The rise and fall of an alien: why the successful colonizer Littorina saxatilis failed to invade the Mediterranean sea

Understanding what determines range expansion or extinction is crucial to predict the success of biological invaders and effectively deal with biodiversity changes. We tackled this long-standing question from an unparalleled perspective using the failed expansions in Littorina saxatilis and investigating its present and past habitat suitability in Europe through Ecological Niche Modelling (ENM). This snail is a typically successful Atlantic colonizer and the earliest confirmed alien species in the Mediterranean Sea, where, 88 however, it failed to thrive despite its high dispersal ability and adaptability. We explored the environmental constrains affecting its biogeography, identified potential glacial refugia in Europe that fuelled its post-glacial colonisations and tested whether the current gaps in its distribution, including most of the Mediterranean Sea, are linked to local ecological features. We compared the analytical power of approaches based on both marine and terrestrial variables (IMs) or marine descriptors only (MMs). Our results suggested that L. saxatilis is unlikely to be a glacial relict in the Mediterranean basin. Multiple Atlantic glacial refugia occurred in the LGM, and abiotic environmental features such as salinity and water temperature have influenced the past and current distributions of this snail and limited its invasion of the Mediterranean Sea. Both modelling approaches showed high predictive performances, with was slightly greater in MMs; however, IMs provided more realistic scenario. The ecological failure of L. saxatilis’ colonization in the Mediterranean Sea is linked to the snail’s inability to rapidly cope with the AtlanticMediterranean large-scale environmental differences. ENM is a powerful method to understand and forecast the behaviour of introduced organisms, particularly when based on biological knowledge of species. Our findings contribute to clarify the processes constraining or facilitating shifts in species’ distributions and biological invasions.