Hunter vs. Wolf
Boyce also discovered that wolves and human hunters influenced elk populations in very different ways.
“Another shortcoming was failure to recognize the strong age selectivity by wolves and hunters,” Boyce wrote. “Hunters preferentially kill bulls, but when they kill cow elk, they primarily kill prime-age females of high reproductive value; cows learn to avoid hunters to the extent that individuals older than about 9-10 are essentially ‘bullet proof.’”
Yellowstone wolf biologist Doug Smith has often referred to the park’s Northern Range herd as some of the toughest elk around.
“In contrast, wolves killed primarily young and old (elk), and as a consequence the per-capita influence of hunters on elk populations was much greater than for those killed by wolves.
“Even after making these adjustments, however, during the second decade after wolf reintroduction, the number of elk averaged lower than predicted by the simulation model.
I believe that this is because bear (Ursus arctos and U. americanus) and cougar (P. concolor) predation was higher than we had anticipated. Grizzly bears have been shown to be highly effective predators on elk calves reducing recruitment by about seven calves/100 cows.”
Other studies have shown that after cutthroat trout numbers in Yellowstone plummeted and with the reduction in whitebark pine seeds, grizzly bears compensated by eating more meat, often elk calves in the spring.
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“One thing he left out, that I and several other researchers believe is important, is the indirect effect of wolves on elk pregnancy rates (the presence of a predator on the landscape reduces prey pregnancy rates — not direct killing),” wrote John Winnie Jr., an associate professor of Ecology at Montana State University in Bozeman, in an email.
Academic debate
Since the reintroduction of wolves, elk populations — which some biologists will argue were unnaturally high in the largest Northern Range herd that migrates into Montana in the winter — have fallen from about 19,000 in 1995 to about 5,000 the last three years. Likewise, wolf numbers have dropped from a high of about 170 in 2003 to roughly 100 now, mirroring a decline in the availability of their main prey — elk.
A debate between scientists continues as to whether or how much the reduction in elk has helped revive tree species like willows and aspen, whose tender shoots are often winter elk food.
Winnie said researchers Scott Creel, of MSU, and Dave Christianson, of the University of Arizona, “pointed out that a decline in elk numbers was sufficient to explain an increase in willow growth noted” by scientist Hawthorne Beyer, of the University of Queensland, Australia, in a study published in 2007. “Beyer was claiming that changes in elk distribution on the landscape due to the risk of wolf predation was leading to willow being released. Boyce argues that (Creel and Christianson) are wrong because elk numbers didn’t decline during the course of Beyer’s study, citing Beyer (2007) to support this argument. Beyer’s figure 2 clearly shows a nearly 50 percent decline in the elk population during the course of his study, making (Creel and Christianson’s) criticism reasonable.”
Winnie also noted that the 1988 fires and regrowth of aspens in Yellowstone, along with the expansion of grizzly bear ranges to outside the park have been other notable changes over the past 40 years.
“So overall, I think Boyce has the bigger picture right, but I think he’s wrong about some of the details,” Winnie said.