Ecology of the common snail, Helix aspersa Müller, in a disturbed dune environment : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Zoology at Massey University
A study was made of the population ecology of Helix aspersa Muller in coastal dunes at Santoft forest, near Bulls. The environment was being altered by afforestation processes. The distribution of the animals is affected by the presence of tree lupin, Lupinus arboreus Sims, to which they are strongly attracted. Areas planted with lupin are capable of supporting a much greater density of snails than non-lupin areas, and this is thought to be due to the nutritive value of this species. Snail population densities appear to increase rapidly after lupin seeding in the dunes and this results in widespread lupin die-back after as little as three years from seeding. After lupin die-back the snail population decreases again. Some suggestions are made as to the origin of H. aspersa in the dune country and on the eventual fate of the populations under the maturing forest. Using shell characteristics, it was found that most juveniles in expanding populations reach maturity in little more than one year whereas those in high density, declining populations generally take two or three years. Individuals in expanding populations also attain a greater size on maturity than those in high density populations. The main factor affecting population density appears to be adult recruitment, which is considerably higher in expanding than in stable or decreasing populations. H. aspersa is found to be socially gregarious and this is particularly marked over the winter period when adults and large juveniles aggregate for hibernation. Hibernation begins in May and reaches a peak in July. Many animals are active again in mid-August. Predation by the song thrush Turdus philomelos was studied in one area. Predation occurs throughout winter and generally increases over late spring/early summer. Snails affect nitrogen fixation levels in lupins before lupin death occurs and it is considered that this is due primarily to disruption of the phloem tissue of the stems during snail feeding. It is suggested that this disruption of the translocation tissues is the ultimate cause of plant death. The possible economic significance of H. aspersa in dune forestry through its effects on lupin is discussed, and the need for further investigation indicated.