Can large carnivores change streams via a trophic cascade? Beschta & Ripple 2020

After having been wiped out by the 1920s, wolves were reintroduced to Yellowstone National Park in 1995-1996. This study assessed the importance of large carnivores to wild ungulates’ behavior and density, with secondary effects on plant communities, rivers and channels, and beaver communities. Focusing on the West and East Forks of Blacktail Deer Creek, the authors summarized the population trends of wolves, elk, and beaver; sampled the heights, recruitment, and browsing intensity of Geyer willow (a common local tall willow); measured dimensions of the channel, and ascertained beaver dam heights.

After the reintroduction of wolves, the Rocky Mountain elk population decreased from 17,000 in 1994 to about 4,000 to 5,000 in recent years. Browsing intensity therefore greatly decreased, leading to taller riparian willow stems, which is an important food web support and physical habitat for both terrestrial and aquatic wildlife species. The willow canopy cover over the water surface has also increased rapidly over the last two decades, which holds a significant role in supporting the aquatic biota:

Canopy cover can reduce the amount of solar radiation reaching a stream, especially important during summertime periods when solar angles are high, day lengths are long, and flows are normally low, thereby mediating potential increases in water temperature. Furthermore, invertebrates in the canopies of near‐channel willows provide food for fish and seasonal leaf‐fall represents an important carbon base for aquatic invertebrates which, in turn, provide ‘reciprocal flows of invertebrate prey’ to adjacent terrestrial consumers [Beschta & Ripple 2020: 8].

Another benefit of protecting the riparian vegetation from herbivores is the improvement of streambank stability. During the period of wolf absence, intensive elk herbivory caused streambank erosion and channel incision (river cuts downward into its bed, deepening the active channel and may lead to dissected landscape), resulting in less frequent overbank flow. The channel incision lowered water tables and reduced subsurface moisture in flood plain vegetation during summer.

The return of wolves started the process of riparian vegetation restoration, which in turn supported stream-dependent species such as beavers. The reduction of elk herbivory increased food sources and materials for beavers to construct dams, while also fostering the narrower and shallower channels preferred by beavers. Thus, along with the recovery of vegetation and channels, beavers have returned in 2018, creating active dams to further rehabilitate the ecosystem.

If beaver populations continue to increase over time, the ecological effects of these ‘ecosystem engineers’ may well have a significant role in restoring riparian vegetation, floodplains, and channel dimensions for at least portions of northern range streams [Beschta & Ripple 2020: 9].