The cavernicolous fauna of Hawaiian lava tubes. I. Introduction

Francis Howarth


The Hawaiian Islands offer great potential for evolutionary research. The discovery of specialized cavernicoles among the adaptively radiating fauna adds to that potential. About 50 lava tubes and a few other types of caves on 4 islands have been investigated. Tree roots, both living and dead, are the main energy source in the caves. Some organic material percolates into the cave through cracks associated with the roots. Cave slimes and accidentals also supply some nutrients. Lava tubes form almost exclusively in pahoehoe basalt, usually by the crusting over of lava rivers. However, the formation can be quite complex. Young basalt has numerous avenues such as vesicles, fissures, layers, and smaller tubes which allow some intercave and interlava flow dispersal of cavernicoles. In older flows these avenues are plugged by siltation or blocked or cut by erosion. The Hawaiian Islands are a string of oceanic volcanic islands stretching more than 2500 km across the mid-Pacific. The western islands are old eroded mountains which are now raised coral reefs and shoals. The eight main eastern islands total 16,667 km 2 and are relatively young in geologic age. Ages range from 5+ million years for the island of Kauai to 1 million years for the largest island, Hawaii (Macdonald & Abbott, 1970). The native fauna and flora are composed of those groups which dis-persed across upwards of 4000 km of open ocean or island hopped and became successfully established. Thus the fauna is remarkably disharmonic. The disharmony on oceanic islands has been discussed by Zimmerman (1948) and Gressitt (1971). Zimmerman (1948) estimated that only 250 successful introductions to the Hawaiian Islands have given rise to the entire native insect fauna of more than 5000 species. The aquatic, soil, and cave arthropods of the continents are conspicuously poorly represented because of their inherent lack of dispersal ability. Although the existence of lava tubes in Hawaii has been known for many years, they remained virtually unexplored biologically unt