How Wolves Change Rivers

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Monday 3 March 2014 15.23 GMT

Wolves are crucial for maintaining a healthy ecosystem — a fact that was conveniently forgotten when they were exterminated from almost all of the continental United States by ranchers, farmers, trappers and hunters.

Meanwhile, the populations of other animals exploded. The entire ecosystem of the American wilderness was changed by rapidly expanding populations of large ungulates — particularly wapiti, Cervus canadensis. (These animals are more commonly known as “elk” — note that the narrator in the accompanying video erroneously refers to these animals as “deer”, implying that they are European red deer, Cervus elaphus, which they most certainly are not.)

After decades of political wrangling with those who traditionally persecuted wolves, conservation biologists and activists who supported restoration of wolves finally prevailed: the grey wolf, Canis lupus, was finally reintroduced several areas in the northern Rocky Mountains of the United States. One of those release areas was Yellowstone National Park. Subadult wolves from several packs in Alberta’s Mackenzie Valley were captured using tranquilizer darts and released in Yellowstone in January 1995 and again in January 1996.

After an absence of nearly 70 years, the beneficial influences of the reintroduced wolves almost immediately became apparent, and continue to be seen to this day. Since they are apex predators that primarily hunt ailing and aging wapiti and other large ungulates, wolves halted these herbivores’ population expansions whilst improving their overall health — and even substantially changed their behaviours. For example, the wapiti stopped munching their way through the valleys and gorges where wolves could easily ambush them. Thus, native flora was able to re-establish and re-grow, thereby increasing biodiversity by providing food and shelter to a growing variety ofplants and animals.

But remarkably, the presence of wolves also changed the rivers. After reintroduction, it was noticed that riverbank erosion decreased so the rivers meandered less, the channels deepened and small pools formed. Why? The recovering vegetation stabilized the riverbanks, which in turn altered the geography of the park itself.