The Hawaiian Islands are the worlds most isolated island chain. Over 2000 miles of ocean separate them from the nearest major landmass. This isolation has severely limited the number of plant and animal colonists becoming successfully established in the islands. This has resulted in a disharmonic flora and fauna, very few major groups are represented, and some that have become established have diversified into a large number of species. The endemic Hawaiian Sap Beetles are one of these diverse groups, with more species descended from a single ancestor than are native to all of North America.

The Hawaiian Islands are formed successively above a relatively stationary hot spot in the earths mantle as the Pacific plate moves to the northwest. The Big Island of Hawaii is the youngest, with progressively older islands to the northwest. Volcanic activity in Hawaii has periods of high and low lava production. All of the current high islands: Kauai (5.1MY), Oahu (3.7MY), Molokai (1.8MY), Maui (1.6MY), Lanai (1.5MY), and the Big Island (0.45MY) have formed during a peak period of lava output which began ~5 million years ago and continues today. Prior to the formation of Kauai there was a period of low lava output when it is believed high elevation forest was relatively rare. From 15.0-10.0 million years ago there was another period of high lava output with islands equivalent to the Big Island today. It was during the end of this earlier peak period of island formation, when a number of large high islands were present, that the Hawaiian sap beetles probably colonized the islands.

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Hawaii's Volcanoes Revealed USGS
By Barry W. Eakins, Joel E. Robinson, Toshiya Kanamatsu, Jiro Naka, John R. Smith, Eiichi Takahashi, and David A. Clague.

Output was extremely low ~30 million years ago. During this time there were no high elevation islands and all high elevation species of plants and animals probably went extinct. All high elevation species now present probably colonized the islands after this time.

Molecular sequence data places a Kauai species in the basal position for 5 of the 6 genera of Hawaiian Sap beetles. In the sixth genus taxon sampling is limited for the molecular data and morphological evidence indicates that a Kauai species is basal. Limited species level phylogenetic data indicates the majority of colonization is from older to younger islands.

The endemic Hawaiian sap beetles are found in montane wet and montane diverse mesic forest types. They are limited at their lower elevation by an introduced predator, the bigheaded ant (Pheidole megacephala), from which they have no means of defense or escape. At elevations above 7,000 feet it is probably too cold for efficient development. While the adults of some species (especially in the genus Prosopeus) are common in fresh flowers, larvae of all, and adults of the majority of species, consume decaying plant material. High rainfall levels insure a steady supply of appropriately rotten food. Mesic forests that receive substantial cloud drip have sap beetle asseblages typical of wet forests.

The vast majority of species of Hawaiian sap beetle are endemic to a single island or mountain range within an island. Most species fly little if at all, and the lower elevations between mountains are too dry for them to hike across. Speciation appears to be common between islands, through colonization and diversification, and within islands through local geographic isolation and host shifting.

The ancestor of the endemic Hawaiian sap beetles probably arrived approximately 9.5 million years ago, in the neighborhood of Gardiner Island. A complex of South Pacific Brachypeplus species with a center of diversity in New Guinea is the probable source of the lucky migrant. Migrants make their way to Hawaii on winds or by rafting in the ocean. Upper level winds do track across Hawaii from the southwest but it is very difficult to survive the cold and dry conditions in the upper atmosphere. Many species of Brachypeplus inhabit wood in the larval and adult stages making rafting a possible method of colonization. In the South Pacific it is probably a common means of dispersal between islands but the currents are not favorable between the South Pacific and Hawaii. If either method was likely then multiple colonizations would have occured. It is not possible to even guess which method accounts for their introduction to Hawaii given that both are so unlikely.

Wing reduction is a common feature of island insect faunas. The Hawaiian sap beetles are no exception, with the condition evolving repeatedly. The only other species of sap beetle known to have reduced wings is from New Zealand (undescribed) and is not closely related to the endemic Hawaiian forms.

Glossary

Montane diverse mesic forest: Elevational range of endemic Hawaiian sap beetles 1,200-5,000 ft., rainfall 1500-2500mm/ year.

Montane wet forest: Elevational range of endemic Hawaiian sap beetles 2,000-7,000, rainfall >2500mm/year.