Friday, August 16, 2013

Ancient Human-Mediated Dispersal of Terrestrial Gastropods

Cepaea nemoralis by L. Holden

                Species have been accidentally or purposely disseminated by humans at an alarming rate over the past century.  Most people have heard about relatively recent invasions and impacts of zebra mussels, Africanized bees, kudzu and other non-indigenous, exotic, or non-native species to name a few.  What many people may not realize is that the true globalization and homogenization of the world’s biota began in 1492 after the “discovery” of the New World by Columbus.  After Columbus, ecosystems met and mixed in an exchange Alfred Crosby referred to as the Columbian Exchange (Mann, 2011).  The “exchange took corn (maize) to Africa and sweet potatoes to East Asia, horses and apples to the Americas, and rhubarb and eucalyptus to Europe- and also swapped about a host of less-familiar organisms like insects, grasses, bacteria, and viruses” (Mann, 2011).  The Columbian exchange had a profound effect on the natural ecosystems and landscapes around the world.  Exotic species became major staple crops that most people do not realize are non-native.  They are here.  We grow them, we eat them or pet them or ride them.  Even fewer people are aware that species were introduced accidentally or purposely thousands of years ago by humans – including terrestrial gastropods!

                In June 2013, Adele Grindon and Angus Davison published a paper in PLOS ONE showing that a peculiar distribution pattern of Cepaea nemoralis land snails in Ireland and the Eastern Pyrenees was best explained by transportation by Mesolithic humans over 8000 years ago.  Apparently, there are a number of species including the Kerry slug, the Pyrenean glass snail and the strawberry tree that are found exclusively in Ireland and Iberia.  This distribution pattern has been referred to as ‘Lusitanian’ and has defied any single explanation.  Grindon and Davison chose to study C. nemoralis because on the West coast of Ireland, C. nemoralis has a large, white-lipped morph that is common and also found in the Pyrenees suggesting some connection that warrants investigation.  The researchers sampled across Europe including Ireland, Britain, northern Spain, southern France and the Pyrenees and sequenced two mitochondrial gene fragments (cytochrome oxidase subunit I (COI) and 16S rRNA) to estimate phylogenies and examine the resultant patterns.  What Grindon and Davison observed was that individuals from Ireland had a mitochondrial lineage, C, that is shared with Central and Eastern Pyrenean populations.  This lineage was absent in most other parts of Europe with minor exceptions.  The authors propose that the best explanation for the disjunct distribution pattern is a single historic long distance dispersal event between the Pyrenees and Ireland.  The species has apparently been a food source in the Pyrenees, so may have been transported live to serve as a source of food.  Grindon and Davison’s paper is not the first documentation of likely human-mediated dispersal of terrestrial gastropods.  Jesse et al. (2011) hypothesized that Neolithic expansion in the western Mediterranean resulted in the expansion of the range of Tudorella sulcata s. str. and Lee et al. (2007) hypothesized pre-historic inter-island introductions of an endemic Pacific island tree snail, Partula hyalina.  It is certain that as other unusual distribution patterns are examined other ancient human-mediated dispersal events will be discovered and shed light on the history of world’s biota including gastropods.  

Grindon, A. J. and A. Davison.  2011.  Irish Cepaea nemoralis land snails have a cryptic Franco-Iberian origin that is most easily explained by the movements of Mesolithic humans.  PLOS ONE 8(6):1-7.
Jesse, R., E. Vela, and M. Pfenninger.  2011.  Phylogeography of a land snail suggests trans-Mediterranean Neolithic transport.  PLOS ONE 6(6):1-7.
Lee, T., J. B. Burch, T. Coote, B. Fontaine, O. Gargominy, P. Pearce-Kelly, and D. O. Foighill.  2007.  Prehistoric inter-archipelago trading of Polynesian tree snails leaves a conservation legacy.  Proceedings of the Royal Society B 274:2907-2914.
Mann, C. C.  2011.  1493: Uncovering the New World Columbus Created.  Vintage Books, New York.