There's no denying that cats can be adorable. But feral and stray cats are wreaking havoc on the global ecosystem. Here's why it's bad to leave domesticated cats running free — and what scientists and management officials are doing about it.

People have long believed that domesticated cats got their start in ancient Egypt, given the felines' prominence in the early civilization. However, recent genetic and archaeological evidence shows that the animals were originally domesticated some 10,000 years ago in the Near East, when agriculture first got underway. At the time, people in the area had just domesticated wheat, rye and barley, which likely resulted in an influx of rodents. The settlers probably welcomed the arrival of the then-wild cats into their villages to control the pests, or so the theory goes.

Since then, cats have lived with human populations and are now one of the most popular pets in homes throughout the world. Of course, cats don't always stay in the home. According to the American Society for the Prevention of Cruelty to Animals (ASPCA), these cats fall into three categories: free-roaming pet cats, stray cats and feral cats. Stray cats are pets that were abandoned or lost — they are typically friendly, can be seen out during the day and will usually be alone. But after living out on the streets for a while, stray cats can become feral cats, which are mostly nocturnal, wary of humans, and live in colonies with other feral cats.


If they are not spayed, wild female cats will give birth to feral kittens. These cats will go through puberty usually at 8 to 10 months old, and give birth to one to five babies after just 9 weeks — and can get pregnant again while still nursing the new litter. With their quick reproduction cycle and natural street smarts, it's easy to see why experts estimate there are 70 million or more feral cats in the U.S. alone.

And this is not a good thing.


Bad Kitty!

One of the biggest issues with wild domesticated cats (ferals, strays and free-roamers) is the way they impact other wildlife. In a widely reported study in Nature Communications earlier this year, researchers estimated that cats kill a whopping 1.4 to 3.7 billion birds in the U.S. each year. Only about a third of the birds killed are non-native (meaning: most of the birds killed are native species).

On top of this, the authors estimate that the cats also kill 6.9 to 20.7 billion mammals, which include rabbits, squirrels, shrews and voles, in addition to mice and rats. Based on data from other countries, they think cats may kill 95 to 299 million amphibians and 258 to 822 million reptiles in the U.S. each year. And, for the most part, it's not our lovable house pets that are to blame (though they are stone-cold killers in their own right). "Un-owned cats, as opposed to owned pets, cause the majority of this mortality," the scientists write in their study.


But it's not just the U.S. that is dealing with feral cat issues — the IUCN actually lists domesticated cats as one of the world's 100 worst alien invasive species (pdf). Research has shown that these animals are especially detrimental when they are introduced to island ecosystems. In 2011, researchers estimated that island feral cats were behind 14 percent of global bird, mammal and reptile extinctions. The authors claim that the felines are also the biggest threat to nearly 8 percent of critically endangered birds, mammals and reptiles in island ecosystems.

Some specific examples: In the Mediterranean, cat predation could lead to the extinction of the endemic Yelkouan Shearwater (Puffinus yelkouan). In Hawai‘i, feral cats pose a major threat to endangered birds (pdf), such as the palila (Loxioides bailleui) and the ‘ua‘u (the Hawaiian Petrel, Pterodroma sandwichensis). In Australia, gigantic feral cats have appeared and may kill some 75 million native animals every day (pdf) — though a recent study suggests that black rats are the biggest threat to native Australian species.


Aside from the danger cats pose to native wildlife, the animals may also be a health risk for humans, according to recent reports. Last month, scientists estimated that cats deposit some 1.2 million metric tons of feces into the environment. The feces often carry the infectious parasite Toxoplasma gondii, which has been linked to schizophrenia and suicide. And earlier this month, a study by researchers from the CDC suggests that feral cats pose a rabies risk to people, despite the efforts of trap–neuter–vaccinate–return programs.


Oh, and then there are those cases when cat gangs maul unsuspecting people (though, to be fair, 885,000 people require medical attention from dog bites each year in the U.S.).


Good Solutions?

There are a few different approaches out there to reduce the number of feral cats.

One option is eradication. Recently on Ascension Island (off the coast of South America), management officials put into effect an eradication program involving traps and poison. The program effectively reduced feral cat populations, promoting the recolonization of seabirds, but it was not without issues — it killed 69 pet cats.


In 2004, a team of researchers reviewed the island eradication programs carried out till then around the world, and found that feral cats had been successfully removed from 48 islands. The most successful programs, they found, were trapping and hunting (often with dogs), but other methods involved poisoning, secondary poisoning from rats and viral diseases. Though the programs were effective, the authors stressed that new techniques would be needed to get rid of the cats on larger islands.

More recently in 2011, other scientists conducted another review and found 87 successful campaigns on 83 islands (pdf). The programs used an average of 2.7 methods, which included leg-hold traps, hunting, primary poisoning, cage traps and dogs. They also found 19 eradication programs that failed due to lack of planning, inappropriate timing and methods.

Killing seems to be an effective way to rid islands of cats, but the authors note that complete eradication can have unexpected ecosystem consequences. For example, the loss of cats on Little Barrier Island left Cook's petrels vulnerable to the Pacific rat, and cat eradication on World Heritage Island allowed rabbit populations to explode and devastate the island's vegetation (though other scientists say there were other contributing factors involved).


Of course, eradication raises concerns among animal welfare groups. For feral cat advocates, the preferred solution is trap-neuter-return (TNR) programs. As its name suggests, this approach involves capturing feral cats, spaying or neutering them and then returning them to the wild and caring for them — without the ability to reproduce, cat colonies will eventually decrease, the reasoning goes.

Such programs have sprung up all over the place and advocates point to selected studies that suggest that TNR programs do work. However, other people counter that there are issues with those studies. A 2009 critical assessment of TNR programs noted (pdf):

Nevertheless, the definition of a successful TNR program for feral cat advocates is almost always different from what a conservation biologist or policy maker might view as a successful feral cat management program. Reduced adverse effects on wildlife and rapid colony elimination are almost never included in the definition of success used by advocates (e.g., No Kill Advocacy Center 2006a). For many TNR advocates, success is not defined by elimination of feral cats in an area, but rather by the welfare of the cats…


A group of conservation biologists has since applauded the assessment (pdf), and added that other conservation biologists need to be more aware of the issues and get more involved, in much the same way that evolutionary biologists have gotten involved to oppose teaching creationism and intelligent design in schools. They also write that conservation biologists need to have a more open dialogue with animal welfare groups to come up with better solutions for the feral cat programs.

One of these solutions, perhaps, may be the trap-vasectomy-hysterectomy-release (TVHR) program. Earlier this month, researchers used computer models to compare the effectiveness of TVHR, TNR and lethal control at decreasing the size of feral cat populations — TVHR is far better than the other two methods, they found.


The researchers say the technique's effectiveness lies in the fact that the cats continue producing their reproductive hormones after the procedure. Male cats' lifespan, sexual drive and social status are not affected by vasectomy, so they will continue to fend off new males who try to join the colony. Additionally, a fertile female who mates with a vasectomized male will undergo a 45-day pseudo-pregnancy period, further decreasing the production of new offspring in the cat colony.

Though the approach seems promising, the researchers only looked at how well TVHR could reduce cat populations over time, and didn't simulate how those decreases would affect the ecosystem at large (which is important for islands, as we saw above). It also still needs to be tested in the real world. In all likelihood, no single program will ever be able control feral cat populations; such a feat will require the combination of different techniques and the cooperation of different groups.

Top image via Martin Lopatka/Flickr. Inset image via jurvetson/Flickr.