Humans as Super-Predators

Human predation is causing remarkably large and rapid changes in physical and behavioral traits of targeted species

02-01-2009 // Laura Tangley

IN THE FIRST STUDY to calculate the pace of evolution in commercially harvested plants and animals, scientists have found that human predation is causing remarkably large and rapid changes in physical and behavioral traits of targeted species. To reach that conclusion, they analyzed the results of 34 individual studies looking at 29 species in 40 different geographic systems. Most of the species were fish, but they also included invertebrates such as snails; two plants, American ginseng and Himalayan snow lotus; and two large mammals, caribou and bighorn sheep.

Their results, published online in the Proceedings of the National Academy of Sciences, revealed that commercial harvesting is speeding up the rate of observable trait changes—namely size and age of first reproduction—by 300 percent above the rate observed in natural ecosystems and 50 percent faster than in systems subjected to other human disturbances like pollution. On average, harvested populations are 20 percent smaller than those of previous generations, and the age of first reproduction is about 25 percent earlier. “The public knows we often harvest far too many fish, says Chris Darimont, a postdoctoral researcher in environmental studies at the University of California–Santa Cruz and lead author of the paper. “But the threat goes above and beyond numbers. We’re changing the very essence of what remains, sometimes within the span of only two decades. We are the planet’s super-predator.”

According to Darimont and his colleagues, fish and wildlife management policies often contribute to these rapid evolutionary changes, by requiring harvesting of the largest animals only. He calls the findings “a wake up call for resource managers. We should be mimicking natural predators, which take far less and target smaller individuals.

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