementary architectural strategies will be found together, allowing many life forms to coexist in the same patch of desert.

Biodiversity in the desert is often measured on a scale that would not be used in the tropical rainforest. Desert ecologists have found twenty kinds of wildflowers growing together in a single square yard (.84 m²), while a single tropical tree might take up the same amount of space. On an acre (.4 ha) of cactus forest in the Tucson Basin, seventy-five to 100 species of native plants share the space that three mangrove shrubs might cover in swamp along a tropical coast. These levels of diversity are a far cry from the "bleak and barren" stereotype, and it may well be that the Sonoran Desert region is more diverse than other arid zones of comparable size.

Consider for example, the flora of the Tucson Mountains, which Arizona-Sonora Desert Museum research scientists recently inventoried with a number of their colleagues. In an area of less than forty square miles (100 km²), this botany team encountered over 630 plant species-as rich a local assortment of plants as any desert flora we know. This small area contains roughly one-sixth of the Sonoran Desert's entire plant diversity. It is disproportionately rich relative to its size, its paucity of surface water, and its elevational range.

Such a diversity of wildflowers and blossoming trees attracts a diversity of wildlife as well. In the Sonoran Desert area within a thirty mile radius of Tucson, you can find between 1000 and 1200 twig- and ground-nesting native bees (all of them virtually "stingless"). As the Desert Museum's research associate Stephen Buchmann wryly notes, "this may mean that the Sonoran Desert region is the richest bee real estate anywhere in the world-the entire North American continent has only 5000 native bee species."

Desert wildflowers attract more than bees. Southern Arizona receives visits from more hummingbird species-seventeen in all-than anywhere else in the U.S. Other pollinator groups, such as butterflies and moths, are well-represented in the region as well. Single canyons near the Arizona-Sonora border may harbor as many as 100 to 120 butterfly species, and moth species may number five to ten times higher than that in the same habitats. When all pollinating organisms breeding or passing through here are counted, it may be that the greater Sonoran Desert has as large a pollinator fauna as any bioregion in the world.

This region is also rich in small mammals and reptiles. Some eighty-six species of mammals have ranges centered within the San Pedro National Riparian Area alone, a record unsurpassed by any natural landscape of comparable size in the U.S.; the area contains half of all mammal species in the binational Sonoran Desert. At least ninety-six species of reptiles are endemic to the Sonoran Desert-found here and nowhere else in the world.

Why is such diversity present in a land of little rain? For starters, our bimodal rainfall pattern brings out completely different suites of wild-flowers and their attendant insects at different times of the year. In addition, we benefit from a more gradual transition between tropical nature and desert nature than does the Chihuahuan Desert on the other side of the Sierra Madre-many tropically-derived life forms reach their northernmost limits in the Sonoran Desert due to its relatively frost-free climes. Of course, tropic rainforests are much more diverse in the total number of species they have throughout their biome, in part because of their ages and their high energy budgets. However, there may be more turnover in species from place to place in the Sonoran Desert than in some tropical vegetation types. That is to say, many desert plants and insects are "micro-areal" - occurring only within a 100 by 100 mile spots on the map. Particularly in Baja California, there are extremely high levels of endemism, including some 552 plants unique to the peninsula.

Nevertheless, it remains true that the highest levels of local diversity in this desert region occur where water accumulates. Some of the highest breeding bird densities recorded anywhere in the world come from riparian forests along the Verde and San Pedro river floodplains. More than 450 kinds of birds have historically nested or migrated along the Colorado, San Pedro, and Santa Cruz rivers. And yet, if riparian habitats were among our richest, what have we lost with the removal of cottonwoods from ninety percent of their former habitat in Arizona? Ornithologists cannot name a single Sonoran Desert bird that has gone extinct with riparian habitat loss, but many of the eighty species of birds dependent on these riparian forests have locally declined in abundance. A single desert riparian mammal-Merriam's mesquite mouse-is now extinct due to the loss of riparian habitat at the hand of groundwater pumping, arroyo cutting, and overgrazing. exican wolves and black bears that formerly frequented our river valleys are among those mammals no longer found in the Sonoran Desert proper.

Conservation International has estimated that as much as sixty percent of the entire Sonoran Desert surface is no longer covered with native vegetation but is dominated by the 380-some alien species introduced to the region by humans and their livestock. Alien plants such as buffelgrass now cover more than 1,400,000 acres of the region, at the expense of both native plants and animals. Tamarisk trees choke out native willow and cottonwood seedlings. Invasive weeds such as Johnson grass and Sahara mustard have taken over much of certain wildlife sanctuaries and parks in the desert, outcompeting rare native species. Other invasive species such as Africanized bees and cowbirds also compete with the native fauna. Biological invasions are now rated among the top ten threats to the integrity of Sonoran Desert ecosystems, whereas a half century ago they hardly concerned ecologists working in the region. These invaders somehow reach even the most remote stretches of the desert, to the point of being ubiquitous.

The wholesale replacement of natives by aliens is enough of a problem, but desert biodiversity has been even more profoundly affected by habitat fragmentation-the fracturing of large tracts of desert into pieces so small that they cannot sustain the interactions among plant, pollinator, and seed disperser. Such fragmentation does not necessarily lead to immediate extinctions, just declines-there is a time lag before a species' loss of interactions with others leads to complete reproductive failure. Fragmentation caused by urbanization is now considered the number-one threat to the biodiversity of the region and is not expected to diminish during our lifetimes. The population of Arizona's Maricopa County in the year 2025 is expected to be two and a half times what it was in 1995, and similar growth rates are anticipated along the entire desert coastline of the Sea of Cortez.

In a sense, humans are making the Sonoran Desert much more like the old (and erroneous) stereotype of a barren wasteland. As more than forty dams were constructed along rivers in this century, old-timers witnessed hundreds of miles of riparian corridors dry up. Groundwater overdraft has also impoverished desert and riparian vegetation, as farms and cities pump millions more acre-feet out of the ground than rainfall in the region can naturally recharge. The roots of plants are left high and dry above the water table. Most of the Sonoran Desert was not at all naturally barren, but our misunderstandings have impoverished one of the richest arid landscapes on the planet. That is why the Desert Museum has endorsed a long-term Conservation Mission Statement which begins with these words from ecologist D.M. Bowman:

"So what is biodiversity?...the variety of life on this planet is like an extra-ordinarily complex, unfinished, and incomplete manuscript with a hugely varied alphabet, an ever-expanding lexicon, and a poorly understood grammar....Ripping the manuscript to pieces because we want to use the paper makes little sense, especially if the manuscript says that 'to survive you shall not destroy what you do not understand'. Our mission as ecologists must be to interpret the meaning of biodiversity. The urgent need for this mission, and our current ecological ignorance, must be forcefully communicated to the public."

Instead of seeing future inhabitants rip out any more pages essential to the desert's story, the conservation organizations of the region have begun to work together to ensure that the most important corridors and secluded refugia for desert flora and fauna are identified and protected or restored. These critical areas - essential to the flow of diversity from source to sink, from headwaters to river mouth, and from tropical wintergrounds to summer nesting areas - must be kept from further fraying if the fabric of the Sonoran Desert is to remain intact. Scientists can prioritize such areas in terms of their value to biodiversity, but they will be safeguarded for future generations only if a broad spectrum of society is involved in endorsing their protection.

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