Introduction of a species outside its native range is all that is required to be qualified as an "introduced species" such that one can distinguish between introduced species that may not occur except in cultivation, under domestication or captivity whereas others become established outside their native range and reproduce without human assistance. 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.