Fragmentation and genetic differentiation among subpopulations of the endangered Hawaiian mint Haplostachys haplostachya (Lamiaceae)

Haplostachys haplostachya was at one time a prevalent species in the mid-elevation (1650-1800 m) xerophytic shrubland between Mauna Loa and Mauna Kea on Hawaii. Grazing pressure by feral ungulates, destruction of habitat from military activity, and conversion of forest to pastures have fragmented its range to small subpopulations restricted to several cinder cones and portions of the remaining shrubland. Some subpopulations are still extensive, while others are reduced to 20 or fewer individuals. Genetic analyses using random amplified polymorphic DNA (RAPD) markers demonstrate that plants in subpopulations with a large number of individuals maintain levels of genetic variation similar to the entire population, whereas plants in a small subpopulation at Pu'u Leilani (14 individuals remaining) have reduced levels of variation. Of the 122 loci identified, 54 (44%) were polymorphic. Two large populations showed variation at 45 and 49 loci, but the Pu'u Leilani plants showed variation at only 37 loci. The mean expected heterozygosity (H) in this subpopulation was also lower (0.137 vs. 0.163 and 0.154) and genetic differentiation (G(ST)) was higher (0.167 vs. 0.018 and 0.052) than in other subpopulations. An examination of variation indicates that although plants of the three subpopulations are genetically similar, there is evidence of genetic restructuring among the subpopulations. The impact of these results towards conservation efforts of this and other endangered species is discussed.