It's long been know that eliminating a predator such as wolves
from an area can affect populations of many other animals in
the area. Now, a study by Oregon State University suggests that
the elimination of wolves from Yellowstone and other areas of
the Rocky Mountains is responsible for a decline in aspen groves
of as much as 50 to 90 per cent.
"This hypothesis is not yet proven, and we're working
closely with National Park Service biologists in more than 115
permanent research plots to test the theory," says Eric
Larsen of the Department of Geosciences at Oregon State University.
"What is clear is that the wolves disappeared during the
same era that the successful development of mature aspen stands
ground to a halt."
Using historic documents, aerial photographs, and ring dating
techniques, it was determined that Yellowstone Park aspen successfully
recruited tree-sized aspen into their overstory until about 1928,
but have been unable to do so since then. Various theories have
been proposed to explain the lack of aspen overstory recruitment,
including the effects of fire suppression and a trend towards
a warmer and drier climate. A key factor on which virtually all
scientists agree is that elk browsing has had a major effect
on suppressing the growth of young aspen in Yellowstone.
"During winter, elk browse off the aspen suckers, preventing
them from growing to a full tree height," Larsen said. "But
elk have been in this ecosystem for centuries, so the question
becomes why are the aspen declining only now?"
One answer is that due to their protected status, elk populations
may now be unusually high. However, there may be additional factors
other than just the total number of elk present.
"The difference between the effect of elk on aspen now,
compared to periods prior to 1900, may be a reflection of both
their population levels and their behavior," says William
Ripple, professor of forestry and director of the Oregon State
University Environmental Remote Sensing Applications Laboratory.
"Foraging behavior of elk may be influenced by the risk
of predation on them."
Wolves are a natural elk predator. Wolf packs not only lower
the overall elk population, but may also change elk behavior
by their very presence. Elk avoid areas frequented by wolves,
which can include aspen thickets, and protect themselves by staying
in open areas. By influencing both the total number and foraging
behavior of elk, the wolf packs may historically have prevented
extensive elk browsing in some of Yellowstone's aspen stands.
The ecological link between wolves, elk and aspen is being
tested with continued research in Yellowstone Park. The study
is comparing aspen growth and survival rates both inside and
outside the territories of Yellowstone's wolf packs.
The researchers will acquire data on the amount of elk use
of those areas and its effect on aspen growth. This project and
others are part of a larger "Aspen Project" at OSU,
focusing on the condition of aspen throughout the western United
States and Canada.