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The saguaro flowers unfurl in darkness only minutes before a shadowy visitor approaches the cactus' curving arm and dips her snout into the pungent white petals. She is a lesser long-nosed bat, and she is pregnant. Already, the expectant mother has traveled more than a thousand miles, following a corridor of blossoming saguaro, cardón, and organ-pipe cacti from southern Mexico northward to Arizona's Organ Pipe Cactus National Monument. She will soon give birth to a single offspring inside an abandoned mine tunnel where she roosts. When her baby learns to fly, and the saguaro and organ-pipe fruits drop and wither under the blistering summer sun, she and thousands like her will continue east across the desert to feast on agaves in adjacent mountain ranges before journeying south again for winter.

The Sonoran Desert contains many ecological communities, including mountains and tropical forests. (Alex Hawes)

An estimated 100,000 lesser long-nosed bats (Leptonycteris curasoae) converge on southeastern Arizona in late summer for the agave bloom, and an even larger number feed on agaves not far south, in the Mexican state of Sonora. These gray and cinnamon-brown creatures are valuable pollinators of the cacti and agaves they frequent, but their long-term survival is imperiled on both sides of the international border.

Lesser long-nosed bats were declared an endangered species in 1988. Conversion of their desert-scrub habitat by livestock grazing is just one cause. In Mexico, the moonshine tequila industry's demand for agaves is driving a decline in this vital resource for bats and other pollinators. In Arizona, the most immediate threat to the bats is the lack of viable maternity roost sites, according to Yar Petryszyn, a curator of the University of Arizona's mammal collection. Only four of these roost sites are known to exist in the entire state, most likely because of human disturbance. In the 1960s, for example, a huge ventilation fan blocked bats' entry and exit from Colossal Cave, a tourist attraction southeast of Tucson, and the bats abandoned their roost there. More recently, border crossers used a roosting cave as a hideout and drove away a colony of bats west of town. "If bats are threatened with people visiting too much, then you are going to lose large numbers of bats," says Petryszyn.

The bats are just one of many species struggling to cope with changes in their Sonoran Desert home. But defenders of the desert seek to conserve not only the stark beauty and acclaimed wildlife of the Sonoran—from its famed rattlers and roadrunners (Geococcyx californianus) to critically endangered species such as the Sonoyta mud turtle (Kinosternon sonoriense longifemorale), whose U.S. population has dwindled to 130 individuals. More broadly, people are seeking balance between habitat and development, rivers and ranching, and consumption and conservation.

Blanketing more than 100,000 square miles, the Sonoran Desert stretches from the Mexican state of Sonora into southern Arizona, southeastern California, the Baja Peninsula, and the islands of the Gulf of California (Sea of Cortez). Illegal immigration joins an ever-growing list of challenges for protectors of this spellbinding landscape, a list that includes an exploding human population, a plunging water table, and an invasion of exotic species.

Roadrunner. (photos.com)

One Arizona community, Pima County, last spring passed a landmark, $174.3 million open-space bond measure that has united environmentalists, biologists, developers, and hunters in pursuit of compromise between humans and nature. People have begun to recognize not just their impact on the desert, but the desert's cultural, spiritual, and economic impact on them.

From the evening cries of quail to the pungent odor of creosote after a late-July torrent, the Sonoran breathes with a vitality few outsiders can envision when conjuring the word "desert." Most think "deserted." Yet this preconceived image of a lifeless expanse, absent animals, absent verdure, may come to pass as the collective efforts of ranchers, miners, builders, and just plain folks to make a life in the Sonoran take their toll, bit by bit, on the fragile landscape.

Interrupting the Flow
Geronimo's surrender at Skeleton Canyon, Arizona, in 1886 signaled the United States' final conquest of the Western frontier. Fifty years later, the completion of Hoover Dam across the Colorado River's Black Canyon signaled a similar conquest of the environment.

Hoover Dam, and less heralded dams on the Colorado River and its tributaries, provided hydroelectric power to draw settlers to the Southwest while preventing flooding and easing irrigation. Following the completion of Roosevelt Dam in 1911, the Salt River Valley became a major cotton producer. Meanwhile, advanced pumping technology put progressively greater strains on water tables in Arizona and in Sonora, Mexico. By the 1980s, rivers that flowed year-round were flowing only seasonally, and some rivers that flowed seasonally rarely flowed at all. "The Colorado and its tributaries, along with the other major rivers that brought water to the Sonoran Desert, such as the Yaqui and the Mayo, became ghosts of the past, victims of the twentieth century, carcasses of sand whose lifeblood had been diverted into cotton fields, copper mines, and vast, sprawling cities," writes Thomas Sheridan of the Arizona State Museum in A Natural History of the Sonoran Desert.

The wildlife of the Sonoran Desert has proven remarkably adapted to the adversity of heat and drought—but tragically unprepared for the novelty of agriculture and urbanization. Most directly threatened by people meddling with the desert's plumbing are the Sonoran's resident fish. The changing flows of rivers managed according to electrical demand rather than fish biology have created stream conditions below dams that favor non-native fish species. Today, exotic species—most introduced for game fishing—outnumber native species two to one. These outsiders often outcompete or even prey upon Arizona's native fish, two-thirds of which are now listed as either threatened or endangered, the highest percentage of any state.

The Gila topminnow (Poeciliopsis occidentalis), a prolific breeder that was once common in the Southwest, has been heavily preyed upon by the mosquitofish (Gambusia affinis), a species introduced from the Mississippi Basin for mosquito control. At the other end of the spectrum, the Colorado River squawfish (Ptychocheilus lucius), which can grow to six feet long and more than 80 pounds in weight, is endangered because dams disrupt its reproduction: Seasonal flooding normally cued the fish to spawn. The squawfish and other big river desert fish have humps behind their heads—"like a sportscar," says Kieran Suckling of the Center for Biological Diversity—that push them toward the riverbottom and thereby stabilize them amid powerful currents. Non-native fish lack the hump and would get swept away during a monsoon. Yet as dams hold back summer floods, native fish can't drive off alien invaders. Squawfish were once so common that farmers speared them from streams with pitchforks to fertilize their fields. They are now nearly extinct in Arizona.

Tapping the region's water not only imperiled fish; it set off a series of catastrophic ecological consequences that reverberate along the Sonoran Desert's waterways, affecting wildlife of all stripes. As pumping technology has enabled aquifers to be more effectively exploited, the region's water table has dropped an average of 160 feet and as much as 1,000 feet in some areas.

The San Pedro Riparian Corridor. (BLM)

That drop puts the water table beneath the root zone of many riparian trees, including Fremont cottonwoods (Populus fremontii) and Goodding willows (Salix gooddingii). Cottonwood-willow complexes are now one of the rarest forest types in the U.S., according to The Nature Conservancy. An estimated 90 percent of the Sonoran's riparian forest—lands adjacent to rivers, lakes, and other aquatic habitat—has disappeared due to groundwater pumping and irrigation. In turn, riparian-living animals, from Mexican garter snakes (Thamnophis eques megalops) and Chiricahua leopard frogs (Rana chiricahuensis) to western red bats (Lasiurus blossevillii), have become increasingly threatened.

More than 85 percent of all Sonoran Desert animals rely on riparian areas at some stage of their life. The San Pedro riparian corridor, which runs 140 miles from its source in the Los Altos and Mariquita mountains in Sonora, Mexico, north to Arizona's Gila River, nourishes about 250 migratory bird species, 80 mammal species, and 45 reptile and amphibian species. Elegant trogons (Trogon elegans)—striking, emerald-headed birds with crimson breasts—forage for insects and fruit above the river's banks, while coatis (Nasua narica) nose about the leaf litter for grubs, worms, and lizards in bands of up to 20 or more.

One of only two rivers to flow north from Mexico—and one of the last undammed rivers in the Southwest—the San Pedro offers particularly critical habitat for the southwestern willow flycatcher (Empidonax trallii extimus). Fewer than 1,000 breeding pairs of this white-throated, olive-brown bird remain in isolated populations across southern Colorado, New Mexico, Arizona, and southern California. Identified by its characteristic "fitz-bew" call, the bird favors dense riparian vegetation—habitat at a premium these days. Declared an endangered species in 1995, the southwestern willow flycatcher is facing off with yet another uninvited guest in the region: the brown-headed cowbird (Molothrus ater), which lays its eggs in the nests of at least 100 bird species, often destroying the eggs already there. Occasionally, a mother might successfully raise both her young and the interlopers, but not in the case of willow flycatchers: A cowbird invasion results in either complete nest failure or the rearing only of cowbird chicks.

Grazing and Blazing: Invasive Species
Yet nothing jeopardizes riparian habitats and the species they shelter, according to many conservationists, more than the animals for whom cowbirds are named. Cows graze greedily upon small saplings and thereby prevent the healthy growth of riparian stands. According to Sheridan, 37,000 cattle roamed Arizona in 1870, but by the turn of the century, following the securing of the Southwest for American frontiersmen, about 1.5 million cattle and one million sheep had been herded into the state.

Fortunately, steps are being taken to protect the vital habitat that has survived a century of ranching and water diversion. The San Pedro Riparian National Conservation Area, created in 1988, safeguards 43 miles of the river's banks from grazing and other harmful activities. The U.S. Forest Service has stopped public grazing near more than 250 miles of federally owned streams in southern Arizona and New Mexico to protect willow flycatcher habitat. "Since the cattle have been removed, it's like a jungle!" exclaims Suckling about some of the sheltered stretches of river he's rafted recently, adding that deeper streams resulting from fortified riverbanks offer excellent native fish habitat as well.

Cattle graze near Tucson, Arizona. (NRCS)

Elsewhere, however, the impact of grazing spreads unimpeded. Ranchers across the region have planted exotic grasses to feed their stock. Conservation International estimates that more than half of the entire Sonoran Desert is now covered by nearly 400 alien plant species. A few species—buffelgrass (Pennisetum ciliare) and Lehmann lovegrass (Eragrostis lehmanniana) from Africa, and red brome (Bromus rubens), a species first brought from the Mediterranean to California in 1848—have predominated. In central Sonora, Mexico, ranchers have cleared more than a half-million acres of desert to plant buffelgrass, some with Mexican government funding.

Exotic grasses have spread across the desert, literally, like a wildfire. Old World grasses thrive after—and even encourage—fire, in stark contrast to the desert-scrub and tropical deciduous forest species native to the Sonoran Desert, which evolved essentially free from flames. Early summer wildfires in the Sonoran, fueled mainly by red brome, have become more than ten times more frequent over the past decade. While red brome is providing a new food source to desert tortoises (Gopherus agassizii), these slow movers often cannot escape the blazes it creates. Tom Van Devender, senior research scientist at the Arizona-Sonora Desert Museum, attests that exotic grass species have converted much of Mexico's Sonoran desert-scrub region into a grasslands landscape reminiscent of the African savanna—without the animals. "The loss of wildlife is tremendous," says Van Devender. "You hardly ever see a diamondback rattlesnake [Crotalus atrox]," a species practically synonymous with the Sonoran Desert.

A Developing Problem
More than alien weeds, it is people who have conquered the Sonoran Desert, and who are arriving every day to lay claim to its spoils. The region's settlement has shifted from more than one-half rural to more than three-quarters urban since World War II, according to Gary Nabhan and Andrew Holdsworth's State of the Sonoran Desert Biome. Between 1970 and 1990, the populations of the desert's twin capitals—Hermosillo, Sonora, and Phoenix, Arizona—each more than doubled. The trend shows no signs of slowing: Maricopa County, which includes Phoenix and already claims more residents than 20 U.S. states, has been forecast to grow 250 percent between 1995 and 2025.

Highways, subdivisions, golf courses, irrigation canals, and other outgrowths of Sunbelt civilization have sliced the Sonoran into isolated islands of vestigial habitat. New construction in Pima County, a sprawling district reaching from Tucson south to the border, eats up an acre of desert every two hours. Tucson's rapid growth has already harmed one hardy mammal, the desert bighorn sheep (Ovis canadensis), now vanished from the Pusch Ridge, Rincon, and Catalina ranges that overlook the Old Pueblo. Superbly adapted to the desert, bighorn are suffering from habitat fragmentation, disease spread by domestic goats, and deadly fires that have grown out of control following years of fire suppression by the U.S. Forest Service, according to Paul Krausman, a wildlife biologist at the University of Arizona. "In a nutshell, wilderness species don't do well with economic development," says Krausman.

Defending a Desert, Sonoran Style
Into the tussle over water, livestock, and land flew a tiny, reddish-brown bird—the cactus ferruginous pygmy-owl (Glaucidium brasilianum cactorum). It has served as an improbable catalyst to unite the defenders of development and the defenders of the desert. Standing a mere seven inches tall, the fierce pygmy-owl preys upon insects, lizards, and even others birds. Historically, the owl has preferred mesquite-cottonwood riparian thickets below 4,000 feet. As its habitat vanished and the owl's numbers declined, however, the nocturnal hunter was forced into ever-greater proximity with people.

A survey in 2001 found only 36 cactus ferruginous pygmy-owls in Arizona, primarily in three locations: Tucson's quickly developing Northwest Side, the Altar Valley southwest of the city, and Organ Pipe Cactus National Monument along the Arizona-Mexico border. A population of 280 pygmy-owls was also found in Mexico's Sonora—including 26 within six miles of the international border, which could eventually bolster Arizona's ranks—while an eastern population occupies southern Texas and adjacent portions of Mexico.

On March 10, 1997, nearly five years after it was first petitioned to list the cactus ferruginous pygmy-owl, the U.S. Fish & Wildlife Service declared the owl's Arizona population endangered. The move could have set off a turf war in southern Arizona between conservationists and developers. Instead, the listing prompted the Pima County Board of Supervisors in 1998 to create the Sonoran Desert Conservation Plan (SDCP), a bold measure that is joining traditional foes to fashion a sustainable future for wildlife and city life.

Bighorn sheep no longer live in their native habitat near Tucson. (photos.com)

"Like many western communities, Tucson has rejected meaningful land-use planning [in the past]," says William Shaw, a professor in the University of Arizona's School of Renewable Natural Resources and chair of the SDCP's Science and Technical Advisory Team. "The pygmy-owl listing was the hammer." Initially, the listing disrupted the construction of homes, roads, and even a new high school in Northwest Tucson. However, the county sought to take advantage of the Endangered Species Act's controversial Section 10, a statute that eases certain restrictions of the Act as long as a municipality creates a comprehensive habitat conservation plan for the greater good of the threatened species. Pima County officials saw beyond the need to salvage land for the pygmy-owl, though. They seized the moment to build a future for the surrounding Sonoran Desert and its threatened wildlife for decades to come.

Members of the SDCP's Science and Technical Advisory Team were charged with not only designating habitat to protect the pygmy-owl, but mapping out an interconnected system of lands—which Pima County could purchase over time—that would protect the desert ecosystem as a whole and put a brake on sprawl in the metropolitan region. For five years, scientists worked to compile a list of 55 "priority vulnerable species" that were particularly imperiled by future development in the county. By preserving habitat for these selected species, planners would inevitably aid plants and animals not chosen as well.

Making the list: familiar faces such as the pygmy-owl, the southwestern willow flycatcher, and the lesser long-nosed bat, as well as the Pima pineapple cactus (Echinomastus erectocentrus)—a small, endangered plant that some have called "the next cactus ferruginous pygmy-owl" for its potential to disrupt construction. Absent from the list were endangered species like the jaguar (Panthera onca) and the Mexican spotted owl (Strix occidentalis lucida) that either range primarily outside Pima County or on protected federal lands, and for whom purchasing acreage would have little impact. Male jaguars, which can roam distances up to 500 miles, are spotted north of the Mexican border about once a year. "The jaguar's survival is not going to depend on Pima County's decision," says Shaw.

Some critics have raised issues with the composition of the list, noting that only eight of the 55 species are federally designated as endangered. Proponents of the SDCP hope that the county's actions will prevent these species from reaching that point, saving the local government and private landowners years of headaches and considerable legal expense, while preserving clean air and clean water for the entire community through the protection of healthy habitat.

Scientists divided the 5.9 million acres covered by the SDCP into a grid of color-coded polygons according to how many of the 55 vulnerable species each polygon may contain. Polygons with habitat suitable for five or more species—frequently along riparian areas—became "biological cores" in the most urgent need of protection. The SDCP's planners also devised corridors of land linking already established reserves to extend the range of protected wildlife much farther.

The map became a wish list for conservationists hoping to secure the Sonoran for centuries to come. All that was needed was money. On May 18, by a two-to-one margin, Pima County voters approved a $174.3 million open-space bond, of which $112 million would fund "habitat protection priorities"—land specifically targeted for biodiversity conservation by the SDCP. Development will certainly continue—there's no damming the flood of people into the Southwest from all corners of the continent—but conservationists hope new construction will shift onto less biologically valuable land.

The SDCP, which has won a host of regional and national planning awards, is serving as a model for land-use schemes elsewhere. "The Tucson population is not as unique as we think of ourselves," says William Shaw. "Communities all over want to do what we're doing." He points to efforts to restore the Los Angeles River as an example. "Well, we may be unique compared to Phoenix!" he jokes. The inexorable rise of metropolitan Phoenix long ago eliminated species such as the cactus ferruginous pygmy-owl from its urban core. Without endangered species listings, there has been no regulatory hammer to drive developers and conservationists to work together, and so Phoenix lacks a long-range plan akin to that of its southern neighbor.

Yet as long as there are people drawn to live in the desert, there will be people drawn to protect it. Nearly 150 miles west of Tucson, Sue Rutman tackles the buffelgrass invasion at Organ Pipe National Monument single-handedly. She is the lone botanist in a park more than seven times the size of the District of Columbia. There's been scarcely any rain this year, leaving Rutman to wonder whether the organ-pipe cacti will provide enough fruit for the lesser long-nosed bats. She frets too that the hundreds of border crossers and smugglers streaming into the park each week by foot and crashing through by car—and the Border Patrol agents chasing after them—may be spooking the 20 Sonoran pronghorn (Antilocapra americana sonoriensis) left in the U.S. that occasionally wander into the park in search of cholla. Still, she remains upbeat.

"Organ Pipe Cactus National Monument is 517 square miles. There's no way that all that is going to be damaged, and so there's an awful lot of research going on that is illuminating aspects of the human-occupied world. That's one of the reasons we need open space," says Rutman.

"Another reason is that I need to breathe!"

—Alex Hawes planted his own alien roots in Tucson two years ago, dreaming of warm winters and never-ending skies.

ZooGoer 34(1) 2005. Copyright 2005 Friends of the National Zoo.
All rights reserved.