Ebbs and flows of aquatic insects in Hawaiian stream ecosystems

The mountain streams of Hawaii are home to many endemic groups of insects and several introduced and invasive species. These streams also have extreme ranges in size, gradient, hydrology and permanence, providing a range of habitats that change dramatically within the life span of most species. This highly dynamic aspect of habitat change imposes temporally-based environmental extremes not common to temperate stream insect assemblages. The objectives of this study were to understand differential native responses to these stochastic variables compared to non-native temperate species in Hawaiian streams. Integrating data from nearly 20 years that included drift, riffle and cascade insect assemblages from several streams of the Hawaiian Islands, we evaluated the effects of stream flow withdrawal and natural spat variation on native and non-native insect assemblages among watersheds of different sizes and degree of anthropogenic impact. Non-native Trichoptera (Cheumatopsyche analis and Hydroptila potosina) have become ubiquitous in Hawaiian streams and occupy most in-stream habitats compared to native species often restricted to cascades. Introduced Trichoptera responses to changes in stream flow are often variable, scale dependent and unique to individual watersheds. Native insect assemblages (e.g., Telmatogeton sp. and Procanace sp.) recolonize quickly after natural spate events but are significantly impacted by long-term anthropogenic water removal. Native species are most often found in habitats rarely studied for Hawaiian insect ecology, but provide opportunity for stream biomonitoring and assessment programs.