Anne Minard, National Geographic News 12 Jan 09;
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.
"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.
"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."
Super-Predators: Humans Force Rapid Evolution of Animals
Robert Roy Britt, livescience.com 12 Jan 09;
Acting as super-predators, humans are forcing changes to body size and reproductive abilities in some species 300 percent faster than would occur naturally, a new study finds.
Hunting and fishing by individual sportsmen as well as large-scale commercial fishing are also outpacing other human influences, such as pollution, in effects on the animal kingdom. The changes are dramatic and may put the survival of some species in question.
In a review of 34 studies that tracked 29 species across 40 different geographic systems, harvested and hunted populations are on average 20 percent smaller in body size than previous generations, and the age at which they first reproduce is on average 25 percent earlier.
"Harvested organisms are the fastest-changing organisms of their kind in the wild, likely because we take such high proportions of a population and target the largest," said lead researcher Chris Darimont of the University of California, Santa Cruz. "It's an ideal recipe for rapid trait change."
Darimont told LiveScience that while he considers the changes to be evolutionary, some biologists consider them phenotypic and, without evidence of genetic shifts, would not call them evolution.
The study found dramatic change in several fish species and creatures as small as snails and as large as bighorn sheep and caribou.
Dominant force
The results, published online today in the journal Proceedings of the National Academy of Sciences, are similar to a host of other scientific conclusions dating back nearly two decades.
In 1990, Douglas Chadwick wrote in National Geographic magazine how trophy hunting - the practice of selecting only the largest beasts to kill -"has caused a decline in the average size of Kodiak Bears [in Alaska] over the years."
By harvesting vast numbers and targeting large, reproductively mature individuals, human predation is quickly reshaping wild populations, leaving smaller individuals to reproduce at ever-earlier ages, Darimont explained.
"The pace of changes we're seeing supercedes by a long shot what we've observed in natural systems, and even in systems that have been rapidly modified by humans in other ways," Darimont said. The study found the changes outpace by 50 percent those brought on by pollution and human introduction of alien species.
"As predators, humans are a dominant evolutionary force, he said.
Others agree the problem is serious. Columbia University biologist Don Melnick recently said trophy hunting is akin to selective breeding and is "highly likely to result in the end of a species."
Surprising ability to change
One surprise: The capacity of creatures to change.
"These changes occur well within our lifetimes," Darimont said. "Commercial hunting and fishing has awoken the latent ability of organisms to change rapidly."
Changes occur in two ways. One is sheer genetics:
Evolution can favor smaller fish able to pass through the mesh of gill nets and survive to reproduce, thereby passing on genes for smaller offspring.
Another change process is called plasticity. Shifts to earlier reproduction, for example, can occur because there is a lot of food and fewer fish to dine on it. The fish eat more and reach maturity sooner.
"Whatever the underlying process, shifts to earlier breeding spell trouble for populations," Darimont said. "Earlier breeders often produce far fewer offspring. If we take so much and reduce their ability to reproduce successfully, we reduce their resilience and ability to recover."
One specific example: the overfished Atlantic cod on the eastern coast of Canada. Less than two decades ago, they began mating at age 6. Now they start at age 5.
Government problem
In some cases, as other studies have found, the problem results from decades of big-game hunting and, more recently, poaching. Some populations of African elephants, for example, have unnatural percentages of tusk-free animals among them now, because hunters and poachers favor the ivory.
But some government rules contribute to the problem.
"Fishing regulations often prescribe the taking of larger fish, and the same often applies to hunting regulations," Darimont said. "Hunters are instructed not to take smaller animals or those with smaller horns. This is counter to patterns of natural predation, and now we're seeing the consequences of this management."
Darimont thinks new policies are in order.
"While wolves might prey on 20 animals, humans prey on hundreds of thousands of species," he points out. "We should be mimicking natural predators, which take far less and target smaller individuals."
Policy shifts may or may not save a species, however.
"It's unknown how quickly the traits can change back, or if they will," Darimont said.
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