Threats to native aquatic insect biodiversity in Hawai'i and the Pacific, and challenges in their conservation

The decline in numbers and diversity of native insects in the Hawaiian Islands is widely recognized by field scientists, but there has been little progress in either documenting the real decline of native species, or in demonstrating specific causes of their overall decline. Additionally, few conservation actions to either restore populations or mitigate actual threats to native arthropods have been mentioned in the literature. Therefore, an examination of relevant aquatic systems and the native aquatic insects dwelling within was necessary to determine whether this is a perceived or real decline of these native Hawaiian aquatic species because of threats from invasive species. One example is the large adaptive radiation of the endemic native damselflies (Coenagrionidae: Megalagrion) in Hawai'i that are now largely found in remote headwater areas of streams, yet they also represent the hope of preserving highly diverse freshwater ecosystems found throughout the Hawaiian archipelago. Differential impacts of introduced fish such as Poeciliidae and Salmonidae were examined on Hawaiian Megalagrion. The lack of impacts on native damselflies by a large, generalist predator such as rainbow trout pointed out a seeming paradox. Whereas the small but ubiquitous mosquitofish appears to have completely devastated native aquatic fauna wherever it has been introduced outside of its natural range, trout, because of their restricted range and smaller population sizes have had minimal impacts on native invertebrates in Hawai'i. Because introduced fish species in Hawai'i are such a severe threat to native invertebrates, long-term monitoring of the remnant populations has become necessary to preserve these remaining populations. The results of monitoring and preserving the remnant Megalagrion xanthomelas population now found in only 95 m of fishless stream at the Tripler Army Medical Center indicate this species still survives. To put the Hawai'i problem into perspective, a review of the impacts of invasive species on native insects in other tropical areas and a synthesis of the problem facing Hawaiian freshwater insects areas was also provided. Finally, drawing on a mixed record of past mistakes and successes, some potential practical conservation measures intended to preserve and restore endemic Hawaiian aquatic insects are recommended.