To call the Chihuahuan Desert a rugged landscape is an understatement. Contained within the 800,000 desert acres of Texas’ Big Bend National Park are prickled bushes, rocks sharp enough to slice skin, heat waves shimmering above the gritty soil, and an exquisite lack of water. It is natural to wonder how life happens there.
If you’ve spent some time in a desert, then you understand: it is by persistence that anything survives, if only for a little while.
Black Bears in the Big Bend
Black bears are endemic to the Chihuahuan Desert’s sky islands, the green-scaped mountains of west Texas and northern Mexico that here-and-there jut a few thousand feet out of the flat, parched desert. But when pioneers settled west Texas at the turn of the 20th century, they systematically extirpated black bears. Pioneers erroneously thought black bears were threatening predators.
By the time Big Bend National Park was established in 1944, “Few or no black bears occurred in the park’s Chisos Mountains or any other mountain range in west Texas,” says Raymond Skiles, the park’s wildlife biologist.
But something wickedly unusual happened in the 1980s. Every now and then, hikers in the Chisos Mountains reported bear sign and sightings to park officials. Official confirmation of the species’ presence there occurred in 1988 when a hiker photographed a female black bear and her cubs.
“It was photographic proof!” Mr. Skiles exclaims about that day during his second year of what would become a career managing Big Bend’s fauna.
Against all odds, black bears were independently repopulating one mountain range of west Texas. More sightings occurred each year in the 1990s and the bears caught the attention of the wildlife biology community beyond the park.
When a large mammal disappears, then reappears on its own, scientists get excited. In 1998, Dave Onorato, then a PhD candidate from Oklahoma State University, began a six-year, intensive field study of Big Bend’s bears. Inquiring minds, including his, wanted to know: who were these bears and how did they get there?
Dr. Onorato conducted DNA testing on much of Big Bend’s bear population and proved what wildlife biologists had been postulating: these were Mexican bears.
Eighty miles southeast of the Chisos Mountains in Mexico rises the Sierra del Carmen, the largest sky island in the Chihuahuan Desert. According to Dr. Onorato, the Sierra del Carmen is like “the Chisos on steroids.”
Despite extirpation on the Texas side of the international border and decades of absence from west Texas’ mountains, black bears still thrived in the Sierra del Carmen. Dr. Onorato’s genetic testing confirmed that, sometime during the 1980s, one or more bears, including a female, successfully crossed the desert separating the Sierra del Carmen and the Chisos Mountains.
His research also showed that a few more bears later joined them, enough to create a breeding population with decent genetic diversity. By March of 2000, says Dr. Onorato, “I estimate we had seen 90% of the park’s bear population. We knew of 29 bears, including 10 females, 11 males, and 8 cubs.”
“To call it an exodus is accurate,” recalls Mr. Skiles about what took place during the fall of 2000.
As of that September, Dr. Onorato outfitted 15 bears with radio collars to study their movements. About the same time, he noticed a suspicious lack of radio signals emanating from the bears’ typical ranges. He took to the road and sky with his telemetry unit and documented a mass movement of bears out of the Chisos and the park entirely.
One bear died in the desert south of the Chisos Mountains, within park boundaries. Some of the bears either died or were killed by humans in Mexico. A few walked so far south of the international border that Dr. Onorato could no longer track them. And, a couple later returned to the Chisos.
Of all the dispersing bears, four took young, dependent cubs with them. When the exodus was said and done by the spring of 2001, Big Bend’s bear population was decimated. Dr. Onorato says that a multi-year mast failure likely led to this dispersal. He means that the bears’ primary fall food source, berries and nuts from juniper, pinyon pine, and madrone trees, didn’t occur in its usual volume during both the falls of 1999 and 2000.
Lower-than-normal regional precipitation values also were recorded in 1998 and 2000, leading the biologist to postulate this as the reason why trees didn’t produce their masts.
In the spring of 2003, I accompanied Dr. Onorato on one of his research outings, an overnight backpacking trip to the Sierra Quemada, the cluster of orange-rocked ridges just south of Big Bend’s Chisos Mountains. He and I had each worked in Big Bend for a few years by that time and had become friends. Though I’m not a wildlife biologist, I schlep research gear well.
On this trip, we sought Elektra, a female black bear that he had previously trapped, radio collared, and nicknamed. He believed that she and her two yearling cubs were denning in the Sierra Quemada.
By 2003, Elektra was an expert wanderer. She was one of the bears who dispersed into Mexico during the fall of 2000. She survived her desert foray, returning to the park in the spring of 2001. Our trip felt like an extended scavenger hunt for one expertly hidden clue, her radio collar’s signal.
When we finally found it, far up a slope riddled with cactus and talus, we also found Elektra and those two cubs snoozing away springtime in a cavernous den. After conducting his requisite tests, Dr. Onorato asked each of us if we wanted a look-see inside the den. When it was my turn, I ducked my head into the cool air. I couldn’t see much because of the dark, but I did hear a rhythmic whumping.
In light of everything that Elektra and the Big Bend bears had been through, this bear-cub version of purring was music to my ears. Dr. Onorato’s research team tracked Elektra for another year before her radio collar fell off. In 2004, they visited her winter den high in the Chisos Mountains and found that she’d birthed three cubs.
No one knows what happened to Elektra’s family after that, but Dr. Onorato believes “(S)ome of her offspring must still be around.”
Black Bears Repopulate, Again
Since Dr. Onorato’s research project ended in 2004 and he moved on to study panthers for the Florida Fish and Wildlife Commission, Mr. Skiles has been monitoring bears using visitor-sighting statistics. He believes the population has been on a steady rise since the 2000 exodus.
“The numbers have been creeping back up," the wildlife biologist says. "Last year, I started thinking that maybe the bear population is about the same as in 2000. Now we’re on track for another record-sighting year.”
But at the same time West Texas is experiencing extreme drought. Mr. Skiles says that the Chisos Mountains went 11 months without a lick of precipitation from September 2010 to August 2011. In the last couple of weeks, portions of the range have received some water. Though desert plants are known for their ability to capitalize on whatever water they get, when they get it, Mr. Skiles says it’s probably too late.
“A lot of oak trees didn’t even leaf out this year,” he told me.
Mr. Skiles believes that a mast failure of some sort will happen in the coming months as a result of drought. Whether this will inspire another bear exodus, he says, “I don’t know. Because the bears are no longer radio-collared, we can’t closely track their movements. That said, we’re still seeing the same bears in the same places.”
And yet, I can hear the concern in his voice as we talk. He’s been hanging out with Big Bend’s bears for more than 20 years.
“I try to remain objective,” the wildlife biologist begins when I ask him about the feelings escaping between his words, “but this could be hard to watch.”
One thing is for certain, among all of the unknowns, is the importance of individual bears.
“Though research has shown that our bears are linked to the regional metapopulation,” Mr. Skiles states, referring to all the bears currently inhabiting west Texas’ and northern Mexico’s sky islands, “there just aren’t many bears in Big Bend.”
When I ask Dr. Onorato about the value of one bear in a population this small, he emphasizes, “One bear has huge value. To lose a female of reproductive age would be devastating.”
The Manager’s Dilemma
Mr. Skiles recently faced this possibility. For the past year, a mother bear and her three yearling cubs had a range that included the developed area of the Chisos Mountains. These four bears had been delighting park visitors with frequent sightings.
On a hot day in June, the four bears were found holed up near a structure at the Chisos Mountain Lodge, in the heart of the developed area.
“The sow was in visible distress,” remembers the wildlife biologist. “The cubs attempted to rouse her by whining and approaching, but she just lay on the ground.
“It’s generally National Park Service policy to let nature take its course,” he elaborates about incidents like these, “but the scene was suspicious.”
Visitors had reported seeing these four bears roaming without issue less than 24 hours before. And, the wildlife camera at the nearest water source recorded a visit from the bears during the previous few days.
“That her illness came on suddenly made me think that she’d ingested something in the developed area.”
Because he couldn’t rule out the possibility that the bear had been inadvertently poisoned by something of human origin, and perhaps because she and her three cubs represented a significant proportion of Big Bend’s bear population, Mr. Skiles intervened.
“If she was ill and unable to travel to the closest water a mile away, minor dehydration could have become deadly," he explained. "So, we set out a couple pans of water.” Each of the bears took drinks from the pans and, by nightfall, the foursome had left the developed area.
Two days later, visitors spotted the four bears, seemingly healthy, two miles away from where the incident took place.
A Bear-Filled Future?
“In the case of black bears in the Chihuahuan Desert, the bigger picture is also important,” Mr. Skiles begins when I ask him the what-next question. “Big Bend’s bear population is not a unit unto itself. It is and always will be dependent upon bear populations in the region’s other sky islands."
Dr. Onorato agrees. “The population needs periodic gene flow from the Sierra del Carmen, by way of bears moving across the desert, to remain healthy," he says.
“The recolonization situation remains tenuous,” Mr. Skiles tells me, “and it’ll stay that way until there are bears again in all the sky islands of the Chihuahuan Desert.”
Dr. Onorato says the Chisos Mountains in Big Bend is a key mountain range because of its location about halfway between the Sierra del Carmen and four or five other west Texas and northern Mexico sky islands. "The Chisos can serve as a geographic stepping stone for bears to recolonize other portions of west Texas," he says.
Bears have probably wandered the Chihuahuan Desert for thousands of years in what seems like persistent disregard of the challenges presented in doing so.
“It’s just what these animals do," Mr. Skiles says.
From the exodus of 2000, Dr. Onorato recalls, “(A) female with cubs in tow went all the way to the Sierra del Carmen. Since only the mother was radio collared, we’ll never know if the cubs made it. If they did, it’s a journey they’ll remember, and a journey that might someday bring them back.”