The cat, also known as the domestic cat or housecat to distinguish it from other felines and felids, is a small domesticated carnivorous mammal that is valued by humans for its companionship and ability to hunt vermin and household pests. Cats have been associated with humans for at least 9,500 years, and are currently the most popular pet in the world. Owing to their close association with humans, cats are now found almost everywhere on Earth. Their adaptability, rapid breeding rate, and predatory instincts make them effective hunters. In some locations, cats have affected indigenous animal populations; this has led them to be classified as an invasive species in some areas. Many problems are caused by the large number of feral cats worldwide, with a population of up to 60 million of these animals in the United States alone. Failure to control the breeding of pet cats by spaying and neutering and the abandonment of former household pets cause the development of such feral colonies.