A new study drives home the destructive power of our species. Not only do we kill other animals at much higher rates than other predators, but our ability to bring down larger adults can make it very difficult for some prey populations to recover. This superpredator status may fill our bellies, but it has darker implications. "Any predator capable of exerting such impact will eventually drive its prey to extinction," warns Gerardo Ceballos, an ecologist at the National Autonomous University of Mexico in Mexico City.
It’s happened before. About 14,000 years ago, humans entering North America caused many large species, such as the mammoth, to disappear. And our hunting technologies have only improved since then, particularly when it comes to catching fish. Overfishing is a severe problem in some parts of the world, and a recent report concludes that because of human activity, more than 90 fish species are at risk of extinction.
The new study originated in a casual observation. Thomas Reimchen, an evolutionary ecologist at the University of Victoria in Canada, has spent years studying how predators impact the stickleback fish on an island 130 kilometers off the Canadian Pacific coast. Over the decades he determined that each species never kills more than 2% of the sticklebacks per year and usually attacks juveniles. Yet off that same island, fishermen seemed to be taking a far higher percentage of salmon, mostly adults. The contrast bothered him, so Reimchen and a few former students searched the scientific literature for data on the rate at which humans and other animals were killing other species.
After a decade compiling and analyzing about 300 studies, the team came to some grim conclusions, says Chris Darimont, a conservation scientist also at the University of Victoria who helped lead the study. Humans and other predators—like lions, wolves, and grizzly bears—kill wild herbivores at about the same rate, but humans kill large carnivores at nine times the rate of other predators, Darimont, Reimchen, and their colleagues report today in Science. We kill those carnivores not for food, but for trophies and—sometimes—to eliminate them as competitors, Darimont says. Because they naturally don’t face much predation, they have not evolved ways to successfully avoid humans or reproduce fast enough to make up for human-induced losses.
But the toll on fish is even greater. The researchers report that people catch adult fish at a rate up to 14 times other predators. Thanks to mechanized fishing, the annual human toll on marine fish may exceed 100 million tons. What's worse, by focusing on catching large adults, fishing removes individuals in their reproductive prime that are needed to replenish diminishing populations. Already, this fishing pressure has caused species to evolve new growth patterns and behaviors.
Marine ecologist Nicholas Dulvy from Simon Fraser University in Burnaby, Canada, is pleased with the work. Hunting and fishing have "not captured the public attention necessary to change the status quo," says Dulvy, who was not involved with the study. "The disparity between human and animal predation rates is a useful way of illustrating how ecologically out-of-whack many exploitation rates and management policies are." Darimont thinks people need to take a lesson from other predators, switching the focus to catching juveniles and lowering catch rates.
But others take issue with the study's approach and conclusions. "I think it’s total rubbish," says Ray Hilborn, an ecologist at the University of Washington, Seattle. An expert on sustainable exploitation, he says that even though humans may take more fish than any one predator, their haul makes up only 40% of total natural predation on fish. Hilborn says this is a reasonable amount given the need to provide food for the human population, and the new work is “fuzzing up what we mean by sustainability.” He says he doesn't think that people can fish less and still provide enough food for the world.
Even so, people should take a really hard look at management practices that go after the largest individuals, says Blaire Van Valkenburgh, a paleoecologist at the University of California, Los Angeles, who was not involved with the work. "Getting that attitude to change is going to be very difficult, but at least [with this paper] we will be able to get some people to talk about it."