Non-native means originally from elsewhere, and it could have reached the new location naturally or by human means. Introduced means transferred to a new locality by humans, either intentionally or by accident. Invasive refers to a non-native species is causing problems in the new location, directly to human health or economy, or to cultivated plants or farm animals. Sometimes the term invasive is applied ecologically to include harm to other local species that may or may not directly affect humans.
Introduced species have often arrived from another continent, such as the Arion spp. slugs from Europe that ravage our gardens, but occasionally they are from other parts of the same continent, e.g., Prophysaon andersonii slugs from the west coast of the USA introduced to Quebec (Nicolai & Forsythe 2020), and Triodopsis hopetonensis that is spreading both northward and eastward from the mid-Atlantic coastal areas where it used to occur {Hubricht 1985, GBIF 2023, iNaturalist 2023).
Introduced species usually arrive at ports or large urban areas, often arriving on cargo, especially agricultural products. There is evidence that the grove snail Cepaea nemoralis has often arrived on tiles from Mediterranean areas (check a vacant lot near a tile store and you might find a colony of C. nemoralis). Many past introductions are likely via the soil of potted plants and modern introductions continue in the horticulture trade. The Cuban snail Zachrysia provisoria was established in the 2000s in the rainforest exhibit in the Pittsburgh Zoo, undoubtedly transferred on tropical plants from Florida, where Z. provisoria has become established. Plant nurseries can be important hubs for the spread of non-native gastropod species (Bergey et al. 2014).
How long must an introduced species be present in a new location before it becomes accepted as part of the native fauna? In North and South America, we can use the year 1492 as the year when modern Europeans began traveling between Europe and the Americas, carrying species between the continents. If a species is known to be present in the Americas before 1492 (many people use the year 1600 after which trade dramatically increased between continents), we can confidently call it native. In contrast, Europe, Asia, and Africa lack such a convenient date for defining introduced species and must define introduced species in other ways (e.g., several species of edible snails in the family Helicidae are known to have been moved around Europe by Romans thousands of years ago).
Even with the convenient start date of 1492 (or 1600) for arrival of introduced species in the Americas, some species can be challenging to classify. For example, Cepaea hortensis appears to have arisen in northern Europe (its range is spread widely across Europe (not including the southernmost parts); in contrast it is spread thinly along the NE coast of North America from Long Island to Newfoundland (and into the St. Lawrence Seaway), suggesting that it arrived in North America relatively recently and has not had a chance to spread. Evidence indicates that C. hortensis was present in North America before Columbus arrived (in 1492) and evidence from a cave on the Gaspe Peninsula, Quebec indicates it was present in North American before the Vikings (Pearce et al. 2010).
How do we know whether a species is native or introduced? Sometimes we do not know for sure, so we rely on clues. Many non-native species occur in urban areas, especially seaports but also airports receiving air freight, or their populations appear to be spreading from those areas. Many of those localities are disturbed areas. Many recently introduced species occur in small, localized populations in contrast to larger widespread populations elsewhere on earth. If a fossil record exists, presence of a species before 1492 (or 1600) can argue for native status. Improved genetics capabilities have allowed us to examine population-wide genetics patterns, which typically show reduced genetic variability in introduced species (resulting from a founder effect), in comparison to much greater genetic variability in populations in the source area; in fact, genetic analysis can sometimes pinpoint the geographic location from which the introduced species was translocated.
Despite these useful clues, in some cases recognizing introduced species can be challenging. Arion spp. are common agricultural and garden pest slugs in North America and they are known to be widespread in Europe (where they are also pests). However, they are also widespread in northeastern North America (including in relatively wild areas), raising the question whether they could be native species that take advantage of the tasty human crop plants that appeared with humans. I’m not proposing that they are native to North America and were introduced to Europe (I expect they were known pests in Europe before 1492). I argue that their widespread occurrence in North America could be via dispersal on logging equipment during the late 1800s and early 1900s when trees across the northeastern United States were cut down for timber. Furthermore, genetic testing shows that North American Arion spp. have reduced genetic variability compared to European Arion spp., arguing that the North American populations originated from (and are a subset) of the European populations.
Another species challenging to classify as native or introduced is Cochlicopa lubrica, a small (5-6 mm) glossy shelled snail common in human-modified areas. It is widely distributed across North America (north of Mexico) and throughout Europe. Many people in North America consider it to be native (Hubricht 1985). However, another possibility is that it might have arrived in North America shortly after the Europeans arrived (unseen), then was noticed later and assumed to be native. I am not aware of a fossil record arguing for its native status and I consider the matter to be unresolved. Another similar (and slightly amusing) story is the genus Hawaiia. These small (2 mm) white snails are actually native to North America but were inadvertently introduced to the Hawaiian Islands where they were first discovered and named Hawaiia after their presumed homeland. Only later were they discovered to be widespread in North America in their true homeland, but in taxonomy (the science of naming species), once a valid name has been established, it persists even if it was misleadingly named.
Literature Cited
Bergey, E.A., Figueroa, L.L., Mather, C.M., Martin, R.J., Ray, E.J., Kurien, J.T., Westrop, D.R. & Suriyawong, P. 2014. Trading in snails: plant nurseries as transport hubs for non-native species. Biological Invasions 16:1441-1451.
GBIF. 2023. Triodopsis (Triodopsis) hopetonensis (Shuttleworth, 1852). https://www.gbif.org/species/2295718.
Hubricht, L. 1985. The distributions of the native land mollusks of the eastern United States. Fieldiana: Zoology, New Series, No. 24:1-191.
iNaturalist. 2023. Observations on Magnolia Threetooth https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=232995.
Nicolai, A. & Forsyth, R.G. 2020. Introduced Prophysaon andersonii (J.G. Cooper, 1872) in Quebec, Canada: first record of Prophysaon (Gastropoda, Eupulmonata, Arionoidea) in eastern North America, confirmed by partial-COI gene sequence. Check List 16 (2): 307–316. https://doi.org/10.15560/16.2.307.
Pearce, T.A., Olori, J.C. & Kemezis, K.W. 2010. Land snails from St. Elzear Cave, Gaspé Peninsula, Quebec: antiquity of Cepaea hortensis in North America. Annals of Carnegie Museum 79(1): 65-78.