A recent study published in Nature Communications estimates that free-ranging domestic cats in the United States kill between 1.4 and 3.7 billion birds and 6.9 to 20.7 billion mammals each year – 2.8 times higher than previously projected. Un-owned outdoor cats (as opposed to owned pets) cause 89 percent of this mortality, according to the study. The findings raise serious questions about the impacts of outdoor cats on wildlife conservation.
The study’s authors – Scott Loss and Peter Marra of the Smithsonian’s Migratory Bird Center and Tom Will of the U.S. Fish and Wildlife Service (FWS) Division of Migratory Birds – place the U.S. cat population at roughly 84 million owned cats plus a range of 30-80 million un-owned feral and colony cats (those fed and sheltered in outdoor colonies).
Loss and his team conducted their research as a meta-study, gathering data from 16 previously published studies on cat population estimates and wildlife predation by cats and then analyzing the larger dataset. “Most of the studies that we used did not distinguish the types of un-owned cat groups,” said Loss. Un-owned cats include feral cats that are not habituated to humans, strays that are habituated, barn cats that live permanently outdoors, and cats fed in colonies maintained by volunteers (sometimes called trap-neuter-return or TNR colonies). Loss and his colleagues hope to do further work to identify which subcategories of un-owned outdoor cats are predating the most wildlife, where hotspots of predation are occurring, and whether those areas overlap with habitat for at-risk wildlife species.
Such data could prove useful for wildlife biologists and communities hoping to manage or curtail outdoor cat populations. “A major reason for the current non-scientific approach to management of free-ranging cats is that total [wildlife] mortality from cat predation is often argued to be negligible compared with other anthropogenic threats, such as collisions with man-made structures and habitat destruction,” wrote the study’s authors. Yet free-ranging cats “are likely the single greatest source of anthropogenic mortality for U.S. birds and mammals,” they wrote. Because of the extreme controversy generated by discussions about cat management, Loss and his team couldn’t comment on the management implications of their study, but did say that they hope the study will encourage greater research into management solutions.
The Search for Solutions
Others share that hope. Grant Sizemore, the Cats Indoors Program Officer with the American Bird Conservancy (ABC), said the study’s results could perhaps be used as an educational tool to inform the public of the risk of predation by outdoor cats. ABC encourages cat owners to keep cats indoors and recommends that outdoor cats be trapped, neutered, and adopted. “Those cats that cannot be adopted,” said Sizemore, “should be placed in a well-managed outdoor enclosure” – one that prevents cats from roaming freely to prey on wildlife.
Across the U.S., various wildlife conservation, advocacy, and research groups have developed or advocated management models for controlling feral and outdoor cat populations. Niels Pedersen, Director of the Center for Companion Animal Health at the University of California-Davis, advocates a trap-neuter-return program and encourages community members to not feed stray or feral cats or put out garbage that may encourage the growth of cat populations.
Pinellas County, Florida, tried a regulatory approach, passing a leash law in 1987 requiring cat owners to leash cats that are let outside. The ordinance also states that if a community member begins feeding a stray cat and doesn’t report it as a stray, the person has assumed ownership of the animal and therefore must keep it on a leash when it is outdoors. Passage of the ordinance was supported by the Board of County Commissioners, the University of Florida College of Veterinary Medicine, and several Pinellas county animal advocacy groups that were concerned for the welfare of a growing population of stray and feral cats in one of the most densely populated counties in the state.
Many researchers and managers are not convinced that TNR and other current management tools are effective or without drawbacks. “Stray and feral cats kill wildlife – I see it every day – and management is not going to prevent predation of other species,” said John Hohenstern, senior animal control officer for Pinellas County, whose team enforces the cat leash policy. Hohenstern said that cats quickly learn to avoid traps, for example, so even if a cat is initially trapped, neutered, and released, it is nearly impossible to catch it again if it needs vaccination or other treatment. For that reason, “management is a fallacy,” said Hohenstern.
Pedersen points to a different problem: the often-overlooked ecological consequences of removing cats from landscapes where they have existed for centuries. “What people don’t understand is that cats are the dominant carnivore in almost all human-oriented ecosystems,” he said. “Every attempt to take cats out of the equation has led to disastrous ecological shifts as far as buildup of rodents as well as other over-populated species.”
Pedersen is also somewhat skeptical of the recent Loss et al. study. “I’m not saying their conclusions aren’t correct,” he said, “but meta-studies often start with a preconceived hypothesis and then cherry pick various published research studies to yield a preconceived conclusion. Such studies have often been proven incorrect upon more in depth field research.” Loss said that the results could be refined by increased field research.
To that end, Loss and his colleagues are brainstorming a database to track cat predation deaths among mammal, bird, and reptile populations. For birds, “it would be ideal if bird mortality data was collected with the same level of rigor that live bird data is collected; this would improve the accuracy and precision of mortality estimates. For example, a major advance would be to develop an online mortality reporting database that is accessible by researchers as well as the general public (this would be analogous to eBird, which gathers live bird observations from across the world),” said Loss. There are also plans to examine cat predation of reptiles and amphibians.
The Wildlife Society has produced news articles, fact sheets, and a position statement on TNR programs and other issues relating to feral cat management. To learn more, please visit the following:
Facts on Feral Cats
Position Statement on Feral Cats
Invasive Species Working Group