Yellowstone elk populations have dramatically risen and fallen in recent decades, but researchers are arguing over the relative impact of wolf predation on elk populations.
For example, Yellowstone's famed northern range elk increased from about 4,000 head in 1968 to some 20,000 by 1988, due to a combination of factors: elk colonized new winter range in and north of the park, wet summers resulted in better plant production, winters were mild, and the fires of 1988 opened forests allowing more ground cover to grow. With the reintroduction of wolves into the ecosystem in 1995, elk populations held their own from 1995 to 2000 (17,000), before they dramatically dropped by 50 percent to 8,335 in winter 2004.
At the same time, researchers note both high human harvest levels and seven years of drought at the same time wolf numbers were growing throughout the Greater Yellowstone Ecosystem.
In a broader context, more than 30,000 elk from 7-8 different herds summer in Yellowstone and approximately 15,000 to 22,000 winter in the park, according to National Park Service biologists.
Are Wolves to Blame?
Doug Smith and Daniel Stahler (NPS wolf biologists) and John Vucetich (Michigan Tech biologist) joined forces to investigate the influence of harvest, climate and wolf predation on Yellowstone elk. The three built computer models based on elk-related data prior to wolf restoration (1961-1995). The goal was to use the best of these models to predict how elk populations might have fared, had the wolves never been reintroduced.
Their models suggested that human harvest (hunting) might be "super-additive," that for every one percent increase in the harvest rate, elk population growth rate would decline by more than one percent.
"According to the best-performing model, which accounts for harvest rate and climate, the elk population would have been expected to decline by 7.9 percent per year, on average, between 1995 and 2004," they wrote in a study published by the peer-reviewed journal of ecology, Oikos. "Within the limits of uncertainty, which are not trivial, climate and harvest rate are justified explanations for most of the observed elk decline. To the extent that this is true, we suggest that between 1995 and 2004 wolf predation was primarily compensatory (of no significance)."
The researchers acknowledge that some wildlife managers and segments of the general public believe the decline of the northern range elk herd is attributable to wolf predation. "Our analysis indicates that there is greater justification for believing that the harvest rate and severe climate, together, account for at least much of the decline," they wrote.
Competing View About Yellowstone Wolves
Yet ungulate biologist P.J. White (NPS) and ecologist Dr. Bob Garrott (Montana State), in a paper for Biological Conservation (2005) contend that the rapid growth of the wolf population has in fact contributed to rapid demographic decline for elk.
White and Garrott have also speculated that as wolf recovery continues, there will be greater numbers of bison and antelope, because of wolf pressure on elk and coyote populations, respectively.
Yet wolves are beginning to take bison in the park's interior. The Pelican Valley wolf pack hunts bison in late winter when they are more vulnerable and migratory elk are not available. White and Garrott suggest that a change of prey preference from elk to bison, by wolves, could lead to stable populations for elk and bison.
"Counts of northern Yellowstone elk have decreased more than predicted," wrote White and Garrott, "and counts will likely continue to decrease in the near future given the strong preference of wolves for elk and the high kill rates."
In a telephone interview, White said he believes that wolves have overshot their favored prey base of elk.
"I don't think (the elk population) decline is entirely due to wolves," said White. A moderate to liberal harvest policy has played a role, he said, as well as predation by a growing population of grizzly bears.
But simple answers are both elusive and often wrong, say scientists, citing the sheer complexity of the northern range ecosystem.
White said ruefully that 10 years after the reintroduction of wolves, "the range of predictions is as large as it was before." Past predictions have been spot-on, but others have been wildly off the mark. Today, there's disagreement on whether wolf predation is negligible or significant, where the elk and wolf populations will eventually settle, and at what level the elk hunting harvest can be sustainable.
What everyone would agree to, is that ongoing research is needed to better understand the complexity of the Greater Yellowstone Ecosystem.
Yellowstone Elk Facts
Elk are the second largest antlered animals in the world, only moose are larger. Bull elk are 4.5 to 5 feet tall at the shoulder and weigh 550 to 800 pounds. Cow elk weigh from 450 to 600 pounds. The National Elk Refuge near Jackson, Wyoming, has an elk herd with consists approximately 20 percent bulls, 65 percent cows, and 15 percent calves.
While most members of the deer family are primarily browsers (feeding on twigs and leaves of shrubs and trees), elk are both browsers and grazers, feeding extensively on grasses and forbs, as well as shrubs.
Grizzly bears, black bears, mountain lions, wolves, and coyotes prey on elk. By weeding out the weak, predators help maintain healthy, vigorous elk herds.
Source: U.S. Fish and Wildlife Service