“Although cats make affectionate pets, many domestic cats hunt as effectively as wild predators. However, they differ from wild predators in three important ways: First, people protect cats from disease, predation and competition, factors that can control numbers of wild predators, such as bobcats, foxes, or coyotes. Second, they often have a dependable supply of supplemental food provided by humans and are, therefore, not influenced by changes in populations of prey. Whereas populations of native predators will decline when prey becomes scarce, cats receiving food subsidies from people remain abundant and continue to hunt even rare species. Third, unlike many native predators, cat densities are either poorly limited or not limited by territoriality. These three factors allow domestic cats to exist at much higher densities than native predators. In some parts of rural Wisconsin, densities of free-ranging cats reach 114 cats per square mile. In these areas, cats are several times more abundant than all mid-sized native predators (such as foxes, raccoons, skunks) combined&” (from Cats and Wildlife: A Conservation Dilemma, by John S. Coleman, Stanley A. Temple, and Scott R. Craven).
In a two-year study, the East Bay Regional Park District compared two grassland parks—one with over 20 cats fed daily, one without. The park with no cats had over twice as many birds as the other park. Ground-nesting birds like the California thrasher and California quail were seen in the no-cat park but never seen where cats were present. In the no-cat park, native deer mice and harvest mice predominated, while 80 percent of rodents in the cat area were house mice, an exotic pest species. (from the Cats Indoors! campaign of American Bird Conservancy)