Patterns of dispersion, density and dispersal in alpine populations of the land snail Arianta arbustorum (L.) (Helicidae)

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Abstract
In studying populations of the land snailArianta arbustorumin the eastern Swiss Alps, juvenile and subadult snails were found to be aggregated all the time, while adults were mostly aggregated during summer, the main mating period. The mean size of aggregations was 25 cm × 25 cm but varied, according to the area a grass tuft covered, while the intensity of aggregations was influenced by weather conditions. Densities of 10–15 subadults and 3–6 adults m−2were found along ditches and streams, 2–25 subadults and 2–20 adults on uncultivated alpine meadows, while on scree‐covered alpine grassland the density of subadults and adults was 0.3–0.5 m−2. The extent of daily movements varied with microclimatic factors and season and depended on the structure of the habitat. The dispersal rate was greatest in June (2.6 m month−1) and least in August (1.1 m month−1). In summer and autumn, active uphill movements were noted, a process which compensates for the loss of altitude by passive downhill displacements. Roiling down on snowfields and being carried away by avalanches and streams were observed.