Loss of apex predators affects river systems in the Western US

Background/Question/Methods

Studies assessing how native ungulates can affect, following the loss or displacement of apex predators, the dynamics of riparian plant communities and the morphology of river channels have been absent in the scientific literature.  Our objective herein is to provide a synthesis of recent research relative to how riparian plant communities and channels are affected by the absence of key predators.  To do so, we focus primarily on results of river systems in three national parks in the western US: Yellowstone National Park in the northern Rocky Mountains, Olympic National Park in the coastal Pacific Northwest, and Zion National Park in the canyonlands of southern Utah.  Within each park, key large predators were extirpated or displaced in the early 1900s.

Results/Conclusions

Following the loss/displacement of large mammalian predators, increased browsing pressure by native ungulates initiated long-term declines in recruitment (i.e., growth of seedlings/sprouts into tall saplings and trees) of palatable plants such as willow, cottonwood, big-leaf maple, and other woody species.  Channel responses to browsing-suppressed riparian plant communities were soon to follow.  Channel changes included increased widths of active channels via accelerated bank erosion, erosion of historical floodplains and terraces, increased area of unvegetated alluvium, channel incision, and increased channel braiding.  A reduced frequency of overbank flows, due to channel widening/incision, indicated these rivers have become increasingly disconnected from their historical floodplains.  Results from Zion National Park also identified major biodiversity affects (e.g., reduced abundance of plant and animal species). Although these studies were conducted in national parks, results may have important implications concerning riparian plant communities, biodiversity, and channel morphology for streams and rivers found on other public lands in the western US.  Introduced ungulates on non-park service lands have often heavily utilized riparian areas for over a century, largely in the absence of key predators, with significant consequences to riparian plant communities and channels.