Such species might be termed "naturalized", "established", "wild non-native species". If they further spread beyond the place of introduction and cause damage to nearby species, they are called "invasive". The transition from introduction, to establishment and to invasion has been described in the context of plants. Introduced species are essentially "non-native" species. Invasive species are those introduced species that spread widely or quickly and cause harm, be that to the environment, human health, other valued resources, or the economy. There have been calls from scientists to consider a species "invasive" only in terms of their spread and reproduction rather than the harm they may cause. According to a practical definition, an invasive species is one that has been introduced and become a pest in its new location, spreading (invading) by natural means. The term is used to imply both a sense of urgency and actual or potential harm. For example, U.S. Executive Order 13112 (1999) defines "invasive species" as "an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health". The biological definition of invasive species, on the other hand, makes no reference to the harm they may cause, only to the fact that they spread beyond the area of original introduction. Although some argue that "invasive" is a loaded word and harm is difficult to define, the fact of the matter is that organisms have and continue to be introduced to areas in which they are not native, sometimes with but usually without much regard to the harm that could result. From a regulatory perspective, it is neither desirable nor practical to list as undesirable or outright ban all non-native species (although the State of Hawaii has adopted an approach that comes close to this). Regulations require a definitional distinction between non-natives that are deemed especially onerous and all others. Introduced "pest species" that are "officially listed" as invasive, best fit the definition of an "invasive species". Early detection and rapid response is the most effective strategy for regulating a pest species and reducing economic and environmental impacts of an introduction  In Great Britain, the Wildlife and Countryside Act 1981 prevents the introduction of any animal not naturally occurring in the wild or any of a list of both animals or plants introduced previously and proved to be invasive. By definition, a species is considered "introduced" when its transport into an area outside of its native range is human mediated. Introductions by humans can be described as either intentional or accidental.