Evaluating Aspen Responses to Changes in Elk Abundance, Distribution and Behavior Following Wolf Reestablishment in West-Central Yellowstone National Park

Scientific Disciplines
Biological Sciences - Terrestrial
Montana State University
Yellowstone national park
Greater yellowstone ecosystem
gray wolf
wolf reintroduction
trophic cascade
trophic interactions
browsing pressure
aspen morphology
Volume 18, No. 1-4

Evaluating aspen responses to changes in elk 
Abundance, distribution and behavior following 
Wolf reestablishment in west-central yellowstone 
National park
Timothy L. Shafer*, Department of Ecology, Montana State University, Bozeman, Montana 59717. 
David W. Roberts, Department of Ecology, Montana State University, Bozeman, Montana 59717
Robert A. Garrott, Department of Ecology, Montana State University, Bozeman, Montana 59717.
Kathryn M. Irvine, Northern Rocky Mountain Science Center, U.S. Geological Survey, Bozeman, 
Montana 59715
The reintroduction of wolves to Yellowstone National Park created a unique “natural 
experiment” to study trophic interactions in a large-scale terrestrial system among wolves, 
elk, and aspen.  This study utilized data from a long-term elk demography study that was 

established prior to wolf reintroduction.  Significant changes in the abundance and distribution 
of the Madison headwaters elk herd were observed following wolf reestablishment.  The 
spatial arrangement of these changes made it possible to directly test for the occurrence of a 
density-mediated trophic cascade.  The objectives of this study were to answer the following 
questions: 1) was there a marked decrease in browsing pressure on aspen where elk densities 
declined, and 2) was there a corresponding plant-growth response indicating that aspen were 
released from browsing pressure?  Historical browsing conditions and aspen height were 
observed for 31 aspen stands to assess the occurrence of a density-mediated trophic cascade 
following wolf reintroduction.  Browse conditions and aspen morphology in stands where elk 
densities declined dramatically following wolf reintroduction were compared to stands that 
experienced persistent heavy browsing throughout this period.  A major decline in browsing 
pressure along with a modest increase in aspen height and leader longevity was detected, 
supporting the hypothesis of a density-mediated trophic cascade.  However, the magnitude of 
the growth response was weak, suggesting that browsing was not the dominant limiting factor 
to aspen growth in the study area and that aspen may be more strongly limited by bottom-up