Our Living Resaiiixes — Hawaii 



i69 



Molecular and cytological (see glossary) 

 research has been paralleled by ecological 

 research on the natural breeding sites of these 

 species on specific Hawaiian plants, such as 

 olapa (Araliaceae) and ohawai (lobelioidsl 

 Extensive surveys have determined that 

 Hawaiian Divsophila are specialized microbi- 

 vores {see glossary) that complete their lite 

 cycle in the decaying tissue of over 40 families 

 of Hawaiian plants (Carson and Kaneshiro 

 1976). This ecological infomiation is the most 

 detailed for any group of native Hawaiian 

 insects, and the combined phylogenetic and 

 ecological data provide a firm foundation for 

 further study of the position and function of 

 these insects in ecosystems. 



Current ecological studies focus on quanti- 

 fying species diversity over ecosystem gradients 

 and evaluating long-term trends in population 

 sizes (Carson 1986; Foote and Carson, unpub- 

 lished data). Among the patterns observed are 

 changes in Drosophila community structure 

 associated with invasions of nonindigenous 

 plants and animals in Hawaii. One dominant 

 trend is the increasing representation of the 

 recently introduced cosmopolitan species of 

 Drosophila in wet forest communities disturbed 

 by feral pigs and alien weeds. A second pattern 

 is the apparent decline of certain guilds of 

 endemic picture-wing Drosophila and their 

 host plants over a 20-year period of observation. 



Close to one-fifth of the world's known 

 Drosophila fauna are endemic to Hawaii. 

 Invasions by nonindigenous Drosophila are 

 adding to the diversity of the group. This abun- 

 dance of species is increasingly useful for track- 

 ing biological diversity at several levels, from 

 changes in chromosome inversion frequencies 

 over altitudinal dines to the measurement of 

 long-term changes in community structure. 



Status and Trends 



Methods 



Beginning in 1971, as part of the 

 International Biological Program, the relative 

 frequencies of populations of 14 species of pic- 

 ture-wing Drosophila were measured in the 

 Olaa Forest at Hawaii Volcanoes National Park 

 on the island of Hawaii (Fig. 1 ). The fly popu- 

 lations were surveyed earlier by using baits 

 placed on tree trunks, vines, and tree ferns, but 

 the most recent survey (1992-93) used tree fern 

 stipes exclusively. Since 1980 the surveys have 

 employed nondestructive sampling where indi- 

 viduals are identified by unique wing and tho- 

 rax markings in the field (Carson 1986; Foote 

 and Carson, unpublished data). 



Since 1982 four fenced feral pig exclosures 

 have been constructed in rain forests where 

 Drosophila surveys have been undertaken. 



These exclosures average about 300 ha (740 

 acres) in size. The impact on Drosophila com- 

 munities of removing non-native pigs has been 

 evaluated through the comparison of recently 

 introduced cosmopolitan and endemic tlies 

 attracted to baits in different-aged exclosures 

 and adjacent forest where feral pigs are still 

 acfive (Foote et al.. unpublished data). 



Llsing Chromosomes To Trace Evolutionary 

 History 



There are 491 described Hawaiian species in 

 the family Drosophilidae. Most of the species 

 belong to one of two genera, Drosophila and 

 Seaptomyza. Among the Hawaiian species, 124 

 have been genetically surveyed, including 106 

 of 1 1 1 picture-wing species in the genus 

 Drosophila (Carson 1992). Most species are 

 single-island endemics, reflecting the forces of 

 geographical isolation imposed by this volcanic 

 archipelago. 



Inversion polymorphisms {see glossary) 

 have been detected within or between popula- 

 tions of about one-third of the species and their 

 frequencies have been measured over environ- 

 mental gradients in several well-studied species 

 on the island of Hawaii (Carson 1992). 

 Variations in the frequency of different poly- 

 morphisms along a gradient are used as an indi- 

 cator of the role of natural selection in main- 

 taining genetic variation. One such genetic gra- 

 dient occurs among populations of Drosophila 



kn endemic Hawaiian picture- 

 wing Drosophila (D. conspicita- 

 left) perched on a Clermoitlia fruit 

 next to a cosmopolitan member of 

 the species complex that includes 

 D. simiilan.', (right), one of the 

 common non-indigenous species 

 in Hawaiian rain forests. The pic- 

 ture-wings are close to the size of 

 common house flies, giants in 

 comparison to their mainland rela- 

 tives. 



Pacific Ocean 

 I I Montane rain forest 



Fig. 1. Hawaii Volcanoes National 

 Park, including the Olaa Forest, 

 where population surveys of pic- 

 ture-wing Drosophila have been 

 carried out. 



