Introduction and Spread of Mangroves in the Hawaiian Islands

Abstract

Introduction and Spread of Mangroves in the Hawaiian Islands Lyndon Wester* The terrestrial flora of Hawaii was derived from a small number of ancestral species as a result of the archipelago's remote location. Great expanses of open ocean separate Hawaii from continents and other islands, and thus water has acted as an effective barrier even to many widespread drift species whose propagules can float and withstand long immersion in seawater. The intertidal zone in Hawaii lacked vascular plants entirely despite the availability of habitats suitable for many of them (Guppy 1906, Egler 1942). The introduction and subsequent naturalization of mangroves and other exotic species has produced significant changes in the coastal environments. Mangroves are opportunist trees that colonize the tidal zone of tropical coasts. The most extensive stands occur on aggrading shores which receive sediments from large drainage systems, but small colonies also can be found on limestone coasts, coral atolls, sand beaches, and lagoons, suggesting that protection from wave action, rather than the presence of fine sediments, is a prerequisite for seedlings to take root (Sauer 1976). *Dr. Wester is an associate professor, Department of Geography, University of Hawaii, Honolulu, HI 96822. 125 126 ASSOCIATION OF PACIFIC COAST GEOGRAPHERS About 50 mangrove species, belonging in some 12 families, include remarkable cases of possible convergent evolution. Seedlings of all mangrove species are well adapted for dispersal over oceans but, under natural conditions, none were successful in colonizing the islands of the central Pacific (Hawaii, Society, Marquesas, etc.). The taxa of the so-called eastern mangroves of Indo-Malaysia are, with one exception, made up of different species from the western mangroves of tropical America and West Africa. Only Rhizophora mangle L., the most common tropical American species, is also found in the eastern mangrove region, notably in Fiji, Tonga, and New Caledonia (Salvoza 1936, p. 199; Hou 1960). The Introduction of Mangroves into Hawaii The history of the introduction of exotic organisms is usually incomplete. The record of mangroves in Hawaii has been especially confused. Of the seven introduced taxa, only two are currently maintaining themselves (Table 1). Rhizophora mangle is the most common species in Hawaii, and the majority of mangrove communities are dominated by it. The American Sugar Company (Molokai Ranch Co.) first introduced the species in 1902 and planted it on Molokai in the belief that it would retain sediment from the eroded southern slopes of the island and supplement the honey flora (MacCaughy 1917, Degener 1940). Seedlings were intentionally transplanted from Molokai to Oahu (for reasons unknown) and have since volunteered elsewhere, including places on the islands of Hawaii, Maui, Lanai, and Kauai (Fig. 1). The Hawaiian Sugar Planter's Association made two attempts to introduce Old World species from the Philippines. The first, in 1908, Table 1. Mangrove species in the Hawaiian Islands' Species Date Origin Comment Rhizophora mangle R. mucronata Bruguiera gymnorhiza B.parviflora Ceriops tagal Conocarpus erectus C.erectus var. sericeus 1902 1922 1922 1922 1922 Before 1910 1946 Florida Philippines Philippines Philippines Philippines Florida? Bahamas Abundant Last recorded 1928 Established Last recorded 1948 Last recorded 1922 Cultivated Cultivated 'Dates of introduction and last record were obtained from MacCaughy (op. cit.), accession records of the Hawaiian Sugar Planter's Association, and herbarium records. YEARBOOK · VOLUME 43 · 1981127 THE OCCURRENCE OF MANGROVES IN THE HAWAIIAN ISLANDS OAHU ^He eia Swamp Pearl Harbc Waikele Delta-y>;Ui Ui O O d ? O t? O 55 o. o O > Ui H O W O O£T) X m¡? Ui YEARBOOK »VOLUME 43· 1981131 Raddi, Lantana cámara L., and Leucaena leucocephala (Lam.) de Wit are found, with a few herbs and the vines Paederia foetida L. and Ipomea alba L. Where tall trees are cut back from a bridge to prevent fouling, the lower growing B. gymnorhiza, T. populnea, and H. tiliaceus flourish. This would suggest that the success of R. mangle in areas frequently inundated by tidewater may be explained by competition for light, since the Rhizophora can grow taller and possibly more rapidly than its associates. Attenuated, mature specimens of T. populnea and H. tiliaceus are encountered occasionally under a mangrove canopy and may be remnants of an earlier littoral community that has...