Tuesday September 02, 2014

Yellowstone: an ecological lesson


It is fairly common knowledge that all the links in a food chain are important to the health of an ecosystem, but a new BBC documentary about Yellowstone Park is a fascinating example of just how dramatic the impact of top predators can be.

It has become readily apparent to most people that removing a top predator, such as wolves, from an ecosystem has a cascading effect on everything down the food chain, but what has happened to Yellowstone since wolves were reintroduced in 1995 is truly astounding.

By the 1920s, wolves had been hunted out of existence in the famous American national park.

The immediate impact was the proliferation of large mammal species such as elk, deer, sheep, moose and bison.

Unchecked, these populations, laid the valleys bare by grazing. Without vegetation, erosion transformed the landscape. Rivers and streams started to meander. Not only did the absence of wolves reduce the ecological niches for all kinds of birds, small mammals, amphibians and reptiles, but it opened up a greater niche for coyotes, a secondary predator, that further reduced the population of smaller prey species.

In short, removing the primary predator not only transformed the ecosystem, but completely changed the geography of Yellowstone and so it went for 70 years despite human efforts to manage it by culling the herds.

Bringing the wolves back, even though they were small in number, has reversed all that. Although there were not enough wolves to significantly kill off the large mammals, they changed the behaviour of large herbivores pushing them out of the open where they are easy prey.

Vegetation immediately made a comeback. Bare valleys rapidly became reforested opening up the old niches. The wolves also killed coyotes or forced them into hiding.

The birds came back, as did the small mammals, amphibians and reptiles.

Riverbanks stabilized providing berries that allowed bear populations to rebound, which reinforced the impact of wolves as bears will also kill small and weak members of the ungulate populations.

The comeback of beavers helped too as their dams created ecological niches for all manner of water species, otters, frogs, fish etc.

This is known as a trophic cascade. It works in either direction. It happens everywhere in large and small scales.

Yellowstone is a dramatic and fascinating example. The full BBC documentary is well worth watching if you can get your hands on it. There are also shorter versions floating around the Internet.



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