Trading in snails: plant nurseries as transport hubs for non-native species

The plant trade provides a major mechanism for the long-distance dispersal of land snails, including slugs, which have low natural mobility. Whereas inspections at national borders intercept many in-coming snails, dispersal within countries is much less well regulated and documented. To investigate the role of plant nurseries as a source for the distribution of non-native invertebrates, particularly land snails, we surveyed snails in 28 nurseries in Oklahoma (United States) and compared our survey with similar surveys worldwide. We found 36 taxa, including 16 species not native to the region; 11 of these were new state records. Snail species richness increased with increasing outside area of snail-appropriate habitat, but not with enclosed greenhouse area. Species composition was similar among nurseries and Oklahoma nurseries shared several species with nurseries in Hawaii and Europe. Appropriate models for the dispersal of snails via plant nurseries are the transport hub model (snails moving as contaminants on plants coming into and leaving nurseries) and, for snail populations already established in nurseries, the stratified diffusion model (contamination of plants by snails within nurseries, followed by long-distance jumps as plants are sold and transported). Potted plants are portable habitats that protect snails from detection, pesticides and desiccation. Dispersing snails may survive in urban habitats, where mulching and watering may ameliorate hot, dry summers and cold winters.