Pacific Island Mangroves: Distribution and Environmental Settings

Abstract

Mangroves, absent from many small, "low" islands and from most of Polynesia, do not cover large areas on Pacific islands, and show rapid decrease in species diversity and stature across the Pacific. Preliminary data indicate that where they do occur they may be as productive, particularly in terms of detritus per unit area, as more luxuriant mangrove forests elsewhere. Oscillations of sea level during the Quaternary have disrupted the distribution of mangroves and present mangrove swamps are shown to have developed and extended substantially during the late Holocene in each of four environmental settings: i) deltaic/estuarine mangroves, ii) mangroves of embayments/harbors/lagoons, iii) mangroves of reef flats, iv) inland mangroves and mangrove depressions. These are ranked in order from i) to iv), from highest to lowest, in terms of landform and mangrove habitat diversity, rates of sedimentation, opportunities for freshwater nutrient input and enhanced productivity, and, it is argued, potential for organic carbon flux and trophic diversity. Structure and functioning of the mangrove ecosystem differs between settings. Restricted stands of mangroves, such as those inland on "low" islands or atolls, are unlikely to export quantities of organic carbon, but nevertheless are productive and support resources which can play an important role in the subsistence economy of the local inhabitants.