Mangrove Rehabilitation and Intertidal Biodiversity: a Study in the Ranong Mangrove Ecosystem, Thailand

Abstract

The diversity, abundance, biomass and community structure of crustacean and molluscan macrofauna were studied in the Ranong mangrove forest ecosystem on the Andaman Sea coast of southern Thailand. After a history of commercial exploitation the mangroves along the Klong Ngao tidal creek have been assigned conservation status within a new Ranong Biosphere Reserve established in 1997. Over the past 12 years, several areas of mangrove destroyed or degraded by wood harvesting, tin mining and aquaculture, have been rehabilitated on a pilot basis by planting monocultures of mangrove seedlings using four common local species (Rhizophora apiculata, R. mucronata, Bruguiera cyclindrica and Ceriops tagal). These plantation forests with different past management histories were compared with a natural, mixed, mature mangrove forest which has been conserved for about 40 years. Macrofauna were sampled within a 100 m² vegetation quadrat in each study site. Crustaceans were sampled quantitatively by 3×15 min timed hand catches per site. Molluscs were sampled in 3× m² quadrats positioned around three randomly selected trees in each vegetation quadrat. The lowest crustacean and molluscan diversity was recorded from the former tin mining site. The highest diversity was recorded from aRhizophora plantation in the natural mixed forest area for both crustaceans and molluscs. The vegetation community structure was not correlated with the environmental variables measured, or with macrofauna community structure. Of the environmental parameters chosen, the crustacean community structure was best expressed by shore level, while for molluscan diversity and abundance it was soil moisture content. The macrofauna community structure at the tin mining site was significantly different to the other sites, and was dominated by a single species of crab, Metaplax elegans. Grapsid crabs, especially sesarmid species, dominated over ocypodid crabs in the mature forest site, whereas Uca species and other ocypodids were more abundant than grapsids in the degraded concession forest area. Snails of the families Neritidae and Ellobiidae were the most abundant molluscs in the mature forest, whereas Littoriinidae, Assimineidae and Potamidae species were more representative of the younger plantation sites. The findings from this study suggest that some of these well represented families of mangrove macrofauna could be used as indicators of ecological change as part of a long term environmental monitoring programme in Ranong and other areas in Southeast Asia where mangroves are being rehabilitated.