Ecology of the introduced brown garden snail (Cantareus aspersus) and its potential impacts to coastal dune flora in the Auckland Region

Abstract

Cantareus aspersus (brown garden snail) is one of the most common introduced land snails in New Zealand especially on the North Island. The snails are well known to gardeners and nurseries, but they also occur abundantly on coastal sand dunes, which are one of the most threatened ecosystems in New Zealand. My study investigated the ecology of C. aspersus on sand dunes in northern New Zealand and potential impacts of this snail on dune flora with a focus on the Auckland Region. Fieldwork at Tawharanui Open Sanctaury and Whatipu Beach investigated distribution, densities and nocturnal activity of C. aspersus. Laboratory trials determined the palatability of 26 coastal plant species (common native, common introduced and threatened native) and investigated food choice behavior. Densities up to 2.5 snails per m2 were found at night with highest densities occurring in back-dunes and dune slack habitat. Mid-dune habitats had consistent but lower densities than back-dunes and dune slacks. Foredunes were the least preferred habitat. High humidity, recent rainfall and high soil moisture were important factors in influencing activity. Shelter habitats and herbaceous weeds encouraged the presence of snails. Decaying plant material, rosette weeds, and species in the legume family were important food sources. In laboratory trials, a number of threatened plant species were found to be palatable to C. aspersus. Generally, common native plant species were unpalatable while introduced plants in the legume family were palatable. C. aspersus also demonstrated a strong preference for decaying plant material and avoidance of harder textured plants (e.g. grasses). Widespread and abundant snail presence among sheltered dune habitats found in this study suggests that C. aspersus is likely to have a significant impact on the structure and composition of plant communities in New Zealand coastal sand dunes. These results suggest that some threatened plant species may have been more common on sand dunes prior to the arrival of C. aspersus. These small herbivores may also facilitate the invasion of unpalatable introduced grasses. Research such as this may assist in rare plant recovery and restoration projects which are becoming widespread and necessary for the survival of these vulnerable ecosystems.