Tuesday :: Jul 31, 2007

a tree grows in Yellowstone


by Christina Hulbe

Back in May I wrote about the roles of large herbivores in shaping terrestrial ecosystems, and in turn the role of predators in regulating the landscaping abilities of those grazers. Mary noted in comments that the complete suite of bottom-up and top-down pressures in an ecosystem is called a trophic cascade.

A new paper in the August issue of Biological Conservation presents a compelling example, the connection between wolves and aspen in Yellowstone National Park (press release from William Ripple and Robert Beschta at Oregon State University). In brief, Yellowstone's aspen trees stopped regenerating (along with cottonwoods and willows) soon after the U.S government eradicated grey wolves from the park in the 1920's. Without wolves, the park's elk population increased and so too did browsing on aspens, cottonwoods, and willows. Tree regeneration plummeted. Aspen are a foundation species in Yellowstone so as the trees declined, change rippled through park ecosystems. Soil erosion also increased.

Grey wolves were reintroduced to the park in 1995. Now that the wolves are back, the elk population is down and the trees are recovering. Significant numbers of aspen have grown from new shoots to heights of seven feet or more in riparian zones along rivers. Seven feet is a survival milestone because it places the crown of the tree above browsing height. Upland trees have not yet begun to recover but researchers expect that they will, eventually. William Ripple speculates that the early recovery in the riparian zone is related to fear: "There are just some places now in the riparian zone that are too risky for the elk; a wolf may be lurking nearby."

Read more at the Oregon State Trophic Cascades Program website (including a short video). Also, the National Academies Press published a book, Ecological Dynamics on Yellowstone's Northern Range, in 2002 that is available online.

Christina Hulbe :: 6:36 PM :: Comments (10) :: Digg It!