UM study links climate change, decline in songbird populations
JOHN CREPEAU/Missoulian University of MISSOULA — A decline in snow at high elevations has led to shrinking songbird populations in the mountains of northern Arizona, a new study by two University of Montana scientists found.
Some of the same bird species inhabit Montana.
U.S. Geological Survey senior scientist and UM professor Thomas Martin, along with UM biology professor John Maron, recently published the findings of their six-year study on indirect effects of climate change on ecosystems.
The abundance of deciduous trees and songbird populations have declined over the past 22 years because of decreasing snowpack above 8,000 feet in northern Arizona. Because of less snow, elk remain at higher elevations for longer periods of time, browsing on plants that provide cover and nesting places for birds.
The study is available now online, Martin said, while a hard copy of the journal Nature Climate Change is due out next month.
“The indirect effects of climate on plant communities may be just as important as the effects of climate change-induced mismatches between migrating birds and food abundance, because plants, including trees, provide the habitat birds need to survive,” Martin said.
In 1982, Martin graduated with a doctorate from the University of Illinois and was immediately hired as a professor at Arizona State University. That same year, he went in search of a biodiversity test site in the mountains of Arizona, a place with a good mix of deciduous and coniferous plants, so he could study birds and their habitat use.
In the West, those places are often in riparian zones offering moisture in an otherwise dry landscape.
What he discovered was the Blue Ridge Ranger Station of the Coconino National Forest. Over time, Martin noticed the area’s habitat was slowly degrading.
The question was whether the lack of deciduous trees in the area and the subsequent decline of bird habitat were the direct result of climate change.
Was there less moisture in the ground because of decreasing snowpack? Or were these ecological changes the result of natural succession, or perhaps changing migration patterns for elk?
The disappearance of low-lying woody shrubs suggested to Martin that increased elk browsing was causing the decline in vegetation and the subsequent decline in songbirds.
For example, the MacGillivray’s warbler now is rarely seen in the northern Arizona mountains. The migratory birds still pass through the area, but the last time they nested in the drainages was 1993, Martin said.
As maple trees in the area declined, so did warbler numbers.
Six years ago, to test the hypothesis, Martin and Maron constructed three 25-acre enclosed test sites using 8-foot-tall fences to keep out the elk. The sites were located in different drainages.
If a lack of moisture in the ground caused the ecological changes, the enclosed areas would look the same as the surrounding areas. If the elk were to blame, then the vegetation would grow denser inside the enclosed areas.
As expected, the deciduous trees rebounded in the test sites, and the scientists saw reversals in songbird populations. In some cases, the number of songbirds inside the test sites tripled, Martin said.
The indirect effects of climate change are not well-studied, but Martin’s long history with the Arizona site made it a perfect study area, Maron said.
“Tom had many years of data on this area,” Maron said. “A long history is what sets the stage for this study.”
There are other places in the western United States where climate change is having similar effects on ecosystems. Yellowstone National Park, for example, is experiencing increased ungulate browsing over longer periods of time in parts of the high country, Martin said. That has had an effect on aspen and willow populations.