A study published last week in the journal Nature Communications caused a big stir by estimating that domestic cats kill up to 25 billion birds and small mammals each year. In fact, it suggests that "free-ranging" domestic cats (this includes both pets that are allowed outside as well as feral cats—though feral cats are responsible for the lion's share of the take) are the single biggest source of anthropogenic mortality for birds and mammals in the United States. In other words, cats do more damage than buildings, cars, pollution, or any of the other hazards that humans have wrought.
Some New Zealand environmental groups consider the toll domestic cats take on wildlife so significant that they're asking the government to take steps to phase out cat ownership completely. (The World Society for the Protection of Animals says New Zealand has the highest percentage of cats per household in the world.)
But cats are not lone actors. Off-leash, free-roaming, and feral dogs do a fair bit of damage to wildlife, too.
In fact, Julie Young, an assistant professor at Utah State University's Wildland Resources department, says canines might munch or harass just as large a slice of the wildlife pie.
An important disclaimer: dog-wildlife interactions have not been nearly as closely or as extensively studied as those of cats, so there is much that researchers do not know about the negative tolls dogs take on wildlife. These tolls are divvied up into three categories: disease spreading, killing/harassing other species, and competing with other species for food.
Young conducted research into the impact that dogs have on three endangered ungulates in Mongolia, which has a significant number of feral and free-roaming dogs (dogs that are free-roaming do have owners who provide them with food and shelter, but they're allowed to move about on their own). In some cases, dogs killed as many as a third of the ungulate populations Young and her partners were studying.
"It’s a different problem in the U.S.," says Young. The conflicts here are in some rural areas, some urban corridors, and in places with large packs of free-roaming or feral dogs, such as Native American reservations. "I've heard of some communities where dogs kill more wildlife than wild animals do," she says. In fact, the findings of a 2010 wolf study suggested that, based on analysis of fecal matter, dogs took more livestock than wolves, which had been falsely blamed for the kills. A 1988 study in New Zealand linked the death of up to 800 kiwis (out of a population of 1,000) to a single dog.
"It's a management issue as much as it is a personal responsibility issue," says Young. "There are laws in 44 states about dogs harassing wildlife, but they're not really enforced."