Differential effect of water removal on endemic and introduced aquatic insects in a tropical, torrential habitat

A legacy of water diversion exists in Hawaii, where water that originally supported native population needs has been co-opted for development, agriculture and tourism. This change in water usage has resulted in up to 97% of baseflow diversion from some Hawaiian streams. This study assessed the impact of reduced flow from water diversion on in-stream cascade insect communities of four West Maui streams. Cascades were characterized by shallow, high velocity flow over large boulders and support several endemic insect species. In August 2007 and May 2008, three cascade microhabitats (i.e., torrential, amphibious, aerial) were sampled in upstream and downstream reaches of the first water diversion on four streams. Data pooled for all three sampling dates showed a significant increase in torrential microhabitat taxa diversity (t=3.864, df=272, p<0.0001), but a significant decrease in the relative abundance of endemic taxa in this microhabitat compared to the amphibious microhabitat (t=9.468, df=269, p<0.0001). There was also an overall reduction of aquatic insect density in both microhabitats associated with water withdrawal, with the decrease in native taxa being significant (t=2.362, df=268, p=0.0010). Further, in the two largest streams total cascade habitat was reduced by 75.4 - 90.6% below water diversions. These findings indicate that insect communities in cascade habitats can be significantly impacted by water removal, especially the most abundant endemic taxa that represent amphibious habitat. In addition, the dramatic decrease in area of available habitat has implications for the survival and proliferation of endemic species in these tropical streams.