A familiar metaphor for nature is the pyramid of life, with large predators living at the peak because they’re few in number and eat species lower on the pyramid. Like most simple metaphors, this one has a perceptual flaw. It creates the illusion that large predators have an effect only on the prey species immediately below them. The truth, as a growing body of scientific studies shows, is that the presence, and absence, of top predators cascades all through nature in surprisingly complex ways.
Our species has done a sadly efficient job of removing top predators: wolves, bears, lions, tigers, sharks and many more. According to the authors of a new article in Science magazine, “the loss of these animals may be humankind’s most pervasive influence on nature.”
The loss of cougars in what is now Zion National Park led to an “eruption” of mule deer, which reduced riverbank vegetation and, ultimately, changed the shape of stream channels. The loss of sea otters along the Pacific Coast led to the destruction of kelp forests and the many creatures they supported. The effect includes herbivores. When disease decimated wildebeest in East Africa in the late 19th century, grassland turned to shrubs and into fuel for wildfires, changing the ecosystem.
In the rare cases where top predators have been reintroduced, the benefit is profound. The success of gray wolves in Yellowstone changed many things. Grizzlies fed on their kills. Coyote numbers dropped and the numbers of small mammals climbed. Elk spent less time in creek bottoms, where they were more vulnerable, and streamside ecology changed as a result.
It is now clear that biological diversity increases when top predators are present. The pyramid is healthiest when its peak is still present and when humans aren’t the only top predators around.
The New York Times http://www.nytimes.com/