How Wolves Change Rivers

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.

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 ungulates — particularly wapiti, Cervus canadensis, whose populations had exploded in Yellowstone — wolves halted these herbivores’ population expansions whilst improving their overall health. (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.)

The presence of wolves even substantially changed ungulate 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 of plants 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.

Basically, humans conducted a huge real-life experiment by removing — and then eventually reintroducing — an apex predator from a large tract of land. Initially, the ecological changes wrought by the lack of wolves were subtle so they were not generally noticed. But the results of this real-life reintroduction experiment unambiguously indicate that wolves are an integral part of the ecosystem; they certainly are essential to restoring and maintaining the natural ecology of the entire Yellowstone region.

This phenomenon is known amongst ecologists as a “trophic cascade”. In this scenario, a top-down cascade was observed when the top predator, the wolf, was exterminated from most of its natural range. Removing the top predator triggered a wide range of effects that cascaded down through the entire web of life. Although only a total of 41 wolves were reintroduced and their total population remains small, they act as ecosystem engineers by creating niches that other animal and plant species can inhabit.


Author: GrrlScientist



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