Feral cats may live alone, but most are found in large groups called feral colonies with communal nurseries, depending on resource availability. Most abandoned cats probably have little alternative to joining a feral colony. Some feral cat colonies are found in large ties such as around the Colosseum and Forum Romanum in Rome.

The Roman cats are not truly feral because they are partly fed and vetted by the local authority. Because cats are adaptable, those in residential areas know that if they are friendly to humans they need not worry about food or shelter. Some urban "stray" cats have many houses/humans to support them.

Although cats are adaptable, feral felines are unable to thrive in extreme cold and heat, and with a very high protein requirement, few find adequate nutrition on their own in cities. They are often killed by dogs, coyotes, and automobiles. However, there are thousands of volunteers and organizations that trap these unadoptable feral felines, neutering them, immunize the cats against rabies and feline leukemia, and treat them with long-lasting flea products. Before release back into their feral colonies, the attending veterinarian often nips the tip off one ear to mark the feral as neutered and inoculated, since these cats will more than likely find themselves trapped again. Volunteers continue to feed and give care to these cats throughout their lives, and not only is their lifespan greatly increased, but behaviour and nuisance problems, due to competition for food, are also greatly reduced.

Feral cats are thought to be a major predator of Hawaiian coastal and forest habitats, and are one species among many responsible for the decline of endemic forest bird species as well as seabirds like the Wedge-tailed Shearwater. In one study of 56 cats' faeces, the remains of 44 birds were found, 40 of which were endemic species.

In the Southern Hemisphere there are many landmasses including Australia where cat species have never been native, and other placental mammalian predators were rare or absent. Native species there tend to be more ecologically vulnerable and behaviourally "naive" to predation by feral cats. Feral cats have had serious effects on these wildlife species and have played a leading role in the endangerment and extinction of many of them. In Australia a large quantity of native birds, lizards and small marsupials are taken every year by feral cats, and feral cats have played a role in driving some small marsupial species to extinction. Some organizations in Australia are now going to effort of creating fenced islands of habitat for endangered species that are free of feral cats and foxes.

There are two divergent views about the relationship of cats with the environment. The first argues that the environmental impact of feral cat programs and of indoor/outdoor cats is a subject of debate. Part of this stems from humane concern for the cats, and part stems from concerns about cat predation on endangered species. The amount of ecological damage done by indoor/outdoor cats depends on local conditions. As suggested above, the most severe effect occurs to island ecologies.

Environmental concerns may be minimal in most of the UK where cats are an established species and few to none of the local prey species are endangered. Pet owners can contact veterinarians, ecological organizations, and universities for opinions about whether local conditions are suitable for outdoor cats. Additional concerns include potential dangers from larger predators and infectious diseases. Coyotes kill large numbers of housecats in the South-western United States, even in urban zones. FELV (feline leukemia), FIV (feline immunodeficiency virus), or rabies may be present in the area. If faced with conflicting evidence, the safe choice is to keep a cat indoors.

Cats present a risk of overpopulation, as well. According to the Humane Society of the United States, 3–4 million cats and dogs are euthanized each year in the United States and many more are confined to cages in shelters because there are significantly more animals being born than there are homes. Neutering pets helps keep the overpopulation down. A study in 1992 found that in the USA, 12,893 (29.4%) of pets, 26.9% of dogs and 32.6% of cats were sterilized Local humane societies, SPCAs, and other animal protection organizations urge people to neuter their pets and to adopt from shelters instead of purchasing elsewhere.