If the Sonoran Desert's a wasteland, why is the vegetation so thick that it’s nearly impossible to see over, much less walk through unpoked? If life here has been sun-blasted to a minimum, why are the sandy washes scribbled with footprints from javelinas, mule deer, ringtails, and so many rodents that as many as 200 rattlers a square mile can make their living with a sit-and-wait hunting technique?
This seems to be an ecosystem in need of a public relations makeover. Yes, the Sonoran Desert does get parched and hot as hell. But its emblem, that saguaro standing with upraised arms, is not some lonely perch for vultures in the barrens but part of a forest of storage tanks juicy with life.
Within hours of a rain, many will be spreading new rootlets to harvest the drops. The plants' accordion-like structure lets them swell with extra liquid, as a Gila monster's extra-large bladder is designed to do, while it packs away food reserves in its expandable tail. During May and June, the driest months, when winter's rains have been all but forgotten by most, the saguaro and its even bigger southern counterpart, the cardon cactus, crown themselves with extravagant, white, nectar-ﬁlled blossoms. These nourish birds, insects, and especially bats, and are pollinated in return. The flowers then develop into succulent fruits, supplying meals and moisture to a still broader array of creatures, from iguanas to kit foxes, until the summer thundershowers begin. Retiring to one of the many small trees that also characterize this desert, such as yellow paloverde, blue paloverde, catclaw acacia, ironwood, or honey mesquite, to rest and digest, the animals leave droppings full of seeds exactly where a saguaro or cardon needs to grow during its tender years: in the shade of a nurse plant. If life here is just hanging on by a thread, how is it that a saguaro may produce millions of seeds a year and live to be 250 years old?
With few winter frosts and dual rainy seasons half a year apart, the Sonoran region supports such a rich variety of flora and fauna that it seems almost lush compared with other deserts. In some experts' view, it isn't really a desert so much as a drier version of the subtropical thornscrub found farther south in Mexico. However you deﬁne the Sonoran ecosystem, it is a spectacular illustration of how communities of organisms facing extreme conditions ﬁnd ways not merely to endure but to flourish.
A good place to rest, this fold of shade at the arroyo's steep edge, and a good place to ponder. Two days ago, a July cloudburst dropped an inch of rain here in Arizona's Saguaro National Park. Water is still seeping down the arroyo and collecting in pools. Bright green algae already coat the bottoms. Mourning doves, cactus wrens, and hundreds of bees sip from the edges. And somehow, among these mountainsides of stones too hot to touch, tadpoles have materialized within the pools. Native Americans who paused beneath the same rock wall long ago left paintings of humans, creatures, and spirits. If, as it seems, the ﬁgures are keeping watch, they have witnessed many times before the drama of survival that is about to play out.
Many frogs and toads need months to develop from eggs into adult form. Spadefoots, which are similar to true toads but have smooth skin, catlike eyes, teeth, and a sharp nail on the hind feet, live by different rules. Adults exist in a dormant condition buried beneath the desert floor like plant bulbs until they sense vibrations from thunder and raindrops. Then they emerge to gather at runoff pools, send ear-splitting choruses into the night, and breed. With metabolisms set on fast-forward, the offspring can transform into miniature grown-ups in as few as eight days. The race is on to become an air-breather hopping about and fattening on insects before the pond dries, leaving the moist-skinned amphibians no choice except to spade back underground and wait for another heavy rain. That might be months. It might be more than a year.
If the tadpoles in the arroyo pools are red-spotted toads, usually found around more permanent water, they're doomed. If they're spadefoots, they have a chance. Every day, they grow a little larger. Every day, the pools are smaller, vaporizing under the midday sun. . . .
"A year ago this month, she was on death’s door. Skin draped over a skeleton. Out moving all the time, got to ﬁnd food. But there was none," says Jon Davis, a graduate student from Arizona State University. "I wanted to take her in, I was so sure she would die." In the light of our headlamps, a 15-inch Gila monster, ﬁrmly held behind the jaw, dangles like a toy dragon covered with pink and black beadwork. The belly is wide, the wriggling tail plump as Polish sausage. We can see baby rabbit fur stuck to the mouth. Life for this reptile is plainly on the upswing. Davis pulls a couple of cactus spines from the female’s toes and puts her in a bag for closer inspection by daylight.
Rising with the calls of doves and quail before dawn, he reaches in to grab the animal and scan her with a portable ultrasound unit. A wake-up cup of coffee ﬁrst is a good idea; the Gila and its closest kin, the Mexican beaded lizard, are both venomous, and their bites can cause severe pain, faintness, or worse. An image of the animal's bladder appears on the screen. This organ has banked a fortune in water, the currency of the Sonoran Desert ecosystem. "Gilas can drink enough in one day to boost their body weight 20 percent," he tells me. "Just like that, they'll go from looking awful to looking great. This is the kind of year that gets everybody through."
Spanning 100,000 square miles, the Sonoran Desert encompasses southeasternmost California, southwestern Arizona, at least half the Mexican state of Sonora, and nearly all of the Baja California peninsula. Although some sections average barely 3 inches of rain yearly, others see 10 to 12, more than parts of Wyoming. Winter storm systems from the Paciﬁc typically supply about half the annual total, but the winter of 2004-05 brought record deluges, breaking the grip of a several-year drought. By spring, the region looked and smelled like a florists' convention. Even plants no one had seen for decades erupted from long-patient seeds. Then summer came. The thermometer hit 100°F or more for 39 straight days in Tucson. Phoenix recorded afternoons of 115° and more than two dozen heat-related deaths. Heatstroke and dehydration were killing at least that many illegal immigrants from Mexico monthly as they trekked north through remote Arizona borderlands.
Lowest and warmest of North America's four major deserts (the others being the Mojave, Great Basin, and Chihuahuan), the Sonoran is also the only one with two distinct rainy seasons. That tremendous summer heat rising off the countryside draws moist air from the Gulf of California and occasionally the Gulf of Mexico. Thunderclouds take over the blazing sky many afternoons, delivering intense downpours, complete with flash floods. The monsoons, as locals call the hot season storms, were late this year. But they have arrived, slaking thirsts and spurring another round of plant growth. Which means more births among rabbits, rodents, and birds, whose young are all favorite meals of the Gila monster. It may eat enough at one nest to increase its weight as much as 50 percent, then retire to its burrow for a week or more. A hormone recently discovered in the monsters' venomous saliva may help regulate this on-and-off pattern of activity. In 2005, a synthetic version of the chemical won approval as a drug that has proved very effective in controlling type 2 diabetes. It could aid patients in losing weight at the same time.
Davis has spent three years tracking Gilas implanted with radio transmitters and miniature temperature recorders. His study site near Picacho Peak State Park, northwest of Tucson, samples a single square mile of Sonoran Desert. It contains at least two dozen Gilas. In an adjoining square mile, another reptile researcher estimates the western diamondback rattlesnake population at 200. That's not counting the tiger rattlers, Mojave rattlers, and sidewinders. A lot of Sonoran wildlife shifts to nighttime activity in summer to cope with the heat, and while tracking Gilas under the stars, we often come upon diamondbacks. Each rests in a motionless coil, scanning the darkness with infrared sensors in pits on its face, until a kangaroo rat or pocket mouse happens by. Scorpions and tarantulas scuttle underfoot as we ease past cactuses furry with long spines. One careless moment, and you jump back with pieces pinned to your hide. Meet teddy bear cholla, possibly the worst-named, least cuddly plant in the world.
One by one, the pools vanish. Shriveled tadpole bodies cover the pebbles like tar. Where a few puddles no bigger than a dinner plate persist near the source of the seep, so do squirming masses of young amphibians, their bodies still growing, backs breaking the surface at times. Although great, luminous clouds billow overhead most afternoons, the showers they yield are spotty. Finally, it does rain on the mountainside. Not much; just enough to swell the pools a fraction. . . .
Even in early May, this desert can draw three gallons of water out through your skin in a day. But if the region is so arid, why are there angelﬁsh swimming with me and curious sea lions swooping by? Half of the Sonoran ecosystem lies within 50 miles of a sea. Salt water surrounds one of the most fascinating portions, which takes in nearly all of the 34 major islands and more than 850 smaller islands and islets in the Gulf of California. Some have labeled this archipelago Galápagos North because so many different species arose from common ancestors as they adapted to island environments, providing a laboratory for the study of evolution.
Steep-sided Isla San Pedro Mártir, a square mile in size, towers among the remote Midriff Islands at the center of the gulf, shimmering ghostly white from guano, blurred by a haze of wings, and moated by cold, upwelling currents where great whales feed on krill and sardines. Large birds, including the world's densest colony of brown boobies and largest colony of blue-footed boobies, rule this strange castle alongside side-blotched lizards, which abound almost a thousand to an acre. The court jesters are striped gnats known as bobitos, or little boobies.
While gnat larvae develop in the boobies' droppings, adults sip moist mucus from the birds' eyelids. They're happy to drink around your eyes, too. And in your ears, up your nose, or on any patch of sweaty skin—by the hundreds, tickling without letup. The green-and-turquoise lizards, Uta palmeri, found only on San Pedro Mártir, are almost as bold. Show a bit of red on your sock, and a dozen emerge from rocks to run up your leg and bite the cloth. Red happens to be the color of a key Uta food: fruit from cardon cactuses, which grow in candelabra forests fertilized by the bird lime. The lizards also dash right in among the birds to snatch ﬁsh scraps and gnats.
There are 115 kinds of land-dwelling reptiles on the gulf's islands. Forty-eight of them are unique to this region. For example, the Midriff island called San Esteban has produced a rattlesnake, whipsnake, spiny iguana, and Gila monster-size chuckwalla, all found only within its 16 square miles.
Ana Luisa Figueroa of the Mexican resource agency CONANP (Comisión Nacional de Áreas Naturales Protegidas) urged me to add this: By all means, go see the wonders of the gulf's isles. But do it with a guide or, better yet, from a boat circling the shores. The birds nest so close together that a single hiker can put thousands to flight. Before that blizzard settles down, every exposed egg or young chick will have been eaten by gulls or ravens.
Many of the gulf islands have native Peromyscus mice. Many host the endemic Mexican ﬁshing bat. But hardly any hold larger predatory native mammals. After seeing me off on my snorkel along San Pedro Mártir's shore, Araceli Samaniego, a biologist with the Mexican nonproﬁt Grupo de Ecología y Conservación de Islas, part of a North American island conservation network, scrambled up steep cliffs where she noticed a dwindling colony of red-billed tropic-birds nesting. She returned fuming, carrying crab shells and infant bird bones, all gnawed. "Rats," she grumbled. "Everywhere!" House mice—Mus musculus—and cats, too, often jump ship to take up island life.
At each island we visit, Samaniego sets out a series of baited live traps. A petite, soft-spoken woman of 28 with an easy laugh, she talks to the wild mice she catches, calling them corazón—sweetheart—and stroking their little feet before letting them go. If the traps hold rats instead, she carefully weighs and measures them. Then she kills them. She doesn’t like it, but she likes the idea of rats gobbling nestlings and rare endemic life-forms even less. Two-thirds of all known extinctions worldwide since the year 1600 have taken place on islands, Samaniego reminds me, and alien species introduced by people are a major cause.
Gathering data by trapping is the early stage of a planned effort by the island conservation group and CONANP to eradicate rats on San Pedro Mártir. A similar program on nearby Isla Raza in 1995 helped restore the globe's premier breeding colonies of elegant terns and Heermann's gulls, though it came too late for the island's nesting black storm-petrels and Xantus's murrelets.
The arroyo has exactly three pools with huddled tadpoles left. Another shower swept the mountainside last evening; too brief to expand the pools, it at least replenished the trickle that sustains them. Other life-forms are growing and changing on all sides. Having turned entire slopes yellow with blossoms in spring then withered into lifeless-looking bundles of twigs, the brittlebush is going green again. Ocotillos on the banks sport new leaves. Wildflowers tint the shade, conducting business not just with honeybees but also with burly carpenter bees that drill into woody plant stalks to build brood chambers for their larvae, and with tiny, emerald, stingless bees that live a solitary existence. An entomologist in Tucson, Steve Buchmann, thinks the Sonoran ecosystem may support the greatest diversity of bees anywhere—around 500 species. Why not? Plants of one kind or another are in bloom almost every day of the year. As temperatures soar dangerously high, honeybees literally keep a cool head by extruding a water droplet from their mouth to shed heat by evaporation.
The afternoon is hotter by the hour. Yet even as some of the tadpoles trapped in cracks thrash half exposed to the air, new ranks of thunderclouds are forming not far to the south. . . .
In 1993, 2,760 square miles of Mexican mainland near the head of the gulf were declared the Pinacate and Grand Altar Desert Biosphere Reserve. The Desierto de Altar segment holds North America’s most extensive dunes—ﬁnally, a setting that matches the Sonoran Desert stereotype. The sands are shavings off landscapes like the Grand Canyon and snowy peaks of the Rockies, swept downstream by the Colorado River, deposited in its delta on the gulf, and carried here by winds. In the adjoining Pinacate segment to the east, magma surging from the Earth’s mantle blew the place up, repeatedly, leaving 400 cinder cones and ten massive craters to explore.
At twilight, the reserve's biologist, Eugenio Larios, leads the way through labyrinths of jagged-edged lava. You fall, you bleed. Each step echoes below in tubular caves. A mile ahead waits the entrance to one tunnel whose inte-rior is the daytime maternity roost for about 150,000 lesser long-nosed bats. Leaving their young clinging to the walls, the mothers fly out each night to lap cactus nectar, sometimes venturing many miles into Arizona before dawn. They are nearing the end of a yearly migration from south to north through habitats with flowering cactuses—a nectar corridor from Central Mexico to southern Arizona.
At dark, a torrent of winged mammals issues from the once molten ground. The cavern’s breath drawn out in their wake is rank but soft with moisture. Bats rely on such humidity to temper Sonoran summer conditions, just as other mammals seek the refuge of damp burrows by day. After sitting a while, I lay one leg flat, and something strikes my calf like a hot needle. My headlamp beam spotlights a scorpion—a small, slender one. They're supposed to be the worst. We're a world away from a hospital. I ask Eugenio what to expect. He tells me that some people come out OK. And the others? He mimics keeling over, then shrugs.
Within minutes, I feel my heart beating too fast and a cold sweat on my skin. So I lie on the stone, elevate my foot, and wait for whatever comes next. Moonrise illuminates a distant mountain range to the south. A pocket mouse appears beside me, gathering dried grass seeds. Around a boulder comes a pinacate beetle, dark as the lava. When threatened, this insect lowers its head to point its abdomen skyward, ready to discharge foul-smelling, oily defensive chemicals. The indigenous Tohono O'odham say the pinacate beetle puts its head down out of shame because when its ancestor was given a load of stars to arrange in the sky, it tripped and spilled them. Above me is the result: the Milky Way, contending with the moon for brilliance. In an inﬁnite universe stuffed with marvels, my own signiﬁcance hardly seems worth fretting over. And after a while, it's apparent that the venom isn't affecting me anyway. My symptoms were from a flush of fear. I feel more alive than ever.
Couch's spadefoot tadpoles have been growing in the water trapped by a small rock pit near a roadside. Two days ago, they sprouted arms and webbed feet. Newly minted mini-adults are basking on the rim this morning. My afternoon trek up the arroyo is rewarding as well. Three pools persist, and so do clusters of tadpoles within them. . . .
The Sonoran pronghorn never asked for much water. Unlike northern pronghorn, this desert subspecies gets almost all the moisture it needs in dry times from vegetation such as the pulpy ends of chain fruit chollas. Still, the population had been on the decline for decades. Sonora, Mexico, supported about 500. Another 150 lived north of the border, mainly in Arizona’s 330,000-acre Organ Pipe Cactus National Monument and 860,000-acre Cabeza Prieta National Wildlife Refuge immediately to the west. Then, after several thirsty years in a row, the rains of 2001-02 failed almost completely. By late summer of 2002, the U.S. pronghorn population totaled 19, vying with Florida panthers and mountain caribou for the role of the nation's most endangered large mammal.
Historically, these desert antelope could always retreat during severe drought to the Colorado River system or east to the Santa Cruz River and its tributaries. People forget that 19th-century trappers caught beavers and otters in streams shaded by ash, oak, cottonwood, and sycamore where only hot sand runs today. Ever increasing numbers of wells sunk into underlying aquifers have dramatically lowered water tables in the region. Excessive livestock grazing stripped away moisture-retaining riparian, or streamside, vegetation. So much surface water is siphoned off upstream for urban and agricultural use that the mighty Colorado's discharge into the Gulf of California has shrunk to dribbles of chemical-laden soup. And all the while, fences and other barriers, especially highways, discourage the shy pronghorn from migrating in search of what water and forage remain.
"We had a sample of nine pronghorn wearing radio collars in 2001 and 2002," says Curt McCasland, Cabeza Prieta's assistant refuge manager. "Nearly every one died. One female spent the last few days of her life right next to Highway 85"—a busy route through Organ Pipe Monument. "It had rained on the other side, but she wouldn’t cross." If McCasland, refuge biologist Michael Coffeen, and others hadn't kept packing coolers of water into the wilderness for the staggering animals to ﬁnd, perhaps none would have survived. The aid efforts didn't stop there. With support from private wildlife organizations, staff from the refuge and Arizona Game & Fish Department fenced a square mile of Cabeza Prieta range, added water, and brought in pronghorns from both Mexico and Arizona to breed in captivity.
Today, the U.S. population is back up to 60 plus, including 14 adults and their offspring in an enclosure. In the ﬁrst light of day, Allen Zufelt of the game department climbs a nearby hill and locates the animals with a telescope, focusing on a doe with twin fawns, the buck that fathered the young, and other captives until he is satisﬁed that all are healthy. The next step in his daily routine is to walk the four-mile perimeter to be sure no openings have appeared in the woven wire fence or the electric fence in place around that. A double layer of security was called for to keep out coyotes, bobcats, and mountain lions and discourage illegal border-crossers from breaking through.
"We get thousands of undocumented aliens a year coming through the refuge and Organ Pipe," Zufelt tells me.
The people smugglers, labeled coyotes, and the drug smugglers, or narcotraﬁcantes, have built up networks with lookouts, satellite phones, night-vision goggles, and modiﬁed high-speed vehicles that have cut tracks all through previously remote terrain. Some days, my naturalist treks have led me to more improvised shelters and abandoned gear—not to mention Border Patrol squads and Black Hawk helicopters—than wildlife. It’s like being in the midst of a guerrilla conflict.
Watching smoke rise from wildﬁres to the north, Zufelt calls it a sign of what some consider the most serious yet overlooked modern threat to the ecosystem: hardy grasses and weedy herbs from the Sahara and deserts in the Mediterranean and Asia. A few, like buffel grass, were introduced as livestock forage. Forming a ground cover dense enough to fuel roaring blazes in an ecosystem never subject to wildﬁre before, the invaders could eventually replace slower growing, woody plants—in other words, the very cactuses and small trees that deﬁne the Sonoran Desert.
Pronghorn mothers and fawns have joined in a nursery band just across the fence. The does move away in an effortless trot when they catch our scent. We catch theirs as the white rump hairs puff out in warning, releasing an almondy aroma from special glands. Zufelt points to a Harris's hawk nest with young atop a saguaro. "A pair of foxes dug into the pen," he recalls. "Half a dozen pronghorn bunched up and started chasing the foxes. One fox busy looking over its shoulder at a doe ran near this saguaro and was whacked on the side by an adult hawk defending the nest. That fox must have been thinking, Man, I've got to get outta here."
Rain has once again refreshed the arroyo pools. Except for a Gila woodpecker in flight, nothing stirs on the sun-stunned mountainsides above. Yet rain did fall, and more is predicted. Mirages dance over the valley, making it seem that the green plants shimmering below are merely an illusion. And yet rain fell. It always will, sooner or later. And thousands of years of wild invention will come into play to extract the most from every drop.