Sierra Nevada Bighorn Sheep
by Robin Meadows
"The wild sheep ranks highest among the animal mountaineers... Their feeding grounds are among the most beautiful of the wild gardens, bright with wild daisies and gentians and mats of purple bryanthus.… Here they feast all summer, the happy wanderers, perhaps relishing the beauty as well as the taste of the lovely flora on which they feed."
John Muir "The Wild Sheep" 1881
Fifteen years ago, the bighorn sheep in the Sierra Nevada seemed to have everything going for them. True, there were only about 300 left in the entire mountain range, which stretches half the length of California. But biologists had recently created several new bighorn populations and all of them were growing nicely. Today the Sierra Nevada bighorn number about 100 and are in such peril that captive breeding may be the only way to save them.
What went wrong? Part of the answer is that everything is obvious in hindsight. A variety of factors--from mountain lions to a series of particularly harsh winters--contributed to the Sierra Nevada bighorn sheep's decline, and it took a while to figure out what was going on, says U.S. Fish & Wildlife Service spokeswoman Jane Hendron.
Another part of the answer is that California state law protects mountain lions, which are not threatened, at the expense of the bighorns, which clearly are. The Sierra Nevada bighorns' recent decline shows what can happen when animal rights are given precedence over the needs of endangered species. "If we had had the authority to control mountain lions four or five years ago, we wouldn't be in this situation," says Steve Torres of the California Department of Fish and Game in Sacramento.
Overall, bighorn sheep (Ovis canadensis) are doing pretty well. Native to mountain ranges in the western U.S. and southwest Canada, bighorns have rebounded from about 9,000 in 1960 to about 70,000 today. But the Sierra Nevada bighorn is an exception. Genetically distinct from the Rocky mountain and desert bighorns, Sierra Nevada bighorns have been declining since the 1849 California goldrush, when miners began hunting them for food and domestic sheep began carrying pneumonia into the mountains. This strain of pneumonia is innocuous to the domestic sheep but fatal to bighorns.
Although hunting bighorns has been banned in the Sierra Nevada since 1878, this has not been enough to protect them. Herd after herd of bighorns died out, presumably due to the pneumonia introduced by domestic sheep, until there were only two small populations left by the 1970s: 30 on Mt. Williamson and 220 on Mt. Baxter. Between 1979 and 1988, biologists moved about 100 bighorns from Mt. Baxter to create four new populations in their historical range in the eastern Sierra Nevada (Mt. Langley, Wheeler Ridge, Lee Vining Canyon, and the Warner Mountains).
Initially, the new bighorn populations flourished, rising from about 250 to more than 300 in the mid-1980s. Biologists thought the Sierra Nevada bighorns were on their way to recovery. But in 1988 disaster struck the Warner Mountains population: They all died after a domestic sheep strayed into their habitat.
And then came a nasty surprise. Mountain lions started killing off the Sierra Nevada bighorns at such a great rate that the population began to drop again. "The collapse was not expected. We thought the bighorns could survive whatever threats as they had for thousands of years," says John Wehausen of the University of California's White Mountain Research Station in Bishop, who has studied bighorns in the Sierra Nevada for 25 years.
While predators and prey typically maintain a balance in intact ecosystems, Wehausen and the other biologists working to save the Sierra Nevada bighorns have learned the hard way that this is not necessarily the case in disturbed ecosystems. "The rules don't apply when you are trying to restore a very small, fragile population," says the state Department of Fish and Game's Torres. "You can't believe it. You say, 'How can this be? How can this system be spiraling down right before our eyes?'"
Once hunted extensively in California, mountain lions have been protected there for decades. The bounty program ended in 1963 and sport hunting was banned in 1972. While there are no good estimates of the mountain lion population, Wehausen and Torres pieced together the available evidence and concluded that the population in the eastern Sierra Nevada increased dramatically during the 1980s. "The moratorium on hunting was followed by exponential lion population growth that peaked when we were restoring the bighorns," says Torres.
The evidence that the mountain lion population surged in the eastern Sierra Nevada during the 1980s was indirect but strong. For instance, while surveying bighorn habitat during that time period, Wehausen observed an increase in both mountain lion tracks and kills of bighorns. Moreover, ranchers reported more mountain lion kills of livestock during the 1980s.
Besides killing bighorns outright, the mountain lions imperiled them by scaring them off the winter ranges. While the bighorns summer as high as 14,000 feet, they typically come down to about 5,000 feet during the late winter and spring. But during the 1980s, the bighorns began spending the winter at elevations above 11,000 feet. "They traded their winter ranges for safety," says Wehausen.
Wintering at higher elevations had a huge cost. Historically, the bighorns descended to the winter ranges in part to graze on the nutritious new growth there. Staying up high during the winter meant that pregnant ewes had to eat poorer quality forage. In the Mt. Baxter population, inadequate nutrition delayed lambing by a month and halved the number of lambs that survived their first winter. In just ten years, the population went from being large enough to supply bighorns for new populations to fewer than 30.
Bad weather compounded the effects of wintering at high elevations. Initially, foregoing the winter ranges wasn't too hard on the bighorns because there were six consecutive years of exceptionally dry winters. Less snow meant that more vegetation was exposed for grazing, so the bighorns could find enough to eat even at higher elevations. But then the weather changed. "In the winter of '95, the snow did them in," says Wehausen. Nearly half of the Wheeler Ridge bighorns died that winter, 12 in an avalanche. And the Lee Vining Canyon population fell by two-thirds to fewer than 30. This past winter was also heavy and the populations declined even further. Today all five populations are tiny.
While people can't control the weather, the other threats to the Sierra Nevada bighorns can be controlled. In the long run, the mountain lion problem may resolve itself. Assuming that the eastern Sierra Nevada mountain lion population is unusually high, Wehausen and Torres predict that it will decrease on its own. Already mountain lions appear to be declining near the Wheeler Ridge and Mt. Langley bighorns, says Wehausen. Encouragingly, these two populations have begun to return to their winter ranges and are beginning to grow again.
Be that as it may, there are too few bighorns to risk waiting for the mountain lion population to stabilize. This means that the state Department of Fish and Game needs to be able to protect those that remain from mountain lions that are known to be picking them off. The problem is that Fish and Game hasn't been allowed to control problem lions since 1990, when California voters passed a mountain lion protection initiative called the Wildlife Protection Act. Protecting mountain lions sounds good except for one thing: They are not in trouble and the bighorns are. "Nobody wants to go in and kill mountain lions," says Torres. "But we need to be able to take extraordinary measures, including killing mountain lions. They are the biggest threat to the bighorns’ overall recovery right now. Not having that available as a tool is absurd. We need to focus on the sheep."
The Mountain Lion Foundation wants Fish and Game to move problem lions rather than shoot them. "Relocation is the most humane [solution] even if some die. If they're shot, they will all die," says Tom Martens of the Mountain Lion Foundation, a Sacramento-based mountain lion advocacy group. But moving mountain lions is not necessarily a better option. "Relocation often results in death and those that survive can return even over hundreds of miles," says Torres.
Ironically, while the state law protecting mountain lions has no provision for killing--or even moving--those that are killing rare species, the law does allow killing lions that are threatening people, pets, or livestock. The state issues about 250 mountain lion permits a year to ranchers. For comparison, Torres estimates that Fish and Game would need to kill only an occasional mountain lion to protect the Sierra Nevada bighorns.
"In California, the issue of lion control is so hot and controversial that biology or scientific research have little to do with what will happen in the real world," says ecologist Mark Jorgensen of Anza-Borrego Desert State Park, who is working to conserve endangered desert bighorns that are also threatened by mountain lions.
While mountain lions are the biggest immediate threat to the Sierra Nevada bighorns, domestic sheep remain a potential problem. Even though the threat of domestic sheep has been recognized for decades, they are still grazed near bighorn habitat. For instance, there are five grazing allotments in Inyo National Forest that abut Sierra Nevada bighorn habitat. While the bighorns are not likely to come down into the allotments, domestic sheep may go up into bighorn habitat because they have a tendency to stray uphill--and all it takes is one stray to infect an entire bighorn population with pneumonia.
For now, though, the domestic sheep allotments are not a dire threat because the bighorns are wintering at higher elevations to escape the mountain lions, says Inyo National Forest spokeswoman JoBeth Brown. However, the Forest Service recognizes that domestic sheep are a potential problem and is considering solutions ranging from establishing buffers between the bighorn habitat and the allotments to grazing cattle there instead of sheep.
Another way to make the winter ranges safer for bighorns is to keep them clear of brush and trees. Bighorn sheep prefer open habitat close to steep rocky slopes because they can't outrun mountain lions and other predators. Instead, they rely on their keen eyesight to spot predators in time to make their escape to the slopes. But when brush grows on the winter ranges, mountain lions can hide and ambush bighorns.
Just about everyone agrees that the problems of domestic sheep and brush on the winter ranges need to be fixed. But actually getting it done has been another matter. This is partly because while the state has jurisdiction over bighorns and, nominally, over mountain lions, the U.S. Forest and National Park services have jurisdiction over the bighorns' habitat. Ensuring the bighorns' survival in the Sierra Nevada will require coordinating all the agencies involved.
Another thing that just about everyone agrees on is that the Sierra Nevada bighorns should be on the federal endangered species list. A petition to list the bighorn sheep was recently filed and the U.S. Fish & Wildlife Service is reviewing it. Because the federal Endangered Species Act would supersede California's mountain lion protection law, the listing would give the Fish & Wildlife Service the authority to control the lions. Listing the Sierra Nevada bighorn as an endangered species could also provide an impetus for grazing reform in its habitat. Just as important, an endangered species listing could result in funding to breed Sierra Nevada bighorns in captivity, which biologists say is their best chance. "We don't want to wait until we have to bring every last animal into captivity," says Torres. "We want to stay ahead of the game."
The California Department of Fish and Game is considering about eight sites for captive breeding. Most of the sites are in the bighorns’ winter range and would require 10-foot fences to keep mountain lions out. To provide as natural a habitat as possible, these enclosures would be as large as a square mile. Such enclosures have been successful in New Mexico and other states. However, the fencing has to be monitored for breaks every day. Another possible site is Paoha Island in Mono Lake, in the Owens Valley due east of the Sierra Nevada. This might seem like an odd choice, but there are plenty of reasons to use the island: it's large (a couple of square miles) and has a spring, some rocky slopes, and winter snow. The bighorns would have to be moved there and, eventually, back to the mountains but the site has the advantage of not needing to be fenced.
Biologists have high hopes for breeding the Sierra Nevada bighorns in captivity. A similar program is working well for the Peninsular bighorns, which live in the desert mountains of southern California and were listed as endangered last year. The Peninsular bighorns are breeding reliably in captivity and those that have been released are acclimating well to the wild. Moreover, if reestablished, the Sierra Nevada bighorn sheeps' chances of survival would be good because their habitat is so remote that there's plenty left.
Ultimately, however, the success of any captive breeding and reintroduction program depends on solving the problems that led to the species' decline in the first place. "I'm optimistic--I think we can still make a difference," says Torres. "But we have to be able to protect the Sierra Nevada bighorns in the wild."
ZooGoer
28(3) 1999.
Copyright 1999 Friends of the National Zoo.
All rights reserved.
