A Seventy-Year History of Trends in Yellowstone's Northern Elk Herd


E-mail: leberhardt@aol.com


ABSTRACT We analyzed counts of northern Yellowstone elk (Cervus elaphus) in Yellowstone National Park, Wyoming, USA, over 70 years to evaluate the effects of changing management on population trends. Population reduction efforts and hunter harvests during 1932–1968 removed 71,330 elk and decreased estimated abundance from 16,000 to 6,000 elk. Abundance increased to approximately 17,000 elk (λ = 1.19) when removals ceased and harvests were very small during 1969–1975. Moderate to liberal hunter harvests of antlerless elk outside the Park during 1976–2004 removed a relatively consistent proportion (26 ± 0.1 [SD]%) of females that migrated outside the park, mostly from prime-age (3–15 yr) classes with high reproductive value. Substantial winterkill was infrequent (1989, 1997), but it significantly reduced calf survival when it occurred. Wolves (Canis lupus) were reintroduced in 1995–1996 and rapidly increased in abundance (λ = 1.23) and distribution. Estimated wolf kill of elk now exceeds hunter harvest, but has a smaller effect on population dynamics because wolves concentrate on calves and older females (>14 yr) with low reproductive value. During 1995–2004, estimated abundance decreased from 23,000 to 12,000 elk. The recent ratio of wolves to elk is relatively low compared to the estimated equilibrium ratio, suggesting that the wolf population may yet increase in the future. Thus, reduction of harvests of prime-aged female elk to decrease removals of animals with high reproductive value and increase adult female survival appears essential. We analyzed the relative impact of removals by hunters and by wolves using Fisher's (1930) reproductive value and found that the impact of hunters is far more important than that by wolves, a finding of broad significance.