Posted on 11 March 2010.
Big Horns battle the extinction vortex
“Stochasticity,” Heather Johnson explained, to a packed house at the White Mountain Research Station in Bishop a few weeks ago, “essentially means random or unpredictable.”
And it just so happens that stochasticity isn’t just a fun word to say, it plays a big role in the survival of the endangered Sierra Nevada Big Horn Sheep, but not as big a role as mountain lions do.
“Most sheep do die from mountain lions,” explained Johnson, who works for the California Department of Fish and Game’s Sierra Nevada Big Horn Sheep Recovery Program and will be defending her doctoral thesis on the furry subject this spring at the University of Montana. But, she reminded, mountain lion attacks are just part of the “extinction vortex” the region’s wild sheep are fighting against.
Back in 2000, the Environmental Protection Agency placed the Sierra Nevada Big Horn Sheep (Ovis canadensis sierrae) on the endangered list. The distinct species of high altitude sheep that was once said to “span the Sierra” was down to about 100 animals scattered in a few remote spots.
Those rugged spots in the High Sierra include Mount Langley, Mount Baxter, Wheeler Ridge and the Mono Basin. Since the EPA is proud to say it’s only lost nine of the nearly 1,300 animal and plant species ever listed as endangered or threatened, the Recovery Program was challenged with the task of finding out why the Big Horn Sheep were literally disappearing off the face of the earth. And this is where the extinction vortex comes in.
The extinction vortex, the sandy-haired Johnson explained, is the downward spiral species take toward annihilation. A process that basically boils down to two factors: stochastic and deterministic stressors.
On the deterministic side, Sierra Nevada Big Horn Sheep are battling against climate change. For example, heavy summer rainfalls help the species by producing healthier high mountain grasses for the wooly mammals to eat; whereas, heavy winter snowfalls tend to weaken animals and lower reproduction rates.
Genetic variation is another deterministic factor. Since herds of Sierra Nevada Big Horns tend to be small and isolated they’ve lost nearly half of their genetic diversity. Naturally, when you continue to reduce the gene pool, sooner or later, the pool becomes empty.
“There are signs of inbreeding depression, but are we just hosed because of inbreeding? Hopefully not,” Johnson declared.
Amazingly enough, disease spread from domestic sheep, which is a big problem in other western states, isn’t much of a deterministic stressor for the Sierra’s wild sheep. Mountain lions are, however, a huge stressor and part of the problem is that the big cats tend to be a little stochastic themselves.
While other big game prey animals like wolves hunt by picking off the weakest animals they can find, mountain lions are “ambush predators.” Meaning, they attack and eat anything that wanders into range, be it an old ram, a lost lamb or the most prolific ewe.
Big Horn Sheep actually aren’t even the first food of choice for the 21 tagged mountain lions in the region, mule deer are. But in the winter months, the sheep are forced to come down to lower altitudes and almost mingle with the mule deer—and where there are mule deer, there are opportunistic mountain lions. So the sheep are left with one of two choices: risk being eaten or don’t eat. Both, obviously, have significant effects on survival rates.
When you add up all the extinction vortex factors facing the Sierra Nevada Big Horn Sheep things certainly don’t look stacked in their favor. But looks can be deceiving.
With the exception of the Mono Basin herd, “everybody else is improving,” said an enthusiastic Johnson, who first discovered her enthusiasm about Big Horns eight years earlier, during her Junior year at the University of San Diego, when she heard a talk on the native species of sheep held in the very same room.
To help keep the Mono Basin herd from going extinct, an augment population program was started last year. Six radio-collared pregnant females were added to the herd and Johnson was happy to report that five gave birth and all are still alive.
Other management actions being considered include predator removal—something mountain lion advocates strongly fight against—and prescribed fires to help improve plant growth.
Whatever the action plan includes, there is certainly reason to be hopeful that Sierra Nevada Big Horn Sheep will be able to overcome the extinction vortex, especially if those working to help save them have the attitude of Heather Johnson. As she said, “We have to find a way to break out of the extinction vortex cycle.”
“The wild sheep ranks highest among the animal mountaineers of the Sierra. Possessed of keen sight and scent, and strong limbs, he dwells secure amid the loftiest summits.”-John Muir, The Mountains of California
Photo courtesy McKenna