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Life in the Fast Lane: Human Predators Change the Traits of Prey at Unnatural Speeds

January 13, 2009
Contact: Aimee Dolloff, (207) 581-3777; Michael Kinnison, (207) 581-2575 

ORONO, Maine - As predators, humans drastically are accelerating the rate at which noticeable trait changes occur in wild fish and other harvested species.  Indeed, harvested species are changing at rates over 300 percent faster than seen in more natural circumstances, according to the results of a new study co-authored by University of Maine biology professor Michael Kinnison.

The report, published Jan. 12 in an online issue of the "Proceedings of the National Academy of Sciences," is the first to combine many separate studies of harvested populations into one combined analysis, and it indicates that the impacts of fishing, hunting and plant gathering are far-reaching. Harvested populations are on average 20 percent smaller in body size than previous generations, and their age of first reproduction is on average 25 percent earlier.

The research builds on previous work conducted by Kinnison and colleagues that has assessed rates of trait change driven by other human activities, such as pollution and the introduction of species to new environments.

While the majority of this study focuses on impacts on wild fish populations, other subjects included intertidal invertebrates such as limpets and snails, as well as bighorn sheep, caribou, and two plant species. The study is the first to assess the
pace of trait change in numerous commercially, recreationally, and scientifically harvested organisms and compare those rates to rates associated with other human activities and natural causes.

By harvesting vast numbers and targeting large, reproductively mature individuals, the research team concluded that human predation is quickly reshaping the wild populations that remain, leaving smaller individuals to reproduce at ever-earlier ages.  Such selective harvesting can result in rapid evolution.  Trait changes can also result in part from a process called phenotypic plasticity – the ability of organisms to respond physiologically to environmental conditions within their own life times. Harvesting often changes the environments experienced by harvested species, such as by reducing densities and competition, and these environmental changes can result in phenotypic plasticity.

Scientists have often become stalled in debates on the relative importance of evolution or phenotypic plasticity in cases of harvest-driven trait changes.  

"We hope our study refocuses the field back on the central problem – the traits of harvested population are changing at unnatural rates regardless of the mechanism" says Kinnison.  "Although such fast rates provide new insights into just how fast species can change, we should also be concerned whether those changes are sustainable".

Kinnison notes that shifts to smaller sizes might change species interaction, by making prey species more susceptible to natural predators or by tilting the balance of competition.  Smaller sizes or earlier ages of maturation also often entails fewer offspring of lower quality which could hasten population declines or slow recovery.

"The public knows we often harvest far too many fish, but the threat goes above and beyond numbers," says the paper's lead author Chris Darimont, a postdoctoral researcher in environmental studies at the University of California, Santa Cruz..

"We're changing the very essence of what remains, sometimes within the span of only two decades," he says. "We are the planet's super-predator."

In addition to Darimont and Kinnison, Stephanie M. Carlson, assistant professor of environmental science, policy and management at UC Berkeley; Paul C. Paquet, adjunct professor of environmental design at the University of Calgary; Thomas E. Reimchen, adjunct professor of biology at the University of Victoria; and Christopher C. Wilmers, assistant professor of environmental studies at UC Santa Cruz, also contributed to the study.