COMPETITION BETWEEN 2 LAND GASTROPODS ALONG ALTITUDINAL GRADIENTS IN SOUTH-EASTERN FRANCE - NEONTOLOGICAL AND PALEONTOLOGICAL EVIDENCE

In south-eastern France, Trochoidea geyeri (Soos) today characterizes open upland environments from about 1000 m above sea level. At lower a ltitude it is replaced by Candidula unifasciata (Poiret)-another membe r of the Helicellinae-while the other members of the community remain almost the same. During the cold stages of the Quaternary, T. geyeri w as also an important element of lowland gastropod communities in Prove nce, while it was always replaced by C. unifasciata during the intergl acial stages. Thus, T. geyeri seems to be a valuable cold indicator sp ecies for palaeoclimatic reconstructions. Some anomalies in this patte rn, however, show that each of these species can live in bioclimatic c onditions which characterize the area of the other. This suggests a co mpetitive 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. geyer i is missing because of historical factors: in such cases, C. unifasci ata occupies the entire gradient, up to 2250 m. Quaternary climatic ch anges produced altitudinal shifts of the contact zone between T. geyer i and C. unifasciata, and, correlatively, repeated isolations of T. ge yeri populations.