Hunters Speeding Up Evolution of Trophy Prey?

Anne Minard
for National Geographic News
January 12, 2009
Human hunters are pushing their prey to evolve faster than they would naturally, resulting in smaller and younger individuals over time, according to a new study.

Hunters' desire for the largest individuals—the "trophies"—influences plant and animal populations faster than natural selection and even other human impacts, such as pollution and habitat destruction.

Such preferences leave a disproportionate number of smaller animals and plants to reproduce.

The phenomenon of human-forced evolution is already known, said study lead author Chris Darimont.

But what's jarring about the new research is the rate at which whole populations are changing.

(Related: "Trophy Hunting May Push Polar Bears to 'Tipping Point'" [November 21, 2007].)

"Human-harvested organisms are the fastest-changing organisms yet observed in the wild," said Darimont, an evolutionary biologist at the University of California, Santa Cruz, whose work was also supported by the University of Victoria in Canada.

A classic example of natural selection is Darwin's finches on the Galápagos Islands, which rapidly evolve different beak styles to exploit varying food sources in periodic droughts. Organisms can also adapt in response to hardship, such as pollution or weather shifts.

Fast and Furious

Darimont and colleagues examined previously documented, hunting-induced changes across 29 species in 40 locations, including commercially targeted fish, bighorn sheep, caribou, and several marine animals such as limpets and snails. Two plant species were also included in the analysis: Himalayan snow lotus and American ginseng.

The researchers compared shifts in those populations with those in 20 species that face only "natural" pressures, such as climate, competition for resources, or animal predators.

The team also compared 25 species that are not hunted by people but that face other human-caused selection pressures.

The findings showed major differences in two areas: the animals' body size and their patterns of reproduction.

In addition, rates of evolution in harvested organisms occurred 300 percent faster than in natural systems, said Darimont, whose results appear in this week's issue of the Proceedings of the National Academy of Sciences.

And trends toward smaller body size and younger breeding ages happen 50 percent faster due to hunting than as a result of other human-caused influences such as pollution and habitat loss, the study found.

The body and horn sizes of bighorn sheep, for example, have declined by about 20 percent in the past three decades as a result of human hunting.

Atlantic cod on the east coast of Canada now breed at five years of age instead of six—a shift that has occurred in only two decades.

Severed Links

Darimont points out that the implications reach beyond puny fishes or sheep with smaller horns—both commercial and trophy hunters are also hindering animals' and plants' ability to recover.

"These types of traits [bigger bodies, longer horns, and so on] are key elements to individual fitness and to population growth rates," said Jeff Hutchings, a marine biologist at Dalhousie University in Nova Scotia who was not a co-author.

In general, humans hunt at a higher rate than natural predators such as wolves or sharks.

Though animal predators may take 10 percent of a heavily harvested fish species, for instance, people may take up to 70 percent.

(Related: "Overfishing is Emptying World's Rivers, Lakes, Experts Warn" [December 1, 2005].)

"Especially in fishes," lead study author Darimont said, "younger and smaller breeders produce less offspring, and this jeopardizes the ability of prey species to recover after harvest."

Such shifts may also imperil other species that have evolved alongside the targeted animals, either as predators, prey, or competitors, Darimont added.

"The concern is that ecosystem function might change when a species shrinks so rapidly," he said. "You can imagine all these ecological links being severed."

Ditch the Trophies?

It's unclear whether such differences result from short-term survival of smaller and younger-breeding animals or longer-term, underlying genetic responses, said Phillip Fenberg of Natural History Museum in London.

Regardless, he wrote in an email, the solution is the same: Cut down on trophy hunting.

Sustainable management, he said, "requires that people stop preferentially removing the larger and most [fertile] animals from populations, and focus more on a strategy that preserves the historic size-structure of the species."

Darimont agreed, adding that the best way to keep prey animals at healthier sizes is to "mimic natural predators."

"That means greatly reducing our captures and forgoing the largest."

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