A Fort Lewis College biology professor says areas hard hit by sudden aspen decline are likely to be home to deer mice, chief carriers of a hantavirus strain that is often fatal to humans.
The more denuded a hillside of aspen, the more deer mice will be found, Erin Lehmer, whose research was featured in a recent ScienceNews article, said in an interview last week. The article is available at www.science news.org by clicking on “past issues” at the left.
Lehmer and her associates reported their findings this month at the annual meeting of the Society for Integrative and Comparative Biology in Seattle.
Sudden aspen decline, the landscape-scale withering of the species, was first noted about four years ago in many parts of Colorado.
While no single cause could be found, conventional wisdom blamed the phenomenon on drought-produced stress from 2000 – or possibly earlier – through 2005. Insects and diseases followed.
Lehmer said she acted on a hunch in exploring the relationship between the loss of aspen and high numbers of deer mice.
“This pattern – forest dieback and prevalence of disease – is global,” Lehmer said. “Climate change can bring change in habitat and disease.”
Lehmer’s field work was done in June and July 2009 and 2010 near Mancos, where she and associates set traps in grids of 100 by 100 meters.
Nine grids were laid out, three each in stands of aspen that had at least 60 percent dead trees, 20 to 59 percent dead trees and less than 20 percent dead trees.
Grids with at least 60 percent dead trees had three times as many deer mice as the other areas, Lehmer said.
While Lehmer’s students concentrated on deer mice and hantavirus, five other students led by FLC biology professor Julie Korb studied how stands of aspen were affected by insects and pathogens that contributed to sudden aspen decline.
They found that the bronze poplar bore, poplar bore, aspen bark beetle and cytospora, a canker caused by a fungus, were causal agents in aspen deterioration.
The hardest hit aspen, Korb said, were low-density old stands on south- and west-facing slopes at low altitudes. In 2008, she said, 17 percent of aspen in Colorado – or more than 543,000 acres – had been affected.
Korb’s team also studied root-system death, the effects of browsing by elk and cattle on new aspen saplings and plant diversity in healthy as well as deteriorating aspen stands.
The loss of aspen is significant because in conifer forests, aspens are the dominant deciduous tree providing biodiversity for small mammals, insects and plants, Korb said.
Mark Krabath, supervising forester at the Dolores Public Lands Office, said Friday that theses prepared by Korb’s and Lehmer’s students based on their research may be synthesized into a guideline for forest management.
“The Forest Service in Gunnison has done some work here but mostly looking at tree health,” Krabath said. “The students have looked at what came after the decline – bird and mammal populations and regeneration of the understory.”
Student contributions are a help because the Forest Service is laboring under time and financial constraints, Krabath said.
Lehmer isn’t the only one to take note of deer mice activity.
The San Juan Basin Health Department in August 2010 cautioned residents that a large population of deer mice presented a potential danger of contracting hantavirus.
Hantavirus pulmonary syndrome is a respiratory infection that proves lethal in 50 percent of cases. It is contracted through contact with mouse droppings or urine or by breathing dust carrying the virus.
The virus was isolated and identified in 1993 in the Four Corners after several otherwise healthy young people died of an unknown cause.
A Montezuma County man who died Oct. 19 of hantavirus was found in his cabin near Mancos.
In the United States, a stand of healthy aspen is the most biodiverse setting in terms of small mammals, including deer mice, voles and shrews, Lehmer said.
But when aspens die, shrews and voles flee to greener pastures, leaving depleted aspen groves to deer mice, who will eat anything, she said.
Large populations of deer mice increase the possibility of passing the deadly hantavirus among themselves and then transmitting it to humans, Lehmer said.
Lehmer’s field crew consisted of seven FLC biology majors, a student from Vassar and an Animas High School student.
The team trapped hundreds of deer mice, she said. Forty percent of those tested in areas with heavy aspen loss carried hantavirus, compared with 11 to 12 percent of the mice found in the other test areas.
Lehmer, winner of the Fort Lewis College New Faculty Award in 2010, wants to return to the area in coming years to further study the phenomenon. She said five years of study would provide a solid base of information.