Super-Predators: Humans Force Rapid Evolution of

Animals       

Robert Roy Britt, Editorial Director, LiveScience.com

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.

·           10 Species You Can Kiss Goodbye 

·           Trophy Hunting Causing 'Reverse Evolution' 

·           Evolution News & Features 

轉貼自︰

http://news.yahoo.com/s/livescience/20090112/sc_livescience/superpredatorshumansforcerapidevolutionofanimals;_ylt=AkwXPE_i3G.K0puqpU0ETZobr7sF

本文於 修改第 3 次