Non-indigenous (i.e. alien, exotic, non-native) species (NIS) are introduced organisms outside their natural (past or present) range of distribution, and outside their natural dispersal potential, which might survive and subsequently reproduce, threatening biodiversity. Species of unknown origin that cannot be ascribed as being native or alien are termed cryptogenic species. In many cases, non-indigenous species do not harm the regional ecology and economics. However, in certain cases, non-indigenous species can become “invasive” species and have enormous and long-lasting impacts on the region. The EU Marine Strategy Framework Directive lists NIS as Descriptor 2 for the evaluation of Good Environmental Status (GES) of marine ecosystems, stating that NIS should be at levels that do not adversely alter the ecosystem.
Invasive alien species (IAS) are a subset of established NIS able to spread in invaded regions and impact biological diversity and/or ecosystem functioning, socio-economic values, or human health.
Also native species can be invasives. Indeed, either alien and native species with invasive potential are species that may undergo pulse-like, periodic exponential population growth (usually days to months) during which they have an impact on biological diversity, ecosystem functioning, socio-economic values or human health.
As an example, the native Mediterranean species Pelagia noctiluca, close relative of P. benovici, is also invasive.
For clarity, the P. benovici species can be regarded invasive just because it may form large population outbreaks invading the new habitat. But it is an alien, as explained in the article.