Sympatry without co-occurrence: exploring the pattern of distribution of two Helix species in Georgia using an ecological niche modelling approach

Abstract

Two species of the genus Helix are widespread in Georgia: H. lucorum has a Mediterranean distribution whereas H. buchii is a Caucasian endemic typically associated with broadleafed forests. In spite of their sympatry within Georgia, they are never syntopic. Furthermore, in contrast with H. buchii, H. lucorum is mainly found in areas subject to human disturbance. Another large helicoid species, Caucasotachea calligera, is widespread in Georgia and usually co-occurs with either Helix. The distribution patterns of these species suggest that interspecific competition might play an important role in shaping the distribution of the two Helix species. In order to see whether their ecological niches were different enough to provide such a distribution pattern, I used predictive ecological niche models (ENM) based on the Maximum Entropy algorithm. ENMs showed that the niches of these species in Georgia were significantly different but not fully separated (� 15–36% overlap). The distributional pattern of H. lucorum should not be considered truly natural in Georgia and may be anthropogenic. The fact that the two Helix species never co-occur may result from factors other than ecological niche differentiation at any macro scale. Since competition remains the most useful and informative assumption to explain the distributional pattern of these congeneric species, microhabitat requirements also need to be tested as a potential driver.