The restoration of wolves to Yellowstone National Park in 1995-96 created a valuable opportunity to understand how predators shape ecosystems. Intense grazing and browsing by elk changed willow dominated riparian plant communities of the northern range. Willow loss lead to beaver disappearance, and the cessation of dam building caused many streams to incise deeply drying the riparian zones.
One theory is that wolf reintroduction rapidly reversed changes to ecosystems that occurred during the pre-wolf period. However, possibly ecosystems had changed so much that wolf restoration alone was insufficient for restoration. In particular, the reduction in water availability to willows caused by the absence of beaver dams and stream down-cutting created moisture stress that prevented willows from rapid growth. We used a factorial experiment manipulating the availability of riparian water using artificial beaver dams, and browsing using fences. There were three treatments (dammed and unfenced, undammed and fenced, dammed and fenced) and a control (undammed and unfenced). Height and production of willows were measured annually in plots receiving each treatment and control.
If browsing reduction due to wolves is sufficient then willows in treatment and control plots should be of similar height and produce similar new plant tissue annually. However, willow height in control plots increased 60 cm in 17 years, while in dammed and fenced plots by more than 500 cm. Willow in dammed unfenced and willows in fenced and undammed sites were intermediate. This means that willows require beaver dams to recover rapidly from historic excessive browsing. However, beaver require willows for dam building. The mutual requirements of beaver for willows and willows for beaver have stabilized the ecosystem in a degraded state. We tested our experimental results by comparing willow heights on 27 other sites on randomly chosen small streams on the northern range. Average heights of willows on control plots and on randomly chosen plots were virtually identical. Moreover, these heights did not differ dramatically from heights measured in the early 1990's, before wolves were reintroduced. Wolf absence created conditions that cannot be rapidly reversed without beaver. Fifteen beaver dams were established in four stream reaches during 2015 - 2016. However, it is unclear whether these colonies can persist.