Jan. 13, 2009 -- Hunting by humans has such a lasting effect on animal populations that the appearance and behavior of some species are changing as a result, according to a new study.
The paper, published in this week's Proceedings of the National Academy of Sciences, is the first to compare the rate of such change among hunted species with changes that have occurred under natural conditions -- such as flood, disease or drought -- and changes due to other human-caused factors, such as pollution and habitat loss.
One explanation for the trait changes seen in hunted animals is that hunters often target prey of a particular age and size, removing them from the breeding population.
"Ironically, many hunting and fishing regulations encourage hunters and fishers to target the larger individuals," lead author Chris Darimont told Discovery News.
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"For example, fish below a certain size often must be released back into the water, so some management prescriptions can actually create these novel evolutionary conditions that lead to the rapid trait changes we observed," added Darimont, a postdoctoral fellow in the Department of Environmental Studies at the University of California, Santa Cruz.
He and his colleagues analyzed 40 "human predator systems" comprising 29 species that included fish, hoofed animals, and even plants.
The researchers found that in 95 percent of cases, hunting by humans was the cause for decreases in body and horn size seen in many of the animals. In 97 percent of cases, hunting was also responsible for animals reproducing at earlier ages.
Darimont said Bighorn sheep of Alberta, Canada, are one prominent example, having seen a 20 percent reduction in body and horn size in 30 years. He also said that, due to fishing, "Atlantic cod of Canada's east coast have shifted the timing of their first reproduction from an average of six years to an average of five years in only 20 years."
In virtually all of the studied scenarios, hunting by humans -- especially large-scale practices, like trawling -- outpaced nature-driven trait changes by over 300 percent. Now the question is: Are hunters permanently shaping animal populations, or do the observed changes reflect temporary demographic shifts rather than genetic evolution?
Both, perhaps.
"Some observed trait changes likely represent underlying genetic changes passed on from one generation to the next," Darimont explained. "In gill net fisheries, for example, evolution can favor smaller fish that pass through the mesh. Those smaller individuals are more likely to survive, reproduce and pass on genes for smaller offspring."
"By contrast, some trait changes likely do not involve genetic changes, a process called plasticity," he added. "For example, shifts to earlier reproduction can occur because of an abundance of food being shared by a much smaller population of fish."
Jeff Hutchings, a biology professor at Dalhousie University in Nova Scotia, told Discovery News that "we have known for some time that harvesting can affect the expression of various characteristics." He was surprised, however, by the magnitude of the difference between hunting-caused rates of change and those tied to other causes.
"It is enormous," he said.
Despite the apparent damage to ecosystems, Darimont said people could still turn things around.
"In the context of human predation, if the goal is to minimize or eliminate harvest-induced changes, the key is to greatly reduce exploitation levels and to target the same size and age classes, that is, smaller and younger (prey), as natural predators do," he explained.
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