Domestic cats have been a primary driver of at least 33 documented extinctions of birds, mammals, and reptiles worldwide, according to the International Union for Conservation of Nature (IUCN) Red List, a figure that accounts for roughly 14% of all modern vertebrate extinctions the organization has recorded.1Nature Publishing Group. The impact of free-ranging domestic cats on wildlife of the United States Nearly all of those losses occurred on islands, where small, isolated populations of species that had never encountered a mammalian predator were suddenly exposed to one of evolution’s most efficient hunters. The number itself, though widely cited, is almost certainly an undercount, because many extinctions go unrecorded and the indirect ways cats suppress wildlife populations are only beginning to be measured.
Where the “33 Species” Figure Comes From
The number traces to a systematic review of IUCN data linking free-ranging cats to extinctions of insular endemic vertebrates. That review, published in the journal BioScience, identified cats as a primary extinction driver for at least 33 island-endemic species of birds, mammals, and reptiles.2BioScience. Feral Cats and Biodiversity Conservation: The Urgent Prioritization of Island Management The figure was subsequently cited in a landmark 2013 study in Nature Communications estimating the total wildlife toll of free-ranging cats in the United States, which placed the island extinction count at 33 species, or about 14% of all modern bird, mammal, and reptile extinctions on the IUCN Red List.1Nature Publishing Group. The impact of free-ranging domestic cats on wildlife of the United States
The list spans three classes of vertebrates. The majority are birds, which makes sense given how many island bird species evolved without ground-level predators and therefore lacked escape behaviors suited to a stalking carnivore. Mammals and reptiles account for the remainder. Some of these extinctions are clear-cut, with cats identified as the sole or overwhelming cause. Others involve cats acting alongside rats, habitat loss, or other introduced predators, making it harder to isolate exactly how much blame falls on the cat. In every case, though, the IUCN assessment judged cats to be a primary contributing factor rather than a background player.
The Stephens Island Wren and Other Well-Known Losses
The most famous story in cat-driven extinction involves the Stephens Island wren, a tiny flightless songbird found only on a small island in New Zealand’s Cook Strait. The popular version holds that a single lighthouse keeper’s cat wiped out the entire species in the 1890s. The reality is a bit more complicated: cats became established on Stephens Island in 1894, multiplied rapidly, and exterminated the wren along with several other species before the cat population was eventually eliminated. Cat predation was probably the main factor in the wren’s extinction, but it was not necessarily the work of one individual cat.3Notornis. The tale of the lighthouse-keeper’s cat: Discovery and extinction of the Stephens Island wren (Traversia lyalli)
Beyond the Stephens Island wren, the list of cat-caused extinctions reads like a catalog of island isolation. It includes birds from remote Pacific atolls, Caribbean islands, and subantarctic territories. Several were rails, a family of birds prone to evolving flightlessness on predator-free islands. Others were petrels and shearwaters whose ground-nesting habits made them easy targets for nocturnal hunters. Among the mammal losses are small island rodents and insectivores whose populations were too tiny to absorb sustained predation. The reptile extinctions include lizards and skinks from islands where cats were introduced centuries ago, sometimes deliberately to control rats on ships.
Why Islands Are the Epicenter
The concentration of cat-caused extinctions on islands is not a coincidence. Island species typically evolve in the absence of mammalian predators, and this leaves them with a profound vulnerability. A ground-nesting bird that has never encountered anything resembling a cat does not flee, freeze, or fight when one approaches. Researchers call this “prey naiveté,” and it is recognized as a key factor in the extinction and endangerment of prey species when introduced predators arrive.4Journal of Applied Ecology. Reversing the effects of evolutionary prey naiveté through controlled predator exposure Island populations are also small by definition. A species restricted to a single island of a few square kilometers may have a total population of a few hundred or thousand individuals. That population can vanish in a matter of years under sustained predation.
Habitat structure compounds the problem. Cats are substantially more effective hunters in open landscapes. Research using animal-borne video cameras showed that once a cat initiated a hunt, the kill success rate was about 70% in open habitat compared with roughly 17% in areas with dense ground cover.5PubMed Central. Feral Cats Are Better Killers in Open Habitats, Revealed by Animal-Borne Video Many island habitats, particularly atolls and volcanic grasslands, offer sparse cover. A flightless bird or ground-nesting seabird in such a landscape is at extraordinary risk.
How Cats Reached These Islands
Cats did not swim to remote islands on their own. Their global spread was a human project. Historical records link the introduction of cats to islands across the Pacific, Indian, and Atlantic Oceans to European exploration and the maritime industries that followed, including whaling, sealing, and pearl harvesting in the 18th and 19th centuries.6PubMed Central. A voyage to Terra Australis: human-mediated dispersal of cats Ships carried cats to control rats in their holds. When the ships anchored or wrecked, cats went ashore and established feral populations. In some cases, settlers deliberately released cats on islands to deal with rodent problems, not anticipating what else the cats would eat. That pattern played out on hundreds of islands worldwide, setting the stage for the extinctions that followed.
Continental Impacts and the Australian Debate
While the 33 documented extinctions are overwhelmingly an island phenomenon, cats have also played a serious role in species decline on continents, most dramatically in Australia. Since European settlement, Australia has lost more native mammal species than any other continent, and the prevailing explanation has long been that predation by feral cats and European red foxes, combined with changed fire regimes, was the primary cause.7PubMed Central. Ongoing unraveling of a continental fauna: decline and extinction of Australian mammals since European settlement Australia’s native mammals evolved alongside marsupial predators but had no experience with placental carnivores like cats, creating a continental-scale version of the same naiveté problem found on islands.
That narrative, however, has recently been challenged. A 2025 review found that for most threatened species attributed to foxes and cats, the claim rested on expert opinion rather than rigorous population studies. Records for about 20% of populations attributed to cat impacts did not confirm that the declines occurred after cats arrived in those areas, and the researchers found no statistical correlation between cat abundance and threatened mammal populations. They concluded that the hypothesis that cats and foxes cause mammalian extinctions in Australia has been accepted with surprisingly little direct evidence.8BioScience. Review of evidence that foxes and cats cause extinctions of Australia’s endemic mammals This does not mean cats are harmless on the continent, but it does suggest the picture is muddier than the standard story implies, and that other factors like land clearing and altered fire patterns may deserve more attention than they have received.
Threats Beyond Direct Killing
The 33 documented extinctions reflect deaths from predation, but cats threaten wildlife in ways that never involve claws. One of the most insidious is disease transmission. Cats are the only animals that shed the environmentally resistant form of the parasite Toxoplasma gondii, and the oocysts they deposit in soil and waterways can travel remarkable distances. In coastal California, Toxoplasma washed from land into the ocean has become a significant cause of mortality in threatened Southern sea otters.9PubMed. Toxoplasma gondii, source to sea: higher contribution of domestic felids to terrestrial parasite loading despite lower infection prevalence Atypical strains of the parasite have been linked to severe disease in both otters and people in those ecosystems.10PLOS Neglected Tropical Diseases. Using Molecular Epidemiology to Track Toxoplasma gondii from Terrestrial Carnivores to Marine Hosts: Implications for Public Health and Conservation
Domestic cats also act as a reservoir for bacterial and viral pathogens that spill over into wild felids. Research on an erythrocytic bacterial parasite found that domestic cats appear to be a source of global dispersal and spillover to wild cat species, primarily through predation-related contact.11PubMed Central. Transmission pathways and spillover of an erythrocytic bacterial pathogen from domestic cats to wild felids In the United States, pumas living near suburban edges showed increased exposure to pathogens like Bartonella species and feline herpesvirus, suggesting that the interface between urban cat populations and wild habitats is a zone of disease exchange.12PubMed. Pathogen exposure varies widely among sympatric populations of wild and domestic felids across the United States
Then there is what researchers call the “landscape of fear.” Even when cats are not killing birds, their mere presence suppresses wildlife reproduction. An experiment showed that the brief appearance of a cat model near songbird nests reduced parental feeding rates by about a third, with no compensatory increase in food load per trip. That magnitude of reduced feeding would cut nestling growth rates by roughly 40%. Worse, the alarm behavior parents displayed while reacting to the cat attracted corvids, which increased daily nest predation rates by an order of magnitude.13Journal of Applied Ecology. Fearing the feline: domestic cats reduce avian fecundity through trait‐mediated indirect effects that increase nest predation by other species Modeling work suggests that even when cats kill fewer than 1% of birds directly, the fear-induced reduction in breeding success can push urban songbird populations down dramatically at high cat densities.14Animal Conservation. Urban bird declines and the fear of cats These sublethal effects are hard to count in extinction tallies, but they suppress populations below their carrying capacity and may push already-vulnerable species closer to the edge.15Frontiers in Ecology and the Environment. Population impacts of free‐ranging domestic cats on mainland vertebrates
Reptile Impacts Are Underappreciated
Most research on cat-caused wildlife loss focuses on birds and mammals, but the toll on reptiles is growing clearer. Experimental work in Australian tropical savannas found that when feral cats were excluded from plots, reptile abundance increased at twice the rate seen in areas where cats could roam freely.16Biological Conservation. Rapid increase of Australian tropical savanna reptile abundance following exclusion of feral cats The researchers noted that cat predation on squamates and amphibians has received far less scientific attention than predation on mammals and birds, particularly in continental settings. Given how many islands host endemic lizard and gecko species, it is likely that the 33 documented extinctions underrepresent losses among reptiles simply because these animals are harder to study and monitor.
The Mesopredator Puzzle
Removing cats from islands seems like an obvious conservation win, but ecology rarely cooperates with simple plans. On some islands, cats exist alongside smaller invasive predators like rats. When cats are removed, rat populations can explode in the absence of their predator, a phenomenon called mesopredator release. On Little Barrier Island in New Zealand, the eradication of cats led to reduced breeding success of Cook’s petrels, which turned out to be vulnerable to predation by Pacific rats that had been partially kept in check by the cats.17PubMed Central. Spatial heterogeneity of mesopredator release within an oceanic island system A global analysis of island mammal extinctions confirmed the pattern: extinction probabilities were actually lower on islands that had both rats and a larger predator like a cat than on islands with rats alone.18Global Ecology and Biogeography. Island mammal extinctions are determined by interactive effects of life history, island biogeography and mesopredator suppression
This does not mean cat removal is a bad idea. It means it needs to be done alongside rat control or other invasive species management. Conservation planners have learned this lesson the hard way and now typically design multi-species eradication campaigns rather than targeting cats in isolation.
What Happens After Cats Are Removed
When eradication campaigns are well-designed, the results can be remarkable. Reviews of feral cat eradication on islands document clear biodiversity benefits, with native species recovering once the predation pressure lifts.19Conservation Biology. A Review of Feral Cat Eradication on Islands On Ascension Island in the South Atlantic, the removal of feral cats led five seabird species to recolonize the mainland in small but growing numbers, re-establishing nesting populations that had been wiped out centuries earlier.20Oryx. The eradication of feral cats from Ascension Island and its subsequent recolonization by seabirds Success stories like this demonstrate that while the 33 already-extinct species cannot be brought back, many species currently teetering on the edge can be saved if cats are dealt with in time.
Eradication campaigns have been completed on islands of increasing size and logistical complexity over the decades. The techniques involve trapping, hunting with trained dogs, and sometimes targeted toxicants. Each island presents unique challenges, from terrain to non-target species to the sheer density of the cat population. But the growing track record of success is pushing conservationists to attempt eradications on islands that would have been considered impossibly large a generation ago.
Genetic Erosion of Wild Cat Species
One threat that does not appear in the extinction count but concerns conservationists is hybridization. Where domestic cats overlap with wild cat species, interbreeding can erode the genetic identity of the wild population. The European wildcat, already represented by fragmented and declining populations across the continent, faces serious genetic threats from crossbreeding with free-ranging domestic cats.21Conservation Genetics. Molecular analysis of hybridisation between wild and domestic cats (Felis silvestris) in Portugal: implications for conservation Over generations, the wild population absorbs domestic cat genes and the distinction between the two blurs. Scotland, continental Europe, and parts of Africa and Asia all face versions of this problem. It is a different kind of extinction — not the disappearance of individuals but the quiet dissolution of a species’ genetic distinctiveness.
Reducing the Toll From Pet Cats
For people who own cats and care about wildlife, the practical question is what actually works to limit hunting. Keeping cats indoors is the most effective intervention, but for owners who allow outdoor access, several tools have been tested in controlled trials.
Brightly colored collar covers, often called BirdsBesSafe covers, reduced the number of birds brought home by roughly 36 to 42% across studies in both continental Europe and the United Kingdom.22Frontiers in Ecology and Evolution. Colorful Collar-Covers and Bells Reduce Wildlife Predation by Domestic Cats in a Continental European Setting 23Current Biology. Diet and object play reduce predation on wildlife by domestic cats in a controlled trial The covers work by making cats more visible to birds, which rely on color vision to spot threats. They did not, however, reduce catches of mammals, which rely more on hearing and smell.
Bells produced mixed results across studies. One trial found that belled cats brought home roughly half as many prey items as unbelled cats.24Journal of Zoology. Bells reduce predation of wildlife by domestic cats (Felis catus) Other trials found no discernible effect of bells on birds, though they did reduce mammal catches in at least one European study.22Frontiers in Ecology and Evolution. Colorful Collar-Covers and Bells Reduce Wildlife Predation by Domestic Cats in a Continental European Setting The inconsistency may reflect differences in prey communities, hunting styles, or how quickly individual cats learn to move without jingling.
One of the more surprising findings came from a dietary intervention. Switching cats to a high-protein, grain-free food reduced prey brought home by about 36%, and just five to ten minutes of daily interactive play cut it by 25%.23Current Biology. Diet and object play reduce predation on wildlife by domestic cats in a controlled trial The implication is that some hunting by well-fed pet cats may be driven by nutritional gaps or unstimulated predatory instincts rather than hunger per se. Puzzle feeders, by contrast, actually increased prey brought home by about a third in the same trial, possibly because they heightened foraging motivation without satisfying it.
The Trap-Neuter-Return Controversy
For feral cat populations on mainlands, the most contentious management question is whether trap-neuter-return (TNR) programs are an adequate conservation tool. Advocates of TNR often argue that feral cats harm wildlife only on islands, that they fill a natural ecological niche, and that managed colonies resist invasion by new unsterilized cats. A critical review of the scientific literature found that each of these claims is contradicted by the available evidence.25PubMed. Critical assessment of claims regarding management of feral cats by trap-neuter-return Sterilized cats still hunt, still carry and transmit diseases, and colonies rarely shrink to zero because new cats continually join from surrounding areas. The debate remains heated because it pits wildlife conservation goals against animal welfare concerns, and the populations of people who care deeply about each overlap substantially.
Where TNR may have a role is in urban settings where full removal is logistically or politically impossible and where the conservation stakes are lower than on islands harboring endemic species. But in areas adjacent to populations of threatened wildlife, the evidence suggests that TNR alone does not reduce the ecological damage cats cause.