Elk, aspen & wolves:
September 19, 2010
In the past decade, separate studies have come to opposite conclusions about the growth of young aspen trees in Yellowstone National Park and how their growth relates to the reintroduction of wolves.
The first study, by Oregon State University researchers, argued that wolf reintroduction had caused an “ecology of fear” among resident elk, keeping them from grazing on young aspen in certain areas for fear of being eaten by wolves. The result was the regeneration of some aspen, the study concluded.
The second study, out of the University of Wyoming, found that new aspen growth has been relatively unaffected by wolf reintroduction and argues that a further drop in elk numbers would be necessary before the trees see a significant resurgence.
The truth is likely somewhere in between the two claims, said Roy Renkin, a park ecologist.
“The studies are two ends of the spectrum,” he said. “Some aspen is growing taller, but it isn’t widespread.”
About 13 percent of 113 aspen stands Renkin is monitoring are growing new shoots, another 15 percent are dying out and many of the rest are simply maintaining.
“Where we used to have 65 to 90 percent utilization of aspen by elk, we’re down to 40 percent utilization,” Renkin said. “Is it because elk are afraid to go in there? I don’t think so.”
Although the two studies try to tie aspen growth, or the lack of it, to wolves’ effect on elk, Renkin says another factor could be at work. Yellowstone has seen an increase of almost two weeks of frost-free weather in the spring and fall. He thinks any additional growth is just as likely to be attributable to the lengthening of the growing season.
“Where we’re seeing those huge changes it’s related to productivity and not to (grazing),” Renkin said.
Warmer temperatures could also mean grasses are available to grazing elk for longer periods.
“They prefer to graze on grasses,” Renkin said. “Elk go to woody vegetation when they don’t have access to grasses.”
Aspen don’t occupy much of the vast landscape of Yellowstone’s 2.2 million acres. But where the trees are found, they are a huge attraction to a variety of species, for everything from birds and insects to big game and fungi.
“They are really diversity hotspots,” Renkin said.
In Yellowstone, elk are wolves’ main food source, especially in the winter. As a result of the wolves’ reintroduction in 1995 and 1996, the park’s northern range herd has plummeted from a high of about 19,000 animals to a current population of 6,000. Wolf numbers have also fallen recently, dropping from a high of more than 170 to just less than 100 now.
“The elk population has declined, so there are fewer mouths to feed,” Renkin said.
Because of their predator-prey relationship, elk have changed their habits. Renkin said elk have abandoned many of the higher drainages they used to utilize, probably to avoid the deeper snow that makes them easier targets for wolves in winter.
“We’re seeing elk select for shallower snow as a way to deal with predation,” he said.
If nothing else, the contradicting studies along with Renkin’s observations point out how difficult it is to attribute changes in the environment to any single factor.
“There were a lot of people who bought into the whole elk-wolf story and the pendulum may be swinging back now,” Renkin said. “Both studies made contributions to us understanding elk-aspen-wolf interaction.
“Biologists will be debating this issue long after I’m gone.”