Seminar Series 2003

Natural and Anthropogenic Biogeography Of Mangroves In The Southwest Pacific

Orlo C. Steele – University of Hawai`i Dept. of Botany

An oceanic dispersal filter is generally used to explain the sharp attenuation of mangrove richness from 35 species in the Indo-West Pacific to 3 species in Samoa. Bruguiera gymnorrhiza and Xylocarpus moluccensis are two widespread Old World mangroves that have an easternmost distribution in American Samoa. Rhizophora mangle is thought to be a New World mangrove with outlier populations in Samoa, Tonga, Fiji and New Caledonia. Several researchers have speculated that the Pacific Ocean creates too great a barrier for propagules of R. mangle to have naturally become established in the SW Pacific and that this species was introduced by early Polynesian navigators. To explain the distribution of these 3 Samoan species and why five other western Pacific species are not found in Samoa, propagules of 8 mangrove species found in Tonga and Fiji were collected and floated for up to 8 months in seawater tanks. Viability was determined by outplanting sub-samples in a greenhouse at monthly intervals. Concomitantly, residents near mangrove areas in Samoa, Tonga and Fiji were interviewed to compare use and importance of relevant species. Results of the floatation experiment indicate that R. mangle and X. moluccensis were viable after 8 months of floatation. This is 8 times longer that B. gymnorrhiza and more than 2.5 times longer than the western species Rhizophora stylosa. Based on household interviews in Tonga and Fiji, there was no preference for R. mangle over R. stylosa. However, B. gymnorrhiza was preferred over both species of Rhizophora in Tonga for tapa dye and uniquely reported as a food source in Fiji. To more closely explore potential patterns of dispersal to Samoa, RAPD molecular analysis of the 3 Samoan mangroves in the region is currently underway.