The roles of terrestrial molluscs in many important ecosystem processes are largely unknown, particularly in tropical forests. It has been suggested from studies in temperate forests that snails/slugs contribute to litter decomposition directly (by their own metabolism) and/or indirectly (by habitat modification enhancing micro-arthropod or microbial activity). Forty-two mesocosms were established at seven Hawaiian rain forest sites to examine the role of the five most abundant snail/slug species: the native Succinea cepulla, and the non-native Arion intermedius, Deroceras laeve, Oxychilus alliarius, and Limax maximus. Controls had no snails/slugs. To assess the contribution of each mollusc species separately, decomposition experiments were conducted in mesocosms containing litter bags and manipulated field densities of the mollusc species. Presence of molluscs increased litter decomposition rates, which were strongly correlated with mollusc biomass. While rates of release of some nutrients (C, K, Mg, and Mn) differed among treatments, multivariate analyses indicated that the different mollusc species influenced nutrient release in a similar way. No increases in smaller invertebrate abundances were observed in treatments containing molluscs, indicating that molluscs do not facilitate smaller invertebrate recruitment and probably influence these ecosystem processes primarily by microbe facilitation. If a major functional role of terrestrial molluscs is to facilitate microbial growth, then maintaining adequate mollusc biomass may be essential for maintaining healthy functioning ecosystems.