How Apex Predators Maintain Ecosystems

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“We all know that wolves kill various species of animals, but perhaps we’re slightly less aware that they give life to many others.” 

(If you’re watching the above video at work, please note the audio starts with several seconds of authentic wolf howls, so adjust the volume on your speakers accordingly)

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An apex predator resides at the top of their food web, with no natural predators in their habitats. The gray wolf was one of the predominant apex predators in North American, but by 1926, wolves had been extirpated from Yellowstone National Park. Within a few years a team of visiting scientists observed “deterioration progressing steadily,” a result of overgrazing by elk, whose populations began to rise once its natural predator, the wolf, was artificially removed from the environment. When wolves were reintroduced to the park in 1995, the phenomenon was quickly observed in reverse: as wolves preyed on elk, the willows and aspens they once grazed with unchecked abandon began to regenerate, bringing with them a host of new life – a process known as the trophic cascade, a phenomenon in which the apex predator suppresses or alters the traits of their prey, thereby releasing life in the next trophic level. 

In the story Yellowstone’s trophic cascade, elk had grazed so much of the vegetation away that the landscape of the park was visibly altered. In areas where wolves were introduced, they “radically changed the behavior of the deer, avoiding parts of the park where they would be trapped the most easily,” in valleys and gorges. Quickly, these areas began to regenerate, with the height of some trees quintupling within six years. As the aspen, willow, and cottonwood began to flourish, the birds – from songbirds to migratory – once again took roost.

Beavers, “ecosystem engineers” themselves, also began to re-emerge, building dams that provide homes for otters, muskrats, fish, reptiles and amphibians – the next low trophic level. But the cascade had other effects, for the wolves began to prey on coyotes, who in term were preying on small mammals like mice and rabbits. As the number of mice and rabbits began to grow, new predators moved in – birds of prey like eagles and ravens, badgers, weasels, and foxes, with whom coyotes typically compete with for food. The carrion left by wolf kills feed not only vultures but other creatures from bears down to ants. Bears, another apex predator, were also being supported by the regenerating vegetation and game around the course of the rivers, which had begun to change as the elk grazed elsewhere and new native populations moved in.

And here is the story of how wolves changed rivers: not just transforming the ecosystem, but shaping the physical geography of a national treasure, Yellowstone Park.

 

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