| Summary: | Climate warming is expected to drive an upward altitudinal shift of species distributions in mountain areas. In this study, we consider how environmental variables constrain the distribution of freshwater mollusks across elevations based on an extensive survey of the entire Pyrenean range. Results show that several altitude-related variables are significantly relevant for the distribution of all mollusks (i.e., temperature, sediment organic content). Others respond more precisely to some variables: fine substrate proportion increases the probability of finding <i>Pisidium</i> sensu lato (mostly <i>Euglesa</i> species), and the latter, the macrophyte presence, and <i>Ampullaceana balthica</i>. Despite the low acid-neutralizing capacity in many of the lakes, only the distribution of <i>A. balthica</i> was significantly constrained by this factor, independent from elevation. The results confirm a likely altitudinal expansion of the distributions of all species, particularly toward lakes with a summer surface temperature increasing above 12 °C. The pace of change is expected to differ among species according to different nonlinear thresholds in thermal response, which temperature value increases from <i>Pisidium</i> s.l. to <i>Ampullaceana</i> to <i>Ancylus</i>, and the taxon-specific sensitivity to substrates and chemical conditions.
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