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Of wolves and willows

  Updated Jan. 18, 2008

Don Despain and Roy Renkin aren’t the first scientists to notice how the climate is changing in Yellowstone National Park. But they are among the first to examine the link between climate change and the growth of certain plants, such as the willow bush.

For the last couple of years, Despain and Renkin have been studying the remarkable growth of willow bushes along places such as Blacktail Deer Creek in the northwest section of the park. During most of the 1900s, willows here averaged around 2 to 3 feet tall, significantly under their usual height. Today, these willows are back to normal height, up to 8 feet tall.

Despain, a retired U.S. Geological Survey scientist in Bozeman, and Renkin, a management biologist with the National Park Service in Yellowstone, aren’t the only ones to ponder the willows’ dramatic recovery. But many scientists connect it to the return of wolves in the mid-1990s rather than to changes in climate. These researchers note the changes in elk behavior since the wolves’ return: Wolf-wary elk spend less time in open riparian areas, choosing safer borders near forests. With fewer elk browsing on streamside willows, the plants have an opportunity to recover. Or so the theory goes.

Whatever is causing the willow to recover is also affecting the whole ecology of the region. Now that the willows are healthy, beaver return and build dams. The resulting wetlands attract insects, which then feed birds such as the warbler. The big question is whether this web of effects is a “trophic cascade” caused by the presence of a top predator, such as the wolf, or whether it's caused by more subtle, pervasive forces, such as global warming. “This is one of the holiest grails of all ecology,” says Doug Smith, a Yellowstone wolf biologist. “Are ecosystems structured from the top down or bottom up?”

At this point, the top-predator theory is receiving the most attention in Yellowstone. More than 30 papers have been published on the relationship between wolves, elk behavior, and plants such as willow and aspen.

But before the wolves returned to Yellowstone, evidence of strong, top-down, predator-caused ecological effects was rare. In general, ecosystem health is considered to be mostly influenced by bottom-up forces such as water, soil, and climate. It’s ironic that Don Despain and Roy Renkin, whose work supports the mainstream bottom-up theory, are actually in the minority among Yellowstone researchers.

One of Despain and Renkin’s first tasks was to see if climate is changing the growing season in Yellowstone. The idea germinated from a scientific paper Despain saw in 2003 that linked the start of willow growth in the spring to the number of days that temperatures stayed above freezing. He wondered if Yellowstone had recently experienced a longer growing season, and if the willows could be affected by it.

Sure enough, Despain and Renkin found a 30 percent increase in the length of Yellowstone’s growing season over recent years. A longer season gives plants more opportunity to grow, but Despain and Renkin needed evidence that the willow’s recovery could be due primarily to that, rather than simply less browsing by elk. Their preliminary research shows that, in fact, willow bushes now grow at rates double or triple those of pre-climate warming days.

They also found that warming temperatures may be affecting the willow’s chemical makeup as well as its growing season.

With the recent extension of the growing season, Yellowstone’s willows can keep their leaves longer, into late autumn. This could allow the plants to create more sugars for future growth, as well as produce more secondary compounds. Secondary compounds are chemicals a plant makes to discourage animals from eating it. Some of these chemicals are toxic; many just taste bad. So a higher dose of secondary compounds, created during a longer growing season, could diminish an elk’s desire to browse. Despain and Renkin believe all this means climate change may be having a stronger effect on willow recovery than the presence of wolves.

Most scientists don’t see the top-down predator theory and the work of Despain and Renkin as mutually exclusive. There’s nothing simple about the relationships between predators, prey, plants, and the changing climate, they say. It will take a lot more research to refine their understanding of these events.

In the meantime, as Despain and Renkin finish up their research and wait for results to trickle in next year, the debate over the willow’s recovery will continue. For now, Renkin is content with his role in unraveling this latest ecological mystery. “I truly believe it’s a combination of everything,” he says. “But this will take a long time to play out.”

Kinna Ohman is an independent journalist who also produces radio stories for The Environment Report.