The cat (Felis silvestris catus), also known as the domestic cat or housecat to distinguish it from other felines and felids, is a small carnivorous mammal that is valued by humans for its companionship and its 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. Due to their close association with humans, cats are now found almost everywhere on Earth. This extreme adaptability and their worrying impacts on native animals has led to them being classed as an invasive species. Most of these 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.