How Wolves Change Rivers

A small number of top predators can reshape ecosystems and landscape-scale geography because their top-down effects alter species behavior and vegetation, which then changes geomorphology through reduced erosion and stabilized banks.

Regenerating forests and shrubs rooted soils on valley sides and banks, which reduced erosion and bank collapse and caused rivers to narrow, meander less, and form more pools and riffles.

Wolf kills and leftover carcasses supplied food for ravens, eagles, and bears, and combined with more berries and weakened deer calves this extra resource base helped bear numbers rise.

By killing coyotes, wolves lowered mesopredator pressure, which allowed rabbits and mice to increase and in turn supported more hawks, weasels, foxes, and badgers.

More woody vegetation gave beavers extra food and building material, which allowed beaver populations to grow and build dams that created pools and wetlands, generating new aquatic habitats.

As trees and shrubs became taller and denser, they provided nesting sites and food, which attracted many more songbirds and migratory birds to those recovering habitats.

When deer avoided certain areas, saplings stopped being constantly browsed, which let trees and shrubs survive and grow much faster—tree height in some spots surged within a few years.

Wolves changed Yellowstone more by inducing fear than by sheer kills because deer avoided risky valleys and gorges, which reduced grazing there and allowed plant communities to recover.