Specially Skilled Cougars Can Wipe Out Small Herds
Bighorn sheep and cougars normally coexist peacefully. But once a cougar learns how to ambush wild sheep, that could begin the demise of a small, isolated herd.
In over two decades of monitoring three populations of wild sheep in Alberta and Montana, biologists found that in most years no sheep were lost to cougars.
Cougars typically prey on deer, paying little attention to bighorn sheep.
Occasionally though, a cougar switched to a diet of Rocky Mountain sheep. One female lived in bighorn range for ten years, feeding on deer, before she suddenly started preying almost entirely on sheep.
Researchers tracked four such instances of cougars specializing in hunting a specific bighorn sheep population. Each stint of heavy sheep predation lasted three to five years, until the cougar died or moved away. It seems that once a cougar becomes skilled at taking down sheep, this becomes its dietary focus.
The cougars took both lambs and adults at unsustainable rates. In one year, 15 out of 36 adult and yearling ewes at Ram Mountain, Alberta were lost to cougars. Adult ewes at Sheep River, Alberta plummeted from 44 to 15 animals during a four-year cougar binge.
The data that the authors of this study collected on sheep and cougar dynamics allowed them to forecast how these episodes of predation could affect the survival of isolated bighorn sheep populations. It turns out that even infrequent periods of cougar predation can decimate bighorn sheep herds when the ungulates live in small, fragmented groups that cannot intermix with other populations.
If bouts of mortality from cougars arose once every two decades in a herd and lasted 3.5 years on average, as observed in this research, then a sheep population would need to number at least 125 individuals to avoid extinction. During the final years of this study, each of the three bighorn sheep populations monitored contained less than one hundred animals.
Marco Festa-Bianchet, Tim Coulson, Jean-Michel Gaillard, John T. Hogg and Fanie Pelletier. 2006. Stochastic predation events and population persistence in bighorn sheep. Proceedings of The Royal Society B. 273(1593): 1537-1543.