Muskoxen (Ovibos moschatus) are thought to be highly effective at defending themselves from predators. However, a decline in muskox abundance in northeastern Alaska, USA, that coincided with several instances of grizzly bear (Ursus arctos) predation observed during 2000–2006 raised concerns about the effects of predation on this population. In response, from 2007 to 2011 we estimated rates of reproduction and survival and determined rates and causes of muskox mortality on the arctic coastal plain of northeastern Alaska. Annual counts of muskox abundance (x̄ = 191) and estimates of population growth (x̄ = 0.94) indicated a stable or slowly declining population. Annual natality ranged from 0.45 to 0.86 (x̄ = 0.68) births/adult female, whereas annual survival ranged from 0.40 to 0.63 (x̄ = 0.49) for calves and from 0.73 to 0.91 (x̄ = 0.83) for adult females. Predation by grizzly bears was the most common cause of death among cases where a cause could be identified, accounting for 58% and 62% of deaths of calves and adults, respectively. Most bear predation occurred during late winter and spring when little other food was available to bears. The importance of predation compared with other mortality factors, and the change from a growing to a declining muskox population, suggest a change in either predator abundance or behavior. There is no evidence that bear abundance changed dramatically during this period, but abundance of moose (Alces alces) and caribou (Rangifer tarandus) declined substantially in the area where the muskox decline was most pronounced. This suggests bears may have increased predation on muskoxen in response to reduced availability of other ungulates. Maintaining diversity of native ungulates may help bears cope with the natural fluctuations in prey abundance often seen in arctic ecosystems.
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Vol. 28 • No. 1