Mountain streams that originally supported native and cultural practices in Hawaii have been diverted for development, agriculture and tourism for over 150 years. In many streams, > 95% of baseflow is removed, modifying the natural flow regime and altering habitat and associated species composition. Benthic community response to water withdrawal was studied in four watersheds of the West Maui Mountains. Specifically, we evaluated the differential response of native and introduced insect species to stream flow removal between riffle and cascade habitats. In 2007 and 2008, physical stream condition, stream discharge, and habitat scale flow were evaluated upstream and downstream the highest elevation diversion in each watershed. This assessment was used to define the physical habitat template for comparing macroinvertebrate community response differences between habitats.

Results/Conclusions

There was a positive and significant relationship between discharge and total benthic macroinvertebrate densities, with 12 – 73% reductions downstream. Two-way ANOVA revealed significant effects of watershed and location relative to diversion (i.e., upstream vs. downstream), including an interaction, suggesting diversion effects on macroinvertebrate density depended on the watershed. Significant changes in diversity and percent native taxa downstream of diversions also varied by watershed, but also by habitat: cascade habitats were home to significantly more endemic taxa compared to riffles. Non-native Trichoptera (Cheumatopsyche analis and Hydroptila potosina) were dominant (> 95% by density) and ubiquitous in riffle habitats compared to native species which were restricted to cascades, specifically amphibious splash zone habitats. Introduced Trichoptera response to changes in stream flow were variable, scale dependent and unique to individual watersheds. Results indicate the importance of watershed and habitat variation when studying the effect of diversions on native and introduced species. The inclusion of cascade habitats in watershed biomonitoring would improve management potential of native and endemic aquatic stream communities.