Human actions can cause unintended effects on nature. Often, these actions can cause natural systems to change abruptly from their historic condition to new conditions that may be undesirable. The removal of wolves from Yellowstone National Park appears to be an action that has caused unanticipated harm to the ecosystems of the park, in particular, the loss of trees and shrubs from stream corridors. These plants enhance water quality, stabilize stream banks, and provide habitat for many species of wildlife. It appears that elimination of wolves allowed elk populations to reach levels where their browsing prevented willow reproduction and increased willow mortality. The absence of beaver with the demise of much of the willow habitat, resulted in fundamental changes in streams, rendering them less suitable as habitat for willows. This study will determine whether the re-introduction of wolves to Yellowstone National Park has allowed the northern range to recover to its historic condition with abundant willow communities and vigorous populations of beaver. Several approaches will be used including assessments of aerial photographs and quantitative modeling of the system.
A National Academy of Sciences report identified understanding the effects of availability of water and browsing by elk on communities of willows as a problem of urgent importance for policy and management in Yellowstone National Park. The proposed work will address a problem of fundamental scientific value in a location where science can be immediately applied. The work provides a valuable opportunity to communicate science to citizens, managers and policy makers.
One of the most important ways that people alter the ecosystems of the earth is by influencing the abundance of species. There are many examples showing that well-intended human actions aimed at eliminating "undesirable" species from ecosystems has caused unforeseen, lasting harm. The elimination of wolves from the Yellowstone National Park in the early twentieth century is one of those examples. Freed from predation by wolves, the park's population of elk increased dramatically in its primary winter habitat, the northern range. These increases caused loss of species of woody deciduous plants from the landscape, notably aspen and willow, as a result of intense browsing by elk. The loss of willows from riparian areas probably caused the disappearance of beaver, which depend on willows for food and materials for building dams. The absence of beaver dams on small streams led to erosion and drying of stream banks. These changes in streams degraded their value as habitat for willows and other species of plants and animals. It has been hypothesized that the reintroduction of wolves to the northern range in 1995-96 reversed the harmful effects on willows that followed the extirpation of wolves. The restorative effects of wolves on the Yellowstone ecosystem has been accepted as scientific fact in both the popular press and in textbooks on ecology. However, there is little hard evidence demonstrating ecosystem restoration following the reintroduction of wolves. This research project examined responses of willows to browsing and to other controls on their productivity, particularly the availability of water. The goal of the work was to determine if reintroduction of wolves has restored willow communities on Yellowstone's northern range, or alternatively, if other factors, notably the loss of beaver dams, is preventing ecosystem recovery. The research team conducted a ten year experiment where browsing by elk was eliminated in some riparian areas with fencing while other areas were left unfenced, allowing browsing. In addition, fenced and unfenced areas were manipulated with simulated beaver dams, while other fenced and unfenced areas were left undammed. Eliminating browsing alone failed to cause ecosystem recovery. All experimental areas had willows that averaged about 40 inches in height at the beginning of the experiment. Unfenced willows growing on stream banks that lacked dams grew only 10 inches in ten years (from an average of about 40 inches to 50 inches). If wolves were "protecting" these willows, it would be reasonable to expect that they would have grown rapidly. Fenced willows without dams grew only slightly taller than unfenced willows without dams and accumulated the same amount of mass as fenced willows over the ten-year experiment. In contrast, willows that were fenced and dammed grew rapidly, reaching heights of almost 100 inches. These results show conclusively that reduction of browsing. which might be expected due to the effects of wolves on elk numbers and behavior, was not sufficient to allow recovery of willows in the absence of damming by beaver. The work showed that the ecosystem is "stuck" in a state where willows are short in height and small in mass because willows require beaver dams for their habitat and beavers require willows for their habitat. It could be that the experiment was not representative of conditions that prevailed across the landscape. To check for this possibility, the research team investigated factors that control height and mass growth of willows at 21 sites widely dispersed across the northern range. Heights of willows at these sites were similar to heights in the experiment. The heights of willows on the landscape during 2010 were not different from heights measured by others during the years before wolves were introduced. There was a large amount of variation in willow growth across the landscape. The research team found that this variation was best explained by availability of water. Browsing played a limited role in controlling willow growth. It was also important to understand variation in willow growth over time. The research team developed relationships between willow height and age by counting growth rings of stems of different length. This analysis included stems that were present long before wolves were reintroduced. Data on yearly variation in climate--particularly the length of the growing season--was better able to explain growth patterns than data on the number of elk or wolves. All of these studies point to a single conclusion--that willows on Yellowstone's northern range are more limited from the "bottom-up" by access to resources like water than from the "top-down" by effects of browsing by elk. As a result, there is no reason to believe that wolves have restored willows in Yellowstone by reducing browsing by elk on willows. It is clear that removal of wolves caused lasting effects on the ecosystem that were not quickly reversed by the reintroduction of wolves.
