All multicellular land species of life in Scandinavia are invasive: the area was covered by kilometres of ice until yesterday, geologically speaking. But some species are more recent invaders than others. Where I live, we currently have three species of large-bodied snail or slug: the Black slug (Arion ater, Sw. svart skogssnigel), the Burgundy snail (Helix pomatia, Sw. vinbergssnäcka) and the Spanish slug, (Arion vulgaris, Sw. spansk skogssnigel). That’s the order in which they arrived: the Blacks during prehistory, the Burgundians most likely during the Middle Ages, and the Spaniards from 1975 onward.
Now, both of our two species of slug vary a lot in their looks, and their ranges of variation coincide to such an extent that the public can’t tell them apart. The only rule seems to be that unlike the Spaniards, some Blacks are really coal black. Out of 1200 specimens of suspected Spaniards submitted to the Gothenburg Museum of Natural History in the 1980s, 60% were actually endemic Blacks. But they behave differently: the Spaniards are serious garden pests and not very cold-tolerant, while the Blacks are less problematic to gardeners but more hardy. Also, the Spaniards are notable for cannibalism, giving them the ominous vernacular name “Killer slug”, Sw. mördarsnigel.
To my annoyance, many people born after the Spaniards came to Sweden now don’t know that the Black slug exists and is no great problem for gardeners. Yesterday I saw a little girl kill a slug she found on a bike path. When I asked her why, she explained that it was a killer slug and must be killed, or it would ruin somebody’s garden. “Killer slugs are brown, have no shell and look like turds.” She’d never heard of the Black slug. When I explained that most slugs that look like turds are not killer slugs, she asked, “So how do I tell them apart?”. “You can’t”, I replied.