Does standing water and predator presence structure a wetland terrestrial mollusc community?

Abstract

Impacts of co-occurring biotic and abiotic environmental variables on terrestrial mollusc communities have rarely been studied. In wetlands, terrestrial molluscs can drown or become stranded by rising water levels and they are often prey for predaceous beetles and small mammals. I used coverboard traps over a three year period to study the biotic and abiotic gradients that may structure a wetland prairie terrestrial mollusc community in western Oregon, USA. A non-metric multidimensional scaling (NMDS) ordination indicated that shrews (Sorex vagrans) and predaceous beetles were the primary gradients associated with the terrestrial mollusc community. Snails were associated with the presence of shrews, slugs were allied with predaceous beetles, and the vectors representing the shrew and predaceous beetle gradients were opposed, suggesting that beetles preyed upon snails while the shrews consumed both beetles and slugs. These hypothesized predator-prey relationships among molluscs, shrews, and beetles coincide with studies of Sorex gut contents and predaceous carabid beetle life history. The mollusc community was not associated with the water cover gradient in the NMDS ordination, but small mammal burrows and predaceous beetles were found in greater abundance in unflooded mound habitats. This suggests that standing water may temporally concentrate both predators and prey in a shared unflooded environment during the rainy season. The high metabolic demands of shrews and an apparent preference for both beetle and molluscan prey suggest that shrews may exert a strong top-down effect on invertebrate communities.