Jan. 13, 2009
Humans as ‘super-predators’ driving evolution
Hunting and fishing are having broad, swift impacts on the body size and reproductive abilities of fish, big game and other commercially-harvested species, potentially jeopardizing the ability of entire populations to recover, according to the results of a new study co-authored by University of Calgary adjunct professor Paul Paquet.
Not only fast, the changes are also dramatic in magnitude: 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, according to Paquet, a biologist and adjunct professor in the Faculty of Environmental Design, and co-authors from universities across North America.
"Harvested organisms are the fastest-changing organisms of their kind in the wild because we are super-predators and we take such high proportions of a population and target the largest individuals," Paquet said. "It's an ideal recipe for rapid trait change and it can be seen in a wide range of species from bighorn sheep in Alberta to commercial fisheries on the coasts."
The study titled “Human predators outpace other agents of trait change in the wild” is the first to calculate the pace of evolution in commercially-harvested organisms and compare the rates to other systems. The team calculated the rates of trait change with a metric appropriately called the 'Darwin,' which allowed the comparison of changes across traits and species among natural and human-modified systems, including 'human predator' systems. It builds on research by coauthor Michael Kinnison and colleagues that has documented the evolutionary impact of other human activities, such as pollution and the introduction of species to new environments.
The findings are based on a meta-analysis of 34 scientific studies that tracked 29 species in a total of 40 specific geographic systems. The bulk of the studies focused on impacts on fish populations, but other subjects included intertidal invertebrates such as limpets and snails, as well as bighorn sheep, caribou, and two plant species: Himalayan snow lotus and American ginseng.
Paquet and his co-authors’ findings also dramatically increase scientific understanding of the capacity of organisms to change. Some observed trait changes likely represent underlying genetic changes passed on from one generation to the next. 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. For example, shifts to earlier reproduction can occur because of an abundance of food being shared by a much smaller population of fish.
Whereas such plastic changes might be readily reversed if exploitation stops, this is likely not the case with genetic changes. For example, commercial fishing has devastated the number of Atlantic cod on the eastern coast of Canada, where cod used to first reproduce at the age of six years. They now reproduce at an average age of five years, a shift that occurred in less than two decades.
Ironically, Paquet said some wildlife and fish management policies contribute to the rapid pace of trait changes, such as declining horn size in Alberta’s bighorn sheep.
"Hunters are instructed not to take smaller animals or those with smaller horns. This is counter to patterns of natural predation, and now we're seeing the consequences of this management," he said. “Average horn length and body mass has dropped by about 20 percent during the last 30 years.”
Paquet and his colleagues say their findings provide lessons for natural resource managers and that hunting and fishing regulations should be made to reflect natural predation behaviour by targeting less and smaller individuals.
“This study shows that species we harvest are undergoing abrupt and significant changes that may not be reversible. Many industries are likely harvesting away future bounties through the decisions they are making right now.”
Paquet's coauthors on the paper are Chris T. Darimont a postdoctoral researcher in environmental studies at the University of California, Santa Cruz; Stephanie M. Carlson, assistant professor of environmental science, policy and management at UC Berkeley; Michael T. Kinnison, associate professor of biology and ecology at the University of Maine; 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.
The paper “Human predators outpace other agents of trait change in the wild” in the Proceedings of the National Academy of Sciences (PNAS) can be found online at: http://www.pnas.org/