Humans are ‘superpredators’

Want to see a “superpredator?” Just look in the mirror.

Scientists have compared the hunting habits of wild mammals, birds and fishes with those of people. Their new study shows that humans are strange predators. Unlike other animals, we target adult prey in large numbers. That is a practice that can push populations of those prey into decline, the researchers warn.

People mainly target — at least among wild mammals and fishes — prey that are old enough to reproduce. Those are the animals that have babies and keep their populations going, explains Chris Darimont. He is a conservation scientist at the University of Victoria in Canada. Many baby animals don’t live long enough to become adults. Killing too many of the few that do can shrink groups of wild animals over time.

To avoid that, Darimont and his colleagues want people to switch to the hunting patterns of other mammals or fishes. That would mean targeting young prey, not adults. Also, we would have to take smaller percentages of animal populations than we currently do. The scientists make their argument in the August 21 issue of Science.

A shift toward younger individuals would be more sustainable, says Thomas Reimchen. Also at the University of Victoria, he worked on the study, too. Being sustainable means that resources are used in a way that allows them to continue to be available long into the future. Reimchen acknowledges, though, that changing human habits would not be easy.

He has been itching to analyze people as a predator since 1976. At that time, he was studying populations of small fish. Called three-spine sticklebacks, these fish were living in a Canadian lake where 22 kinds of predators feasted on them, including trouts and loons. But the stickleback population didn’t change much in number from year to year. All those predators were eating less than 5 percent of the stickleback mass in the lake. Mostly, they ate the youngsters.

In the ocean nearby, he saw commercial fishing. These fishermen were catching 40 to 80 percent of all of the nearby adults among the species they were targeting. Reimchen recalled thinking that it seemed such targeting of adults would not allow fish populations to remain stable.

This difference between how humans and wild animals hunted stuck with the scientist. About seven years ago, he finally got his chance to investigate these lopsided fishing rates. He recruited Darimont and two other former students. The team collected rates of human hunting or fishing. They found records for 399 mammals and fishes. Then they compared the rates by humans against those of wild mammals and fish.

Human fishing, they found, takes a median of 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.

In hunts for grazing mammals on land, people kill about 6 percent of adult prey a year, the researchers calculated. 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.

Humans are primates without fangs, claws, horns, much running speed or a great sense of smell. But people have guns, nets, vehicles, refrigeration and other technologies. The technologies are used on hunts, Darimont explains. And that has given people predatory superpowers.

Killing animals doesn’t always shrink a population over the long term. Yet by killing large carnivores, such as wolves in the Northern Rockies, people usually alter population trends, says Scott Creel. He works at Montana State University in Bozeman. As a behavioral ecologist, he studies how an animal’s behavior relates to its environment.                              

People kill plenty of animals besides mammals. When hunting  amphibians, for instance, people largely target a few species of large-bodied frogs. One example: American bullfrogs, “themselves terrific predators,” says David M. Green. He studies amphibians at the Redpath Museum at McGill University in Montreal, Canada.

At least in places like North America, governments tend to regulate the hunting of birds. That means human predators are “unlikely to have much population impact,” says David Blockstein. He is a scientist at the National Council for Science and the Environment in Washington, D.C. People may have a bigger effect, though, in places where people hunt birds for food.

The biggest effect that people have on wildlife — bigger than any hunting — is their destruction of animal habitat. The concept of people as superpredators is useful, Blockstein says. But people are also “super destroyers.”

Power Words

(for more about Power Words, click here)

amphibians  A group of animals that includes frogs, salamanders and caecilians. Amphibians have backbones and can breathe through their skin. Unlike reptiles, birds and mammals, unborn or unhatched amphibians do not develop in a special protective sac called an amniotic sac.

behavior The way a person or other organism acts towards others, or conducts itself.

behavioral ecologist A scientist who studies how animal behavior relates to where animals live.

carnivore An animal that either exclusively or primarily eats other animals.

conservation  The act of preserving or protecting something. The focus of this work can range from art objects to endangered species and other aspects of the natural environment.

habitat The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

loon   A temperate zone water bird that tends to live on lakes and is known for its eerie call.

mammal  A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding the young, and (typically) the bearing of live young.

mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from. For objects on Earth, we know the mass as “weight.”

median  The value or quantity that lies at the midpoint of a group of numbers listed in order from lowest to highest.

population  A group of individuals from the same species that lives in the same area.

predator (adjective: predatory) A creature that preys on other animals for most or all of its food.

prey  (n.) Animal species eaten by others. (v.) To attack and eat another species.

primate  The order of mammals that includes humans, apes, monkeys and related animals (such as tarsiers, the Daubentonia and other lemurs).

species A group of similar organisms capable of producing offspring that can survive and reproduce.

sustainability  (n: sustainable)To use resources in a way that they will continue to be available in the future.