Genetic Diversity in Two Avian Species Formerly Endemic to Guam

Abstract

-We examine genetic diversity within endemic island avian species that recently went extinct in the wild and compare results with nonendangered, nonendemic, and nonisland species. The Micronesian Kingfisher (Halcyon cinnamomina cinnamomina) and flightless Guam Rail (Rallus owstoni) once were endemic to the Pacific island of Guam and now survive only in captivity. Horizontal starch-gel electrophoresis and minisatellite DNA profiles were used to measure genetic diversity within and among the rails, kingfishers and closely related species. Allozyme analyses of Micronesian Kingfishers revealed no genetic diversity (29 enzymes screened). DNA profiles were variable, although similarity (S) among founders to the captive kingfisher population was high relative to nonendangered birds (S = 0.56 ? SE of 0.02; 89 bands scored/individual). Three other Halcyon species examined had average allozyme heterozygosity and DNA similarity. Guam Rails had average to high allozyme heterozygosity as measured by four polymorphic loci (H = 0.03 to 0.05) relative to four other rail species (3 Rallus spp., 1 Porzana sp.). However, S (0.62 ? 0.02) was higher than in Micronesian Kingfishers. Further analyses indicated a significant deviation from Hardy-Weinberg equilibrium in two of three polymorphic loci for captive Guam Rails. Our results suggest that caution be taken in making generalizations about genetic diversity in island species. The type and number of genetic techniques used, number of species or populations examined, and variance in life-history traits among species must be considered. Received 14 March 1994, accepted 25 January 1995. ONE PREDICTION regarding island or small populations suggests that both founder effects and inbreeding due to population bottlenecks ought to reduce genetic diversity. The magnitude of the effect depends on factors such as severity of the bottleneck, population growth rate, and mutation rate (e.g. Wright 1931, 1940, Mayr 1963, Nei et al. 1975, Chakraborty and Nei 1977). This prediction has been studied in some detail for other taxa, but surprisingly few avian data are available (Boag 1986). Conclusions from avian research vary depending on the number of populations and measures of genetic diversity examined (Corbin et al. 1974, Yang and Patton 1981, Gyllensten et al. 1985, Parkin and Cole 1985, Baker and Moeed 1987, Baker et al. 1990a, b, Fleischer et al. 1991a, b, Triggs et al. 1992, Browne et al. 1993, Fleischer et al. 1994, Rave et al. 1994). Furthermore, many studies of island organisms have examined genetic diver3Present address: National Biological Service, Oregon State University, 3200 SW Jefferson Way, Corvallis, Oregon, USA 97331. sity for species that were introduced by man and do not reflect natural founding events. Meanwhile, avian species on islands continue to decline. For instance, 93% of avian species that went extinct between 1600 and 1980 were island endemics (King 1980) and 73% of birds that went extinct in North America and U.S. territories from 1492-1987 were island species (Williams and Nowak 1987). Thus, understanding processes contributing to loss of population viability takes on immediate importance. Introduction of the brown tree snake (Boiga irregularis) to the Northern Marianas island of Guam during World War II caused a precipitous decline or extinction for all of Guam's native forest birds, including Guam Rails (Rallus owstoni) and Micronesian Kingfishers (Halcyon cinnamomina cinnamomina; Savidge 1987, U.S. Fish and Wildlife Service 1984a, b). In this paper, we examine genetic diversity in these two recently extirpated endemic species. We then compare genetic diversity found in these island species with levels in mainland forms of similar species. Guam Rails are flightless and have life-his-