Latitudinal and altitudinal diversity patterns and Rapoport effects in north-west European land snails and their causes

The latitudinal and altitudinal range sizes of north-west European land-snail species increase with increasing latitude/altitude. These Rapoport effects are not caused by northern/high-altitude species with wider latitudinal/altitudinal ranges and southern/low-altitude species with narrower latitudinal/altitudinal ranges, as predicted by the climatic variability hypothesis. They are instead caused mainly by different northern/upper borders of species occurring in the south part of the study area or at low and intermediate altitudes, respectively. This pattern indicates that the observed Rapoport effects are the result mainly of differential northward/upward expansion of species that were restricted to southern/low or intermediate altitude refugia during the glacials. Although all species occurring in a refugium experienced the same climatic conditions, there is stochastic variation in their climatic tolerance. Species with broader climatic tolerance were able to expand farer northwards/upwards postglacial. The altitudinal distribution of species richness in the analysed alpine faunas cannot be explained by the Rapoport-rescue hypothesis, because species richness peaks at intermediate altitudes and because there is no negative correlation between the number of range borders and altitude. The Rapoport-rescue hypothesis alone is probably also insufficient to explain the decrease in species richness with increasing latitude.