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.
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