Phylogeography of an invasive land snail: natural range expansion versus anthropogenic dispersal in Theba pisana pisana

The invasive land snail Theba pisana pisana has been widely dispersed through human activities. The Mediterranean and adjacent Atlantic coasts are considered as natural range of this species, which also occurs northwards to the Netherlands, the British Islands and in Ireland. In addition, T. p. pisana has been introduced to some Macaronesian Islands as well as to Argentina, Australia, South Africa, and the United States. One important part of understanding biological invasions is the history of the processes including the routes of spread and introduction. Reconstructing the phylogeography based on COI sequences, we asked to which extent it may still be possible to trace natural as opposed to human causes for the current distribution. In addition, we attempted to infer the origins of the apparently recent human-mediated introductions. Tree reconstructions and demographic analyses showed that apart from the origin in NW-Africa, where T. p. pisana already underwent considerable differentiation, and an initial spread to the Iberian Peninsula, the phylogeographic signal was largely blurred. All available evidence indicated that primarily human activities rather than natural causes have shaped and still shape the current distribution. Already the spread through the Mediterranean basin may be anthropogenic and even the origins of our samples from South Africa and Australia could not unambiguously be traced. One clade was geographically particularly diverse suggesting that different lineages differ in their adaptive and invasive potential.