GREVY'S ZEBRA Over the past few decades, the Grevy's zebra, an important herbivore that ranges throughout the Horn of Africa's savannas, has suffered severe habitat loss and become critically endangered.
Courtesy of Caroline Fraser

Editor's Note: The following is an excerpt from Caroline Fraser's book Rewilding the World.

Over the years, coyotes ate many of Michael Soulé’s cats. For most people, this might have been the end of the story, a nasty reminder of nature’s darker proclivities. But Michael Soulé is not most people.

Soulé is a biologist. At the time, he was a professor at the University of California at San Diego, living in the chaparral canyons outside the city. He had grown up in the canyons, poking around in the leaf litter, catching lizards. When the boy became a biologist, he recognized that the chaparral was a unique ecosystem, with its own suite of interdependent plants and animals, the coastal sage scrub home to fox and bobcats, wrentits and spotted towhees, cactus mouse and California quail. But to real estate developers, the canyons were empty wasteland, waiting to be turned into homes.

As he watched the progressive paving of the canyons, Soulé found himself even more distressed about the big picture, the loss of the ecosystem, than about the cats. Recent breakthroughs in biology had suggested that fragmentation of habitat inevitably threatened species. As developers carved the canyons into suburban lots, leaving behind islands of isolated brush, Soulé was alarmed enough to investigate that theory, and he sent students to compile data on the disappearance of birds from 37 forlorn chaparral islands. He also had them collect data on local carnivores, to see if predation was a factor. After two years, as expected, data showed that the number of birds and other species in each patch was diminishing.

But the data revealed something else, something counterintuitive. In canyons with coyotes, a greater diversity of birds survived. Canyons without coyotes supported fewer species. Having seen ample evidence that coyotes were responsible for his disappearing pets—cats flying through the cat door as if “chased by the devil”— Soulé had a theory: more coyotes meant fewer cats. Fewer cats meant more birds. Coyotes were eating not only cats but also other midsized predators, such as foxes. Coyotes were acting as a control. Without that control, the midsized carnivores ran wild in an orgy of predation that Soulé termed “mesopredator release.” Another study confirmed it: one in five coyote scats contained domestic cat.

Before long, scientists were realizing that much of the country was suffering from a bad case of mesopredator release. The artificial absence of wolves and other large predators gave cats, dogs, raccoons, and foxes license to grow fat on wild birds from the beaches to the mountainsides. Soulé had observed just one manifestation of a crucial new scientific discovery: predators do not merely control prey. They control other predators, and by doing so, they regulate species with which they never directly interact. They regulate biological processes down the food chain.

As scientists study the unbalanced and fragmented systems humans create as they alter the environment, they are realizing how interdependent species are. In a way, all of us are now living in a scientific experiment similar to that which San Diego developers created by carving up the canyons. We have unleashed forces we are still struggling to comprehend.

View Slide Show: Animal Victims of the 6th Extinction

This global experiment is comparable to the one Americans unwittingly set in motion in the 1950s by sowing the land with toxic pesticides. In the fable that opens Silent Spring, Rachel Carson described a “strange blight” settling over a town. Birds fell silent, bees vanished. There was no pollination, no fruit. “Everywhere,” she wrote, “was a shadow of death.” Carson helped keep that pall from settling over the whole country by inciting a national debate that led to the banning of DDT.