Trophic Transformations

According to research being conducted by scientists from Oregon State University (OSU), dramatic shifts in wildlife populations inhabiting Yellowstone National Park have the potential to alter the landscape, restoring it to conditions not seen in almost a century. Elk populations in their historic winter range in the northeastern corner of the park have decreased, while their numbers are increasing outside of park boundaries. These changes in elk numbers and their distribution are linked to the reintroduction of wolves in 1995-96. Wolves affect elk behavior and numbers which reduces the impact of elk on vegetation in what is termed a trophic cascade effect. Improving grizzly bear populations are also affecting elk resulting in vegetation returning to its lush density in some places, although not everywhere.
Luke Painter, OSU instructor and lead author on 3 recent studies monitoring vegetation regrowth in the park, states that without wolves this shift would not be occurring; “Wolves caused a fundamental change, but certainly they are interacting with other factors such as bears, climate, fire and human activity.” Similarly interconnected are bison populations, which appear to have increased four-fold as elk have decreased. Bison will browse as a group, feeding on aspen, willow and cottonwood which partly makes up for the decline of elk, however, they cannot reach as high as elk and so trees are able to reach maturity.
Between 2010 and 2012 Painter hiked the Yellowstone back-country and re-measured 87 stands of aspen trees which had previously been monitored by predecessors at the university from 1997-1998. Painter also performed a regional survey of the northern part of the park and the Shoshone National Forest, adjacent to the park’s western border, where hunting and cattle grazing are permitted. The findings from this study have been published in the journal “Ecology” (http://www.esajournals.org/doi/abs/10.1890/14-0712.1), and illustrate how much can change from an ecosystem perspective over the course of 15 years. While aspen populations were in serious decline over much of the 20th century, recovery is observable and widespread throughout much of the northern range. Where elk are still present in large numbers much of the aspen remains heavily browsed and stunted. Similarly with bison herds; in the Lamar Valley bison have taken over as the dominant herbivore and are suppressing aspen recovery.
Recovery of aspen stands is happening, but it is still in the early stages and is not ubiquitous. However, Painter found that 25% of the stands he surveyed had at least 4 or 5 young aspens tall enough to escape elk browsing, which is something that has not been observed in recent decades. Additionally, 46% of stands have at least 1 tree that has outgrown the elks’ reach entirely. While changes in climate and burn regime would also be valid explanations for the aspen recovery, Painter did not find evidence during his surveys to support these methods, he maintains that wolf introduction is the driving force behind the shift. In the 1990s the general feeling amongst ecologists was that the introduction of wolves would not incur widespread change, but as it happens, they were able to cause a trophic cascade with positive results.