Dr. Dave Christianson is the lead author on a study just published in the journal PLoS ONE. The article, titled "Ecosystem Scale Declines in Elk Recruitment and Population Growth with Wolf Colonization: A Before-After-Control-Impact Approach" is the largest analysis to date on the effect of wolf reintroduction on elk in Yellowstone National Park.
Dr. Christianson uses a long-term data set to show that wolves have led to dramatic (as much as 50%) declines in the elk population in the Greater Yellowstone Ecosytem since their reintroduction in 1995-96. Elk populations outside the Greater Yellowstone that remained wolf-free do not show a similar decline. The study also shows that climate, elk hunting, grizzly bears, and other factors could not explain the dramatic decline in the Yellowstone ecosystem.
Dr. Christianson and his fellow authors also found that the decline in elk population could not be explained using known rates of wolf predation. Wolves do not kill enough elk annually to explain the negative trend in population size. Rather, the study found that elk pregnancy rates and subsequent production of newborn elk calves have declined in response to wolves and that this is the primary catalyst behind the decline in elk population size since wolf reintroduction.