13 January 2009

We should let the big ones get away

Predatory humans are seriously affecting the balance of nature, a recent study suggests. As predators, humans are a dominant evolutionary force.

"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," say researchers who did a review of 34 studies that tracked 29 species across 40 different geographic systems. The team also compared 25 species that are not hunted by people but that face other human-caused selection pressures.

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. In addition, rates of evolution in harvested organisms occurred 300 percent faster than in natural systems.

The study found the changes outpace by 50 percent those brought on by pollution and human introduction of alien species.

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

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.

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.

"Whatever the underlying process, shifts to earlier breeding spell trouble for populations. 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.

"It's unknown how quickly the traits can change back, or if they will."

The study included 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.

Sustainable management "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."

Full articles on the wildsingapore news blog.

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