The abrupt removal of a prolific predator from an ecosystem initiates a cascade of changes that ripple through the entire food web. Domestic and feral cats (Felis catus) are present globally and act as non-native mesopredators in many local environments, exerting a sustained, significant predatory pressure. Understanding the ecological consequences of withdrawing this specific, high-volume predation is crucial for managing biodiversity in both urban and wild spaces. The sudden absence of cats disrupts the established balance, leading to predictable yet complex shifts in the populations of prey, plants, and competing carnivores.
Defining the Cat’s Role in the Ecosystem
The domestic cat is classified as a generalist predator, meaning it hunts a wide variety of prey rather than specializing in one or two species. This broad diet includes small mammals, birds, reptiles, amphibians, and insects, making its ecological footprint expansive across multiple lower trophic levels. In most mainland environments, the cat functions as a mesopredator, a mid-level predator whose own population is often controlled by larger carnivores or humans. However, in isolated environments like islands, where native predators are often absent, the cat can effectively assume the role of an apex predator. The sheer volume of consumption defines their ecological impact, with estimates suggesting that free-roaming cats kill billions of birds and mammals annually in the United States alone.
Immediate Effects: The Release of Prey Populations
The first and most direct consequence of cat removal is the immediate survival benefit for the species they primarily target. This phenomenon is known as “prey release,” where populations previously suppressed by predation experience a surge in numbers due to increased survival rates for both adults and juveniles. Small mammals, such as rats and rabbits, are often the first to exhibit this population explosion because of their high reproductive capacity and the sudden drop in mortality. For instance, on islands where cat eradication has been attempted, invasive rat populations can rapidly increase, sometimes resulting in a higher predation threat to native wildlife than the cats originally posed.
Ground-nesting and juvenile birds also benefit from the cessation of cat predation. Studies show that even well-fed pet cats hunt, and their predation rates can be high relative to the annual productivity of certain songbird species. The removal of this threat increases the number of young birds surviving to maturity, allowing local populations to rebound quickly. On Little Barrier Island in New Zealand, the population of the stitchbird increased significantly within a few years following the removal of cats.
Secondary Impacts: Trophic Cascades and Vegetation Change
The increase in herbivorous prey populations, such as rabbits and certain rodent species, initiates a complex, indirect consequence known as a trophic cascade. A trophic cascade describes how changes at one level of the food web—in this case, the removal of the predator—can propagate down to affect non-adjacent levels, like the plant community. When herbivores are released from predation pressure, their population density can become so high that their feeding habits begin to alter the landscape’s vegetation structure. This overabundance leads to increased herbivory and grazing pressure on plant life, which is particularly evident in sensitive ecosystems.
A clear example of this occurred on Macquarie Island, where the successful eradication of cats led to a massive increase in the rabbit population. The resulting overgrazing by the exploding rabbit numbers caused significant defoliation, fundamentally changing the composition of the island’s plant communities and leading to land degradation. This intense pressure on vegetation reduces seed dispersal and can eliminate plant species preferred by the herbivores, effectively converting complex ecosystems into simpler, more heavily grazed landscapes. Furthermore, a high density of rodents can increase seed predation, reducing the germination success of native flora.
Effects on Competing Predators and Scavengers
The removal of cats significantly impacts other animals that share the same ecological niche, primarily through reduced competition for food resources. Native predators, such as small raptors, foxes, weasels, and small wildcats, benefit from the immediate availability of the newly released prey populations. These animals now have access to a larger food base without the resource competition previously presented by the dense and widespread cat population. This can lead to an increase in the abundance or a change in the foraging behavior of native carnivores, which may then exert a new level of control over the rebounding prey.
The dynamics of the scavenger community are also affected by the cat’s absence. Cats are both predators and scavengers, and their removal alters the use of carrion in the ecosystem. The reduction of cats as competitors for carcasses increases the access of native avian and mammalian scavengers to available food sources. Conversely, the removal of a dominant carnivore can sometimes result in increased scavenging by other mesoscavengers, which may fill the vacated role. The overall effect is a shift in community structure, favoring native species that are no longer competing with a large, non-native predator.