Clawing Its Way to the Top

The cougar's burgeoning population is proving a challenge for some other creatures

• Martin Fortenzer
• Feb 01, 2000

Sierra Nevada bighorn sheep were in trouble. Their population had been knocked down to 250 in the late 1970s, largely because of diseases contracted from domestic sheep. So California officials began an intensive reintroduction effort in an attempt to establish new herds and bolster the creature's numbers.

At first, the program seemed to be working: Bighorn numbers rose to 310 by the mid-1980s. But at the end of the decade the population began to plummet. Last April, when only 100 Sierra Nevada bighorn were left, federal officials listed the species as endangered. The cause of the sheep's recent downfall was one that few biologists could have imagined two decades ago: cougars.


"Cougars could have literally driven this bighorn population into extinction," says John D. Wehausen, a researcher at the University of California's White Mountain Research Station who has been studying bighorns since the 1970s.

After two centuries of persecution, the cougar recently has come back with a vengeance. The feline's numbers have increased dramatically across the western United States. In many areas, there may be more cougars today than there were before Europeans came to America, experts say. Attacks on humans have multiplied in the past three decades, in part because increasing numbers of people are spending more time in cougar habitat. These encounters have received wide coverage, but the media have spent less time looking at how expanded cougar populations are affecting prey species. In recent years, according to some biologists, the big cats have decimated local populations of some animals in the West.

"Are cougars out of balance in this region?" asks Steven Torres, a biologist with the California Department of Fish and Game. "I would say in many circumstances, like with bighorn sheep or porcupines, yes."

Cougars--also known as mountain lions, pumas, panthers and other names--are among the largest cats in the New World. Tawny colored and lithe, the animals may reach a length of nine feet and weigh more than 150 pounds. Cougars are also one of the most widely distributed mammals in the Americas, inhabiting mountains, forests, deserts and grasslands from Chile to Canada. The solitary cats once roamed the United States from coast to coast, but now are found west of the Mississippi River and in Florida, where a remnant population of 50 panthers hangs on.

Unlike other large carnivores in the New World, cougars are solely meat eaters. Their favorite prey is deer, but they will feed on elk, bighorn sheep, small mammals and birds. A cougar hunts by stalking silently until it is within about 10 yards of its victim, then uses its powerful rear legs to put on a burst of speed to attack. Sprinting as fast as 40 miles an hour, the cat then jumps on its victim's back and sinks its large canines into the animal's neck, breaking the prey animal's spinal cord or suffocating it.

Throughout the 1800s and much of the 1900s, the big cats were considered "outlaw" predators, and they were hunted until their numbers were severely reduced. The felines were essentially eliminated east of the Mississippi, and by the early 1900s most states in the West had begun paying bounties for killing them. But in the 1960s, after sensibilities toward predators had begun to change, mountain lion bounties were eliminated and hunting of the cats was greatly restricted. From a low of perhaps only 6,500 in the late 1960s, the country's cougar population has rebounded to an estimated 20,000 or more today--although accurate counts are unavailable because the cats are reclusive and often inhabit inaccessible places.

Another factor in the recent increase of cougars in the West has been the virtual absence of grizzly bears and wolves, which compete with the cats. Wolf packs and grizzlies harass cougars, take over their kills (causing some of the cats to starve) and sometimes kill cougars outright.

But human tinkering may be the biggest reason for the cougar's dramatic rebound. From 1907 to 1963, when California paid a bounty for the cats, between 200 and 450 cougars were killed in the state every year. Then a moratorium on hunting the felines was put into effect in 1972, followed by a successful 1990 ballot initiative that outlawed killing of the cats, even by state wildlife officials, except in extreme cases. This reversal in the cat's status from persecuted to protected--along with high numbers of potential prey--may have allowed cougars to expand beyond their historic population levels, says Wehausen.

In recent years, bighorn sheep have been severely impacted by cougars in several areas of the West, but nowhere more than in California. In the Sierra Nevada, the increasing numbers of cougars affected the sheep in two ways. Cougars preyed on the sheep directly, and they also caused bighorn to behave differently in winter. "The sheep don't come down to the base of the mountains to get the forage they need, and they suffer for it," Wehausen says.

The decline of another federally endangered bighorn population, the Peninsular bighorn, which ranges between Palm Springs and Mexico, has been attributed to a combination of viral disease and cougar predation. The big cats have also impacted bighorn in the eastern Mojave Desert. Between 1988 and 1993, Wehausen found that cougars were able to depress a bighorn population in the Mojave and hold it at a low level. Eventually, "cougars ate all of my radio- collared bighorns, and that was the end of the study," says Wehausen.

Another bighorn herd in the San Andres Mountains in southern New Mexico was all but eliminated by a combination of disease and cougar predation. The population dropped from 280 in the 1970s to 30 or 40 because of a scabies epidemic. Then a drought occurred in the mid-1990s, killing mule deer and forcing cougars to prey more heavily on bighorn. By 1998, all but one of the San Andres bighorn were gone, mostly because of cougar predation.

Bighorn sheep are not the only animals affected by an expanded cougar population. Another biologist discovered that the cats virtually wiped out an entire population of porcupines in the northwestern corner of Nevada. The biologist, Rick A. Sweitzer, now at the University of North Dakota, was conducting a study of porcupines when suddenly, the animals vanished.

"In three years, the population declined from a high of 82 individual animals to fewer than five, and maybe fewer than three," he said. "Based on my observations at kill sites, it appeared to be almost entirely mountain lion predation."

These and other observations have led some biologists to speculate that cougars might now be having more profound effects on ecosystems than was previously believed. "There may be other species that we just don't know about," Sweitzer says. "The only reason I knew about this was that I had an intensive study going on for a significant period of time when this extinction occurred."

Mule deer herds have also been significantly reduced by a combination of drought and cougar predation in some places in the Southwest. But biologists generally do not find that worrisome because deer have high reproductive rates and easily colonize new ground. Biologists are concerned about other prey species, however. Not only can creatures such as the Sierra Nevada bighorn be driven to near-extinction, but a cougar-prey imbalance could result in a loss of genetic diversity in the prey animals. "If we're having a lot of local extinctions, we're losing all the genetics in those sub-populations. You start adding that up and it could actually be causing an overall loss of genetic diversity," Wehausen says.

Still, biologists say they are not suggesting a return to discredited ways of thinking about cougars as "evil" predators, and they caution against generalizing about the cats. They stress that what may be true of cougar impacts in the Sierra Nevada might not be true elsewhere. Also, biologists believe that cougar numbers will eventually come down when the numbers of their prey diminish.

"Wildlife professionals need to be careful about getting caught up in the 'cougars-are-eating-all-our-wildlife' bandwagon," says Ken Logan, a biologist with the Hornocker Wildlife Institute in Idaho. "There are still a lot of unknowns about the effects of cougar predation on prey populations, so it would be prudent for us to gather the facts first."

In the meantime, California wildlife officials are now contemplating last-ditch efforts to save the Sierra sheep from extinction. One plan under consideration is to begin a captive-breeding program for bighorn on a lion-proof island in the middle of Mono Lake. But some biologists believe that will not be sufficient. "To restore a predator-prey system that is out of balance, it's necessary to kill some cougars," says Wehausen. "It's not going to do the predators any good to not have bighorn sheep in the Sierra anymore. Our business is restoring predator-prey systems where we have disturbed them."

Experts say that removing just a few cougars could make a great difference in the survival of threatened prey. A 1997 study of female cougars and bighorn sheep in southwestern Alberta, for example, found that two of the cats killed no sheep, while a third killed only one. But a fourth cougar killed 11 adult bighorn and 6 lambs. Another study in New Mexico by Logan produced similar results. "During our ten years of research, we found that the greatest impact of cougar predation could be done by just a single individual cougar. If you went in and removed that one cat, you would significantly improve the survival rate," Logan says.

But that may not be as simple as it sounds. Cougar management has become a highly politicized issue in the West. Many people feel strongly that the cats should not be killed for any reason, some out of a mistaken belief that the cougar is endangered. In California, cougar advocates have consistently fought recovery plans for Sierra Nevada bighorn when they included a provision for killing some cats. "People identify with cougars because they're pet-like. It's probably the most politically managed mammal there is," Torres says.

Many people favor relocating cats instead of killing them, but experts say that is rarely feasible. "More often than not, the cougar dies," Torres says. "An animal has to learn to make a living in a dramatically new area, and with cougars, the majority of them die within a year. Another factor is that cougars exhibit tremendous homing abilities for moving over 200 miles back to where they came from." Some have suggested putting them in zoos, but most zoos will not take an adult lion.

In some places, reintroducing competing large predators could restore balance to the ecosystems. But experts say that is only practical in a few places. Wolves cannot be reintroduced in the Sierra Nevada, for example, because they are not native there. And the dangers that grizzly bears pose to people limit the number of places where these predators can be reintroduced.

At the other end of the argument, some ranchers, deer hunters and others are pressing wildlife agencies to reduce cougar numbers for the good of livestock and game numbers. Seeking to reconcile sound biological management of cougars with the diverse social views toward the predators, Logan has developed a cougar management plan for New Mexico that could be adapted for use in other states. It divides New Mexico into zones, some where cougar populations would be reduced to minimize threats to humans, livestock and endangered species. Other zones would allow limited sport hunting of cougars. And other zones would be set aside as cougar refuges, which Logan calls "biological savings accounts," where the cats would be completely protected.

In the end, biologists and wildlife managers are dealing with the realities of ecosystems that have been radically altered by humans. "People tend to believe that we have these natural systems, natural predator-prey relationships," Torres says. "But if you look out at the landscape and compare it to what used to be, the bottom line is, we don't have natural systems anymore.

"How do we manage for long-term viability of wildlife populations under the constraints we have? That's the real challenge."

California writer Martin Forstenzer lives on the eastern slope of the Sierra Nevada.

NWF Conservation Hall of Fame 
Since 1963, 23 esteemed environmentalists and pioneers in natural resource protection have been inducted into the National Wildlife Federation Conservation Hall of Fame. One of them, Ernest Thompson Seton, extolled the benefits of cougars and other large predators during a period when most Americans wanted to exterminate the creatures.