This Old World rodent is the common pest found living in houses, barns, and other human habitations. It arrived in the New World as a stowaway on board ship and has lived successfully around people for centuries.
In association with humans
Norway Rat (Rattus norvegicus)
This animal is a good swimmer and climber whose opportunistic behavior has led to its successful worldwide distribution. This Old World rodent did not originate in Norway but in China. It is a serious pest of granaries and other food storage sites and a vector of numerous diseases communicable to humans. Most sources state strongly that efforts should be taken to control of Norway rat populations whenever possible.
Black Rat (Rattus rattus)
This species is thought to have been extirpated from (forced out of) the Commonwealth by pressure from the more aggressive Norway rat. Although some individuals may continue to find their way to coastal seaports such as Philadelphia via incoming ships, it is unlikely that populations will be successfully re-established.