Reduced snowfall in mountains enables elk's winter browsing, causing decline in deciduous trees, bird communities, finds study

Lorena Madrigal

MISSOULA, Montana , January 11, 2012 (press release) – Climate change in the form of reduced snowfall in mountains is causing powerful and cascading shifts in mountainous plant and bird communities through the increased ability of elk to stay at high elevations over winter and consume plants, according to a groundbreaking study by University of Montana scientists.

The study, “Climate impacts on bird and plant communities from altered animal-plant interactions,” was published online Jan. 8 in the journal Nature Climate Change.

The study not only shows that the abundance of deciduous trees and their associated songbirds in mountainous Arizona have declined over the past 22 years as snowpack has declined, but it also experimentally demonstrates that declining snowfall indirectly affects plants and birds by enabling more winter browsing by elk. Increased winter browsing by elk results in trickle-down ecological effects such as lowering the quality of habitat for songbirds.

The authors are U.S. Geological Survey scientist Thomas Martin, who also is a UM biology professor, and UM biology Professor John Maron. They mimicked the effects of more snow on limiting the ability of elk to browse on plants by excluding the animals from large, fenced areas.

They compared bird and plant communities in these exclusion areas with nearby similar areas where elk had access, and found that during the six years of the study, multi-decadal declines in plant and songbird populations were reversed in the areas where elk were prohibited from browsing.

The study demonstrates a classic ecological cascade, Martin said. For example, from an elk’s perspective, less snow means an increased ability to freely browse on woody plants in winter in areas where they would not be inclined to forage in previous times because of high snowpack. Increased overwinter browsing led to a decline in deciduous trees, which reduced the number of birds that chose the habitat and increased predation on nests of those birds that did choose the habitat.

“This study demonstrates that 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.

“This study illustrates that profound impacts of climate change on ecosystems arise over a time span of but two decades through unexplored feedbacks,” USGS Director Marcia McNutt said. “The significance lies in the fact that humans and our economy are at the end of the same chain of cascading consequences.”

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