Seeing humans as superpredators

To get a glimpse of a superpredator, just look in the mirror. Comparing hunting habits of mammals and fishes reveals humans as Earth’s most dangerous, oddball predator — one that targets adult prey in large numbers, a practice that can push populations into decline.

Humans’ main prey are reproductive adults, the animals that replenish populations, explains conservation scientist Chris Darimont of the University of Victoria in Canada. He and his colleagues call for people to switch to the hunting patterns of other predatory mammals or fishes. Such a change would entail targeting the young instead of the adults and taking smaller percentages, the scientists say in the Aug. 21 Science.

A shift toward the young would be more sustainable, “consuming the reproductive interest rather than the reproductive capital,” says study coauthor Thomas Reimchen, also at Victoria. He acknowledges, though, that changing human habits would not be easy.

Reimchen has been itching to analyze people as just another predator since 1976, when he was monitoring populations of small fish called three-spine sticklebacks in a lake in the Haida Gwaii archipelago off Canada’s western coast. Trout, loons and 20 other predators feasted on these fish, yet stickleback population size didn’t change much from year to year. These combined predators were eating less than 5 percent of the stickleback mass in the lake, and mostly the youngsters. But not far away in the ocean, Reimchen saw predatory humans catching 40 to 80 percent of mostly adult commercial fishes — a harvest that seemed unlikely to leave these fish populations stable.

About seven years ago, Reimchen finally got his chance to follow up on lopsided fishing rates. He recruited Darimont, also with the Raincoast Conservation Foundation in Sidney, Canada, and two other former students to search out rates of human hunting or fishing recorded for 399 mammals and fishes.

Marine fishers are “the planet’s dominant predator of adult prey,” Darimont says. Human fishing takes on average about 14 percent of the total weight of adults out of the sea annually. That’s 14 times as high as the median rate of what predatory fish take, the analysis shows.

In hunts for grazing mammals on land, people kill about 6 percent of adult prey a year, the researchers say. That’s roughly what wild carnivores kill. What’s unusual about people, though, is their power to turn those other predators into prey. Human predators kill carnivores at about nine times the rate that carnivores kill each other.

On its own, Homo sapiens is just a primate without fangs, claws, horns, much running speed or a fabulous sense of smell. But the guns, nets, vehicles, refrigeration and other technologies that people have brought to their hunts have given humans predatory superpowers, Darimont explains.

Killing animals doesn’t always shrink a population; more deaths by human hand could just mean fewer caused by other menaces. Yet humans’ killing of large carnivores, such as wolves in the Northern Rockies, usually does affect population trends, says behavioral ecologist Scott Creel of Montana State University in Bozeman.                                

Humans kill plenty of animals besides mammals. Among amphibians, the impact falls mainly on a few of the large-bodied frog species, such as American bullfrogs, “themselves terrific predators,” says David M. Green of the Redpath Museum at McGill University in Montreal. And for birds, at least in the developed world, sport hunting is highly regulated and “unlikely to have much population impact,” says David Blockstein of the National Council for Science and the Environment in Washington, D.C. (Netting songbirds for food in Malawi is another matter.)

So for birds and other land animals, the main threat that humans pose is destruction of habitat. The concept of people as superpredators is useful, Blockstein says. But people are also “super destroyers.”