Black Bear Biology - Osos del Desierto

The Black Bears of the Chihuahuan Desert

Two mountain ranges frame the interior of Mexico, the Sierra Madre Occidental on the west and the Sierra Madre Oriental on the east. Between them lies the Chihuahuan Desert, an arid ecoregion composed of high mountains and mesas separated by wide basins. It is the largest desert in North America, encompassing approximately 250,000 square miles from central Mexico into the southwestern United States. In the U.S., portions of this desert ecosystem fall within the borders of Arizona, New Mexico, and Texas, particularly the region of Texas occurring west of the Pecos River known as the Trans-Pecos. Among arid ecosystems globally, the Chihuahuan Desert is one of the most biodiverse. Even still, a desert seems like a strange place to go looking for bears, but that is exactly what a team of researchers in Texas is doing. 

The Chihuahuan Desert lies at the southernmost extent of the American black bear’s historic range. Like much of North America, bear populations in this region declined dramatically through the early 20th century due to over-harvest and predator control efforts. By the 1940s, black bears were essentially extirpated (locally extinct) from the Trans-Pecos. Four decades later, they began to naturally repopulate the far reaches of west Texas through dispersal from remnant populations in northern Mexico. 

In 1987, black bears across Texas were given formal protection by the state when the species was listed as endangered. It was reclassified to a threatened status in 1996, which remains in effect today. Researchers and wildlife managers have continued to monitor the natural reestablishment of the Trans-Pecos bear population and to study the adaptations that allow these desert bears to persist, teetering at the edge of their ecological limits. 

“You would never look out there and think there were bears there,” commented Dana Karelus, PhD.  

Karelus is the State Mammal Specialist for the Texas Parks and Wildlife Department (TPWD). In that role, she oversees conservation efforts for a wide range of mammals, including black bears. I spoke with her recently about new research efforts that are underway for the Trans-Pecos bear population and the agency’s goals for bear management.  

Past research suggests that black bears in the Chihuahuan Desert ecoregion rely largely on ‘sky islands’ as a means to withstand the desert’s inhospitable conditions. In other words, these bears concentrate themselves in pockets of habitat where food, water, and denning resources are more abundant. Those pockets are often high elevation chaparral and oak woodland communities surrounded by vast expanses of seemingly unsuitable, low-lying terrain. Ecologically speaking, they are the landlocked equivalent of islands. The size and distribution of these insular habitats determines the number of bears that an area can support and influences their rate of dispersal. Of course, for every rule there is an exception, and more recent studies have identified viable bear populations becoming established at lower elevations than previously predicted. Whether they are taking advantage of natural or manmade features, the exceptional adaptability of black bears continues to challenge our understanding of suitable habitats for them. 

While this ecologic flexibility allows bears to make the most of marginal habitats, resources in this ecoregion are still naturally limited. As a result, population densities tend to be lower than in other ecosystems. It’s also been suggested that desert bears may have a smaller body size than bears in other populations, though it is not clear whether this is consistently associated with resource limitations or possibly genetics.  

The dependence of desert bears on isolated pockets of habitat also leaves them vulnerable to the fluctuations of Mother Nature. To illustrate this point, Karelus shared a study conducted in Big Bend National Park in the early 2000s. After multiple years of apparent stability, the majority of bears in the study population vacated the park in a single season and moved south into Mexico, undertaking some of the longest migration movements documented in the species. Observations at the time suggested that a combination of drought and mast failure was the likely cause for this mass exodus. As Karelus points out, adaptive behaviors like this may help bears survive in the desert but they also turn traditional ecological measures like home range and carrying capacity on their ears. 

To manage this dynamic population, TPWD needs a deeper understanding of how black bears use the Chihuahuan Desert ecoregion. They have teamed up with scientists from the Caesar Kleberg Wildlife Research Institute at Texas A&M University-Kingsville and the Borderlands Research Institute at Sul Ross State University to update their current population estimates and examine how bears use different habitat types within the region. While older studies relied on radiofrequency collars that were detected periodically using aerial surveys, newer satellite collars allow researchers to track bear movements and habitat use with far greater frequency and accuracy. The resulting data will be used to identify important habitat characteristics and to inform population models and biologically relevant management strategies. 

Karelus says the highest priority for bear management in Texas is to recover the population to a level that will allow black bears to be removed from the state’s threatened species list. But she also points to an important secondary goal—preventing human-bear conflict. As the bear population continues to grow and expand its range, communities on the leading edge of that expansion can find themselves interacting with bears for the first time in a generation. TPWD works with these communities to educate residents and help them implement strategies to prevent conflicts that could slow recovery. Karelus emphasizes that, while the population is growing, there’s still a long way to go and every individual is important at this stage. 

Another change that has come about since the last time black bears roamed the west Texas landscape is the widespread distribution of deer feeders and other artificial food sources. Karelus acknowledges that the supplemental nutrition these can provide may be helping to facilitate the growth of bear populations in this resource-limited landscape, but they can also be a source for human-bear conflict, as bears can be destructive in their search for food and cannot be legally harvested in Texas. Karelus emphasizes the importance of proactive steps to prevent damage, such as using electrified fencing around deer feeders that exclude non-target species like bears and feral hogs. 

As we wrapped up our conversation, I asked Karelus what she thought of these rugged little bears fighting their way back to repopulate an unforgiving landscape.  

“It speaks to how amazing bears are,” she told me. “They find a way.”  

And I couldn’t agree more. 


Sources Cited: 

Hellgren EC, Onorato DP, Skiles JR (2005) Dynamics of a black bear population within a desert metapopulation. Biological Conservation 122:131-140. 

Yancey III FD, Kasper S (2023) Reproductively viable population of American black bears (Ursus americanus) in lowland Chihuahuan Desert habitat of Trans-Pecos Texas. Western North American Naturalist 83(3):345-354.