Counting Sheep
Conservation efforts seem to be paying off for bighorns in the West, where the animals face a range of threats from disease to habitat loss to global warming
- Paul Tolmé
- Jun 01, 2008
THE HELICOPTER CIRCLES above a herd of desert bighorn sheep perched on Old Dad Peak in California's Mojave Desert. Over the whir of chopper blades, John Wehausen counts rams, ewes and lambs as part of an annual population survey. It is gut-churning work. "It makes most people queasy," says the population ecologist with the University of California's White Mountain Research Station in Bishop. "When I first started, I was good for two hours before I got sick to my stomach."
Wehausen, who began studying bighorns while a doctoral student at the University of Michigan in 1974, conquered motion sickness long ago, but he is nevertheless troubled by what he sees on Old Dad Peak. He counts just four lambs in a herd that, in years past, produced dozens of young. Poor rainfall across the Mojave in 2007 limited vegetation necessary for lambs and lactating ewes. "There simply wasn't enough nutrition," he says.
Rainfall levels have decreased about 20 percent in the Southwest during the past century, and some climate-change models predict precipitation will drop another 10 to 20 percent by 2100. If that happens, low lamb survival rates could become the norm. Global warming already appears to be taking a toll. Thirty of 80 desert bighorn populations in southeastern California have gone extinct during the past 60 years, according to a 2004 study by University of California–Berkeley researcher Clint Epps, Wehausen and their colleagues. "Many of those areas are simply too dry now," Wehausen says.
That study remains the only research ever published about global warming and North American bighorn sheep, a species that provides clues to climatic changes in the continent's deserts and higher mountains. "Bighorns could be canaries in the coal mine for climate change," says Wehausen.
Hundreds of thousands of bighorn sheep (some speculate more than a million) once roamed western deserts and mountains from Canada to Mexico. Hunting and habitat loss reduced that number to an estimated 15,000 by the early 1900s. Thanks to a massive reintroduction and relocation effort, about 75,000 now occupy 20 states and provinces. There are three subspecies. Rocky Mountain bighorns live in the high peaks of the intermountain west, while desert bighorns inhabit arid canyonlands and mountains in the Southwest. Sierra Nevada bighorns inhabit portions of California's High Sierra and are federally listed as endangered, along with an isolated population of desert bighorns in Southern California.
Named for the rams' curled horns (ewes have pointy horns like mountain goats), bighorns are nimble on treacherous slopes thanks to a stocky build and padded hooves that grip rock. Males and females live apart except during the breeding season, when rams crash horns in thunderous head-butting competitions to win mating rights. Ewes give birth to one lamb in spring, a low rate of reproduction that makes bighorns susceptible to population declines.
Bighorns require steep, treeless terrain to spot and elude predators. This type of open habitat occurs in two ecosystems--mountains above the timberline and deserts. Vegetation is limited in both by harsh weather and short growing seasons. Fire suppression has eliminated open terrain, and predation by resurgent mountain lion populations has been implicated in the disappearance of small herds. In the late 1990s, Wehausen began refining methods to extract DNA from bighorn feces, and subsequent studies indicate that isolated herds are losing genetic diversity as human developments have cut off migration corridors with fences and highways, blocking genetic exchange. Global warming threatens to exacerbate all these stresses.
Hoping to minimize the effects of decreased precipitation, Wehausen advocates the construction of artificial drinking-water sources. So-called "guzzlers" have been used in parts of the Southwest since the 1960s, and more than 100 exist across the region. "In places where water is sparse, these guzzlers are basically what is keeping some bighorns alive," says NWF regional representative John Gale, who is working with the Arizona Wildlife Federation and the Arizona Desert Bighorn Sheep Society to build new guzzlers and refurbish old ones. Two guzzlers were added in Arizona's Kofa Mountains last summer. Cisterns collect rain that fills troughs, providing a crucial summer water source when seasonal watering holes run dry. The water table is sinking across the West because of groundwater pumping, extended drought and water diversion projects designed to quench the thirst of the Southwest's booming population. "These guzzlers replace surface water sources that, because of the footprint of man, no longer exist," says Neil Thagard of the Foundation for North American Wild Sheep, a Wyoming-based group that is a leading source of bighorn conservation funds. Adds Wehausen, "By building more of these, we might be able to prevent some populations from winking out."
The biggest impediment to increasing bighorn populations is domestic sheep grazing on bighorn range, including public lands such as national forests, Gale says. Bighorn rams are attracted to these distant cousins and contract illnesses that they pass along to their herds. Goats raised by homeowners for milk and meat are another pathway for diseases. Conservationists say the solution is to remove sheep flocks from public lands, such as national forests, near bighorn habitat. Many woolgrowers who for generations have run livestock on public land are reluctant, however, to give up their grazing rights. This conflict has led to high-profile clashes in places such as Idaho's Hells Canyon, where hundreds of bighorns died from pneumonia in the mid-1990s--nearly wiping out the region's population--after contracting respiratory illnesses from domestic sheep in the Payette National Forest. Conservation groups are suing to force the U.S. Forest Service to remove the sheep. Bighorn advocates in November won a court battle to remove domestic sheep from a grazing allotment in Idaho's Nez Perce National Forest where bighorns roam.
Another flashpoint is California's Sierra Nevada, where the U.S. Fish and Wildlife Service is designating a 400,000-acre critical habitat area for endangered bighorns. Sheep grazing would be banned. Domestic sheep grazers, who have powerful allies in Congress, call the move a "land grab."
In some areas, conservation groups and bighorn supporters have successfully retired federal grazing permits by helping woolgrowers find land away from bighorn territory and by offering cash payments. While buying grazing rights has worked in some areas (three grazing parcels were retired in recent years in Arizona's Apache-Sitgreaves National Forest), Gale worries that these payments are a slippery slope. In Arizona's Chevelon Canyon, for instance, one woolgrower has refused an offer of $400,000 to buy an allotment appraised for far less--apparently in an effort to gain more money. Gale says the Forest Service should be more collaborative in working with woolgrowers and conservation groups. When wildlife and wildlife habitat are deemed the highest uses for public lands, he says, the Forest Service should be more aggressive about moving or retiring grazing allotments while giving the grazer an allotment of equal or greater value elsewhere. The vast majority of Rocky Mountain bighorn habitat is in national forests.
The Forest Service encourages wool growers to move sheep away from sensitive areas during mating season, says Melanie Woolever, director of the Forest Service's Full Curl initiative, a program to increase wild sheep populations. However, the agency has no interest in buying out allotments or forcing grazers to move. "Grazing is one of our multiple uses," she says. "It's a tough balancing act."
Full Curl instead works to improve habitat. For instance, prescribed burns are being used in the Sierras and Rockies to reduce conifer encroachment--the migration of trees into bighorn terrain due to fire suppression. "You can look at photographs from the 1950s and see that tree cover has increased dramatically in some regions," Woolever says.
Not all the bighorn news is bad. Sierra Nevada bighorns have increased from fewer than 200 when listed in 1999 to more than 400 today. It remains to be seen how climate change, which threatens to diminish glaciers and mountain snowpack, will affect this population. "If those glaciers and snowfields disappear, it's possible that alpine meadows will dry up, and there will be less summer forage," says Tom Stephenson, program leader for California's Sierra Nevada Bighorn Sheep Recovery Program.
Through his annual population surveys, Wehausen hopes to gather a long-term data set that will allow him to see how bighorns react to changing weather patterns over time. "It's these creatures that live in extreme environments that will tell us what's happening first," Wehausen says. "If they start going downhill, that's a major wake-up call."
California journalist Paul Tolmé wrote about wolf myths and misconceptions in the February/March 2007 issue.
NWF in Action: On Bighorns' Behalf
Working closely with the Western Governors' Association, the Association of Fish and Wildlife Agencies, and state and federal wildlife and land agencies, NWF is seeking to protect bighorn sheep habitat across the West. NWF has been especially active in bighorn protection in Arizona, partnering with the Arizona Wildlife Federation, a state affiliate, and the Arizona Desert Bighorn Sheep Society, in an effort to win state support for bighorn conservation, including measures that provide water in arid regions and restore sheep to historic range. For more information on NWF work for wildlife, go to www.nwf.org.
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Bighorn Sheep: Creatures of the American West
Bighorn sheep once ranged throughout the Rocky Mountains from southern Canada into Mexico, numbering perhaps 2 million animals. Today, probably no more than a tenth that number survive. Bighorns are descended from wild Asian snow sheep that crossed the now submerged Bering Land Bridge into Alaska about 750,000 years ago. Another descendant of those pioneer animals is the Dall sheep of Alaska and northwestern Canada, a white sheep that biologists classify as a distinctly different species, although bighorns and Dalls can interbreed.
Scientists have classified bighorns found in Colorado northward as the Rocky Mountain bighorn subspecies. In the arid reaches of the Southwest and Mexico lives another subspecies, the desert bighorn. A third subspecies, the Sierra Nevada bighorn, occurs in the California mountain range that gives the animal its name. An extinct subspecies that lived in the Black Hills of South Dakota was wiped out by habitat loss and uncontrolled hunting that began in the 19th century. Of the surviving subspecies, the Rocky Mountain is the biggest, with females weighing up to 200 pounds and males in excess of 300.
Female bighorns, or ewes, grow short, curved horns, but males, or rams, grow massive armaments that can spiral into a complete curl, spread 33 inches wide and weigh up to 30 pounds. Rams are not territorial but do fight to establish dominance and access to ewes during the autumn mating season. Rams use their horns in the fights, which can last for hours. Dominance is based in part on horn size and on age, with few males under 7-years-old breeding. Females begin breeding at 2 or 3 years old. Breeding usually takes place in autumn and early winter, with births in spring. Bighorns can live up to about 20 years.
In spring, bighorn sheep live in groups of eight to ten animals composed of ewes, lambs, and 1- and 2-year-olds. Rams move to higher elevations in groups of two to five. In winter, ewe bands, joined by breeding males, sometimes combine to form large herds of more than 100. They feed in summer mainly on grasses and sedges, and in winter on woody plants, twigs and shoots. They migrate between winter range, on mountain slopes 2,500 to 5,000 feet high, and summer range at 6,500 to 8,500 feet.
Desert bighorns in summer get much of their water from the vegetation they eat, including cacti. When water sources dry up, the sheep rest during the day in shaded areas and feed at night. Summers are hard on newborn desert bighorns--only about a third survive the dry season.
The U.S. Fish and Wildlife Service has listed both the Sierra Nevada bighorn and a population of desert bighorns in southern California as endangered.--Roger Di Silvestro
