"Any predator – as an agent of natural selection – can shape the traits of its prey," Chris Darimont of the University of California and University of Victoria told environmentalresearchweb. "Because we take so much and target the largest, our predation is changing the very essence of the individuals that remain, and very quickly."

Together with colleagues from the University of California, US, University of Maine, US, University of Victoria, Canada, and University of Calgary, Canada, Darimont looked at 40 systems of exploited prey, including fish, ungulates (hoofed animals), invertebrates and plants. Human harvesting caused an average decline of almost 20% in size-related traits and shifts in life-history traits of nearly 25%. And it led to changes 50% faster than other human influences such as pollution or introducing species to islands.

"I don't think we knew how fast vertebrates and invertebrates were capable of changing in the wild," added Darimont. "It appears that predation by humans has awoken the latent ability of organisms to rapidly undergo trait change in the wild."

According to Darimont, the rapid shifts to smaller sizes and earlier breeding the researchers observed could lead to similarly rapid declines in reproductive output of populations. "Smaller and younger females often produce offspring in lower numbers and of lesser quality, especially in fishes," he explained.

Such a trend could limit the ability of populations to recover from the high levels of harvest that are so common, especially in fisheries. It could also affect food-web relationships between harvested and non-harvested organisms, which appear to be changing more slowly.

"Take body size for example – it is critical in maintaining interactions among species like competition and natural predation," said Darimont. "If an exploited fish is shrinking rapidly in size, its links to competitors and its natural predators might be severed. What might happen if these relationships unravel is anyone’s guess, but I suspect it would not be a good thing for the function of the ecosystem."

And how might these changes interact with other environmental pressures? "We can only imagine that they might conspire to form a bigger problem than that predicted by the sum of the threats independently," said Darimont. "For example, theoretical insights from quantitative genetics inform us that continued strong directional selection (which 'harvest selection' imposes) erodes genetic diversity. With decreased diversity, populations are less able to cope with changes to their environment such as habitat degradation and climate change."

Now the team is using time series analyses to find out exactly how the traits of exploited populations have changed over the periods of exploitation. "We are asking questions like, 'did technological advances that allowed for greater harvest elicit particularly rapid phases of change for populations?'," said Darimont.

The researchers reported their work in PNAS.