The domestic cat, Felis catus, has secured a prominent place in American homes, with tens of millions existing as cherished companions. This widespread presence presents a complex duality when considering the animal’s impact outside the household environment. Evaluating this issue requires moving beyond their status as pets to examine their ecological role and the specific criteria used to define an invasive species.
What Defines an Invasive Species
The formal classification of a species as invasive is based on a specific set of ecological and governmental criteria. Under U.S. Executive Order 13112, an invasive species must satisfy two conditions. The first is that the species must be non-native, or alien, to the ecosystem under consideration, meaning it did not evolve within that region.
The second criterion is that the introduction of the species must cause, or be likely to cause, harm. This harm is typically defined as a negative impact on the economy, the environment, or human health. This definition distinguishes a benign non-native species from an invasive species, which actively causes damage to its new environment and threatens native biodiversity.
The History and Scale of Cat Populations in the US
The domestic cat is a non-native species, tracing its ancestry to the Near Eastern wildcat domesticated in the Fertile Crescent thousands of years ago. Cats were first introduced to North America by European settlers, traveling on ships to control rodent populations.
The current scale of the cat population in the US is substantial, but the division of this population fuels the invasive species debate. The number of owned cats kept as pets is estimated to be around 90 million. These owned animals become controversial when they are allowed to roam freely outdoors.
The population that poses the most significant ecological concern is the unowned, free-roaming segment, which includes stray and feral cats. Estimates for this unowned population are highly variable, potentially ranging from 30 million to 80 million individuals. The sheer number of these uncontained, self-sustaining colonies ensures continuous interaction with native wildlife.
Ecological Damage Caused by Feral and Stray Cats
The introduction of cats has had a measurable and severe impact on native wildlife populations, fulfilling the “harm” criterion of the invasive species definition. The primary ecological damage is caused through predation, driven by the cat’s innate hunting instinct, which persists even when the animals are well-fed. Scientific studies estimate that free-ranging domestic cats annually kill billions of animals in the contiguous United States.
Annual mortality estimates range from 1.3 to 4.0 billion birds and between 6.3 and 22.3 billion small mammals. These figures position cats as the largest human-related source of mortality for birds and mammals in the country. Unowned cats, including feral and stray individuals, are responsible for up to 70% of the total wildlife deaths.
Cats also prey on other vertebrates, with estimated annual kills of reptiles ranging from 228 million to 871 million and amphibians between 86 million and 320 million. This sustained, large-scale predation threatens local biodiversity and places pressure on vulnerable species. The domestic cat’s hunting efficiency, coupled with its artificially high population density, has led to its inclusion on the International Union for Conservation of Nature’s list of the 100 worst invasive alien species globally.
The Ongoing Policy Debate Over Classification
The classification of cats as an invasive species is fiercely debated because they are also domesticated animals subject to animal welfare concerns. Many conservation organizations and wildlife management agencies treat free-roaming cats as an invasive threat, advocating for policies that prioritize native species protection. They point to the scientific evidence of predation and the cat’s non-native status as justification for control measures.
Conversely, animal welfare groups often resist the invasive species label, emphasizing the cats’ status as companion animals and promoting non-lethal population control methods. The most common method is Trap-Neuter-Return (TNR), where feral cats are sterilized and then released back into their colonies. Proponents argue that TNR stabilizes populations and is the most humane approach to management.
Conservationists often criticize TNR programs, citing evidence that they are frequently ineffective at reducing colony sizes because of continuous immigration of new individuals, known as the “vacuum effect.” They argue that returning sterilized cats to the environment does not eliminate their predatory behavior or the ongoing risk they pose to wildlife. This fundamental policy disagreement—between lethal removal for conservation and non-lethal management for welfare—defines the current conflict over how to manage the millions of outdoor cats.