SCENIC DRIVE

Smoke-hued clouds break over the highlands of northern Sonora, Mexico. Two Tucson biologists and two Mexican cowboys hurry their sure-footed ponies up a rocky trail, hoping to beat a storm. Clumps of red asters cling to the side canyons; yellow daisies and tangerine-tinged poppies grow in the flatlands. The air smells of rain and horse sweat.

The horses navigate through grass that grows so high it slaps their undersides. An occasional misfit from the Sonoran Desert - a lone saguaro, an isolated organ-pipe cactus - peers down from the slopes at yawning canyons of giant oaks and sycamore trees. Mexican blue jays, startled by the unexpected clink of horseshoes on stone, scold the sky.Any jaguars, ocelots, mountain lions or bobcats hiding nearby must also be surprised by the offbeat posse scrambling up the mountainside.

The horses climb for perhaps a half-hour, then snort with heartfelt gratitude when their riders rein them in and dismount. The four equestrians stretch their legs, drink from water bottles, shade their eyes, and gaze directly northward into Arizona, which lies just 5 miles away. From there, at 5,200 feet, it's easy to see that Arizona and Sonora share this one delicate sky island. In Arizona, it's called the Tumacacori Highlands. In Mexico, it's Sierra Esmeralda, the Emerald Mountains.

Today, border turbulence and population pressures threaten this once-isolated landscape and all the creatures and plants that live in it. The biologists and the cowboys have teamed together to fight almost insurmountable odds to save something greater than themselves that could be lost forever.

SERGIO AVILA is a 36-year-old Mexican biologist who specializes in the study of wild cats in the borderlands. He's an outwardly cheerful, inwardly intense fellow with lively eyes and a thick, shoulder-length ponytail. Avila saw his parents, both medical doctors, pour their energies into raising him and his brother. Now Avila has decided not to have children because raising a family, he fears, will distract him from his life's work of conservation and grassroots organizing. He could have chosen an easier life, but he yearned for a connection to the land and rural people. Avila now heads the Northern Mexico Conservation Program sponsored by the Sky Island Alliance, a privately funded Tucson-based grassroots group formed in 1991 to protect and restore the sky islands of the American Southwest and northern Mexico. The region's biodiversity is the stuff of legend - home to a stunning variety of desert, mountain and subtropical species. Like jaguars.

Although jaguars reportedly once roamed as far north as the Grand Canyon, recent jaguar sightings have occurred in the Tumacacori Highlands. Avila has discovered that ocelots also wander in northern Sonora.

The Sonora side of the sky island is largely held by private ranchers, so it's more isolated than most of the public lands on the Arizona side. Mexican ranchers, eager to learn new ways of using their lands in the face of declining cattle-ranching revenues, have welcomed Avila and his fellow scientists onto their spreads.

The ranchers are particularly keen on a project called Cuatro Gatos (Four Cats). The unique scientific study, which falls under the umbrella of the Northern Mexico Conservation Program and is funded by groups as diverse as the Phoenix Zoo and Turner Foundation Inc., seeks to identify the international travel corridors of wild cats by capturing their photographs with motion-detector sensor cameras strategically placed on the Sonoran side of the borderlands.

It's not an easy task.

Avila begins his hunt for cats with satellite maps, identifies canyons and mountains where cats might live, interviews local ranchers and cowboys, then hikes with them looking for clues - a bit of cat hair caught in a fence, probable bedding places, scat, tracks, dead prey. Using this detective work, he has mounted several cameras on trees near water holes, streams and other secluded places where a shy cat might wander.

His cameras have captured the rare ocelot, the bobcat and the mountain lion. His cameras have yet to capture photos of jaguars, although he's seen their tracks. The photographs help Avila map out possible travel corridors that must be preserved on both sides of the border if the wild cats are to survive in this remarkable biome.

Back in Arizona, Avila shares his data with conservationists studying animal corridors on the American side of the border.

By learning more about wild cats and their travel routes, Avila and his colleagues can gauge the health of the mountains on both sides of the border. Healthy predators require good water, their own territories, abundant prey that has ample forage, and clear pathways from one sky island to another. If the biome thrives, cats thrive. If the biome suffers, cats suffer.

LIKE SO MANY CATTLEMEN who struggle to keep their land on both sides of the border, Roberto Corella has a side job. The 55-year-old Mexican rancher owns a company that sells and installs solar panels. Years ago, his family's Rancho La Esmeralda supported a thriving cattle business. In the 1960s, profits from the sale of 30 Mexican calves could buy a new pickup truck. Now, 100 or more calves must be sold to fund a new pickup. Stagnant cattle prices, drought and increased expenses all threaten this traditional way of life in Sonora.

Corella's family has owned Rancho La Esmeralda for almost a century. It's said that in the mid-1700s, a Yaqui Indian found a 2,500pound slab of silver in nearby Planchas de Plata Canyon. Hoping to draw bird-watchers and ecotourists, Corella built an elegant solarpowered lodge flanked by a pink-cliffed canyon and a clear creek. Groups rent the lodge for secluded retreats.

Just down the road, Corella's friend John Ochoa has built a comfortable tin-roofed house in an oak-lined valley where exotic birds are almost as common as sparrows. Arizona may have received its name from pioneering Basques who settled the area in the early 18th century. In the Basque language, the word "arizona" reportedly means "place of the good oak trees."

Ochoa's home is decorated with antique saddles he has found on his land - he often wonders what happened to the riders - and maps that show that the ranch is situated at the tip of the Sierra Madre. His many guests have left lists detailing the diversity of birds, mammals and insects they've sighted here, and he sometimes pulls those lists out of a drawer and reads them with pleasure.

Ochoa is an American citizen with Mexican residency, a Tucson contractor who grew up in Sonora and Arizona. He says he descends from Basque settlers, and one ancestor was an early mayor of Tucson. He grew up understanding that Sonora and Arizona share the rich biome, where man-made borders are not understood by wild cats and other animals that must navigate from one sky island to another in order to thrive.

THE HORSES ARE REFRESHED and the riding party remounts. The peaks of Arizona disappear as the equestrians descend into a rocky canyon in Roberto Corella's Rancho La Esmeralda.

Sergio Avila and his fellow biologist, Jessica Lamberton, have many questions for the two cowboys, Remolino Noriega, who works for Corella, and Sergio Garcia, who works for Ochoa. Both men understand this wild, threatened land in ways that will help Avila and Lamberton search for cats.

All four riders know they must bridge an unspoken gap; cowboys have traditionally been at odds with environmentalists.

Cowboys view mountain lions as threats to livestock and often kill them. It's Avila's job, as a community organizer and biologist, to help the Sonoran cowboys see wild cats differently. Avila notes, for instance, that killing one lion does not preclude others from moving into the territory, and that big predators may benefit ranchers by keeping other animal populations in balance.

A day earlier, Avila and Lamberton had hiked into remote areas of Ochoa's ranch with Garcia's son, Alex, to inspect three cameras strapped to sturdy tree trunks. They'd checked the batteries, removed and coded the used film, replaced it with new film, scribbled abundant field notes and interviewed Alex about the animals and plants in the area. Before leaving, Avila sprinkled the dirt in front of the camera with bottled bobcat urine, hoping to stop a cat long enough for the camera to take its picture.

Now the two biologists survey Corella's ranch on horseback, descend ing into a creekbed lined with sycamore trees. In one tree, a tropical bromeliad-like plant, a tropical refugee, hangs from the branches.

The horses drink the clear creek water, scattering tiny red-spotted toads. The two Mexican cowboys sit in the shade, surveying the biologists as they perform the routine camera-check and bobcat-urine ablutions.

If the cowboys have reservations about the biologists pouring bobcat urine onto the sand, they won't express them. They understand that cowboys and conservationists must set aside their differences and join together to fight for something greater than themselves - the wild cats that roam this fragile, wild place that knows no borders.