People tend to think of “invasive species” only in negative terms, but the phrase has several degrees of definition. Toward the “OK” scale it refers to “introduced species or nonindigenous species that are rapidly expanding outside of their native range.” A not-so-OK connotation is, “alien species whose introduction or spread threatens the environment, the economy, and/or society, including human health.”
Degrees of the definition are appropriate. For example, an argument can be made that the introduction of most exotic species has richly enhanced biodiversity. Consider traveling plants; more than 4,000 species introduced into North America during the past 400 years make up nearly 20 percent of our current plant biodiversity. And, with no evidence, according to some biologists, that a single resident species has been driven to extinction. In other words, with some pesky exceptions, plants that invade fit in.
As divers we tend to think negatively about the arrival of nonindigenous marine life, yet the opening of the Suez Canal in 1869 introduced 250 new fish species into the Mediterranean from the Red Sea, reportedly resulting in only a single extinction. So there’s that.