"Whatever the underlying process, shifts to earlier breeding spell trouble for populations," said Darimont. "Earlier breeders often produce far fewer offspring. If we take so much and reduce their ability to reproduce successfully, we reduce their resilience and ability to recover."
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, some wildlife and fish management policies contribute to the rapid pace of trait changes. "Fishing regulations often prescribe the taking of larger fish, and the same often applies to hunting regulations," said Darimont. "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." In Alberta, Canada, for example, hunters who are permitted to target the largest specimens of bighorn sheep have caused average horn length and body mass to drop by about 20 percent during the last 30 years.
"Although this is the first study of its kind, we assume this human impact is broad, because our predatory niche is so wide," he said. "While wolves might prey on 20 animals, humans prey on hundreds of thousands of species."
In addition, researchers don't know how these rapid changes will impact larger ecosystems, added Darimont. "Size really matters in nature, in terms of interactions with natural predators and competition for resources," he said. "Will ecological links unravel as exploited species continue to rapidly shrink?"
"This should be a wake-up call for resource managers," he said. "We should be mimicking natural predators, w
|Contact: Jennifer McNulty|
University of California - Santa Cruz