Believe it or not, fish don’t care what we call them. However, terminologies associated with certain species can affect how society perceives their importance and impact.

Naturalists have been interested in the effects of species introductions just as long as we have been moving species around. In that time, species introductions have had a variety of consequences—sometimes the alteration of entire ecosystems, but often with little appreciable effect.

This has led to a numerous terms associated with non-native species, some of which overlap and some which are belligerent or misguided. Here are a few common words that dominate the lexicon of biodiversity research…

Invasive. Probably the most common word used to describe nonnative species, this is actually a quite specific definition. To be termed invasive, a species must be “non-native (alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm, or harm to human health”. Invasive is often synonymous with noxious and nuisance species. Unfortunately, this definition is quite constrained to a human-based perspective of harm or damage, either based on our economic values or how we perceive/quantify ‘ecological harm’. As scientists, we use the term invasive as a specific subset of the following terms.

Nonnative and Nonindigenous. These are simply species that are not native to a focal ecosystem. All species are native somewhere, and most have not been introduced by humans anywhere. All invasive species are nonindigenous to a focal ecosystem, but are not necessarily exotic.

Exotic and alien species are nonindigenous at a large spatial scale—typically continents. All exotic species are nonindigenous, but not all nonindigenous species are exotic; some are simply native to adjacent watersheds or regions.

Introduced species have been transported by humans from their native range into a new ecosystem in which they are nonindigenous, but not necessarily exotic. Most species introduction fail, being thwarted by an unsuitable new environment or  hostile native species (or existing invaders). This can also be a tricky term, as it often implies human intent. However, large events can also create ‘natural introductions’, such as the 2011 tsunami in Japan that transported nearly 600 marine species across the Pacific via debris rafts.

Established or naturalized species are (a) nonindigenous species that have been (b) introduced and have established self-sustaining populations in the new ecosystem. Depending on the context, established species may or may not be considered invasive.

Nuisance or noxious species are unwanted by society. Invasive species are typically considered nuisances, but this term varies with time. For instance, the US government stocked nonindigenous Common Carp throughout the US in the late 1800s. Once established, they were first condiered an economic and recreational boon. However, we now view this species as a harmful invasive nuisance species and seek to eradicate them or prevent their spread.

Unfortunately, society often views native species and persecutes them in favor or nonindigenous species. For example, in the 1950s native fishes in several streams in the Great Smoky Mountain National Park were wiped out with rotenone to ‘make way’ for nonindigenous Rainbow Trout that easily became established. Luckily, recent efforts to reintroduce the native fishes to the area have been successful.