HABITAT USE OF AN ENDEMIC MOLLUSC ASSEMBLAGE IN A HYDROLOGICALLY ALTERED REACH OF THE SNAKE RIVER, IDAHO, USA

摘要

Major waterways have been dramatically altered by dams some of which have operated with consistent and significant flow alterations annually since the early 1900s. Re-operation strategies are increasingly being considered for such dams in regions where biodiversity hotspots of sensitive taxa remain. Understanding the impacts of current dam operations on sensitive species is vital to designing re-operations that effectively promote species conservation. Our objectives were to examine the effects of consistent and dramatic flow alterations from a major dam on the distribution of a highly diverse endemic mollusc assemblage in the middle Snake River, Idaho, USA. Quarter metre plots were surveyed along transects with a Venturi suction dredge and classified as permanently or seasonally wetted habitat by depth. Species were classified by degree of endemism. Chi-square analysis was used to compare species presence between habitat types and analysis of variance was used to assess the significance of habitat type as a predictor of mollusc abundance, species richness and Shannon's H. Multivariate analyses were used to investigate mollusc assemblage composition and correlations with environmental parameters. The majority of mollusc species, including species of conservation concern, were found more frequently and in greater abundance in permanently wetted habitat. In addition, the mollusc assemblage in permanently wetted habitat was more diverse. Species presence and abundance patterns were consistent with source sink population dynamics with permanently wetted habitat serving as a source for seasonally wetted habitat. Results indicate that most mollusc species do not use seasonally wetted habitat even four months after it becomes available despite viable environmental conditions, highly mobile species and colonization sources. Our findings suggest that dam re-operations which reduce the magnitude and duration of seasonal low flows may improve habitat availability for sensitive benthic assemblages.