Wildlife aficionados have long known that allowing cats to roam outdoors is a bad idea – they kill millions of small animals annually, and there's even evidence that runoff from their feces is sickening sea otters. But new two new studies show that household pets don't just harm wildlife through these sorts of direct measures: Just having cats and dogs around is enough to scare birds off and reduce breeding success. In June, researchers announced a model showing that the presence of cats could result in wildlife behavior changes and drastic population declines. And this month, scientists in the UK released a study that found bird numbers declined by 40% in areas where people walked their dogs. Though introduced predators like domestic dogs and cats are harming native wildlife populations, the "ecology of fear" works both ways, having positive ecological effects where native predators are allowed to roam: Reintroduced wolves in Yellowstone have scared elk away from aspen groves, allowing young trees to thrive for the first time in 70 years.

Sources: Dog walks prompting bird flight. BBC News, September 4, 2007; Nick Atkinson. Felix elicits avian response. Journal Watch Online, June 1, 2007; William Ripple. Presence of wolves allows aspen recovery in Yellowstone. Oregon State University Press Release, July 26, 2007. Photo courtesy of the American Water Spaniel Club.