Population regulation in the land snail Arianta arbustorum: density effects on adult size, clutch size and incidence of egg cannibalism

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Abstract
The relationships between local population density and adult size, clutch size and spatial distribution of egg batches were investigated in 11 natural populations of the land snail Arianta arbustorum in a forest near Uppsala, Sweden. Shell size of adults decreased with increasing population density as did clutch size. Within populations, clutch size scaled allometrically with shell size indicating size-specific fecundity. It is hypothesized that food unpalatability caused by mucus deposition slows down juvenile growth rate in high density populations, resulting in small adults and thus reducing their fecundity in subsequent years. The influence of the distance between batches on the incidence of egg cannibalism by hatchlings was examined in a laboratory experiment. In this experiment the number of eggs cannibalized increased with decreasing distance to the batch of hatching snails. Thus, in the field, eggs of highly aggregated batches suffer a high risk of cannibalism. In the 3 populations with the highest snail density, 21–39% of all batches were deposited close to each other (nearest neighbour distance ≤5 cm, i.e. less than hatchlings more within 1 day). These findings indicate that egg cannibalism can act as a population regulating factor.