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Natural selection gives way to human selection

Practice of hunting and harvesting the biggest animals or plants is changing species much faster than nature, researchers find

From Tuesday's Globe and Mail

January 13, 2009 at 4:15 AM EST

Humans have become "superpredators," speeding up the evolution of the species they hunt and harvest at rates far above what is found in nature, according to new research, some of it conducted by Canadian biologists.

The researchers believe that many recently observed changes in species, ranging from the shrinkage in the horns of bighorn sheep in the Rockies to the reduction in the size of caribou in Scandinavia, are being driven by humans.

The biologists estimate that hunting has caused such characteristics as body size and reproductive age to change at a rate that is a staggering 300 per cent above the pace that would prevail in nature. This figure is even greater than the change attributed to other human interferences, such as pollution, which was estimated to alter species 50 per cent faster than what normally happens.

"The implications are pretty wide and profound," said Paul Paquet, a University of Calgary biologist who dubs humans "superpredators" for this outsized impact.

A paper outlining the findings was posted yesterday in the online edition of Proceedings of the National Academy of Sciences.

In it, Dr. Paquet and others argue that current hunting and harvesting practices are inadvertently causing dramatic changes in the behaviour and appearance of species. The reason: Humans tend to "high-grade" nature, taking out the biggest and best in a species.

The approach is clearest in trophy hunting, where animals with the biggest horns or bodies are killed, while inferior ones are left. But the practice also occurs in commercial fishing, where mesh openings in nets capture the biggest fish while smaller ones escape.

The human approach is the opposite to what happens in nature, where predators kill the easiest-to-catch animals, such as the young, the old and the sick, but are unable to take out the fittest adults, which then reproduce and pass their desirable characteristics on to future generations.

Many hunting rules are based on a view that bagging mature animals is the best approach, but Dr. Paquet says humans should be trying to emulate nature, even though at first glance it doesn't seem sporting to kill younger creatures. "That's clearly the best management regime," he said.

The researchers, who are affiliated with the University of Calgary and the University of Victoria as well as two U.S. universities, looked at 34 studies that examined physical or biological characteristics in 29 species subject to heavy pressure from human predation. Besides the bighorn sheep and caribou, they included snails; a pair of medicinal plants; and various commercially caught fish.

"Harvested organisms are the fastest-changing organisms of their kind in the wild because we are superpredators and we take such high proportions of a population and target the largest individuals," Dr. Paquet said.

The rate of change is staggering from an evolutionary point of view, where the alteration in species is often thought to occur slowly. Alberta's bighorn sheep, for instance, have experienced a 20-per-cent drop in the length and size of their horns in only the past 30 years. The sheep are prized for the distinctive curve in their horns, making them sought-after trophies.

Atlantic cod, whose populations have collapsed because of overfishing, now reproduce at an average age of five years, rather than six, as was the case previously. The change, which in humans would amount to children reaching sexual maturity and having offspring at ages of around nine to 11, occurred in only two decades.

Among the species harvested or hunted by humans and reviewed in the paper, body size or horn size decreased by an average 18 per cent. There was also a pattern of reproduction at an earlier age or smaller size.