COMPETITION BETWEEN TWO LAND GASTROPODS ALONG ALTITUDINAL GRADIENTS IN SOUTH-EASTERN FRANCE: NEONTOLOGICAL AND PALAEONTOLOGICAL EVIDENCE

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Abstract
In south-eastern France, Trochoidea geyeri (Soós) today characterizes open upland environments from about 1000 m above sea level. At lower altitude it is replaced by Candidula unifasciata (Poiret)—another member of the Helicellinae—while the other members of the community remain almost the same. During the cold stages of the Quaternary, T. geyeri was also an important element of lowland gastropod communities in Provence, while it was always replaced by C. unifasciata during the interglacial stages. Thus, T. geyeri seems to be a valuable cold indicator species for palaeoclimatic reconstructions. Some anomalies in this pattern, however, show that each of these species can live in bioclimatic conditions which characterize the area of the other. This suggests a competitive dominance phenomenon from a threshold situated around 1000 m on the altitudinal gradient. The influence of competitive interaction is demonstrated using similar altitudinal gradients in which T. geyeri is missing because of historical factors: in such cases, C. unifasciata occupies the entire gradient, up to 2250 m. Quaternary climatic changes produced altitudinal shifts of the contact zone between T. geyeri and C. unifasciata, and, correlatively, repeated isolations of T. geyeri populations.

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