Re-colonizing Mangrove species in tsunami devastated habitats at Nicobar Islands , India

The mega earth quake and the subsequent tsunami in the Indian Ocean during December 2004 had an adverse effect on the mangrove forests of Asian countries such as India, Sri Lanka, Thailand, and Indonesia. Mangrove forests suffered severe damage during tsunami by breaking and uprooting (UNEP 2005; IUCN 2005; Sankaran 2005; Giri et al. 2008). The Nicobar Islands of India situated very close to the epicenter of the earthquake suffered a major loss in terms of human lives and biodiversity. The mangrove forests of Nicobar Islands considered to be one of the pristine forests in India had been badly ravaged during tsunami. It was variously estimated that 62 – 70% of mangrove forests of Nicobar Islands were destroyed (Ramachandran et al. 2005; Roy and Krishnan 2005; Sankaran 2005; Sridhar et al. 2006). The resilience and dynamics of mangrove forests to human induced disturbances have been studied in many parts of the world (Ball 1980; Sherman et al. 2000; Giri et al. 2008), but the information available on the succession of mangroves after natural disasters like hurricane and tsunami are scanty (Roth 1992; Ross et al. 2000). The present study describes the species richness of mangrove re-colonizing habitats in the Central Nicobar group of Islands, which were completely destroyed by the tsunami, 2004.

The mega earth quake and the subsequent tsunami in the Indian Ocean during December 2004 had an adverse effect on the mangrove forests of Asian countries such as India, Sri Lanka, Thailand, and Indonesia.Mangrove forests suffered severe damage during tsunami by breaking and uprooting (UNEP 2005;IUCN 2005;Sankaran 2005;Giri et al. 2008).The Nicobar Islands of India situated very close to the epicenter of the earthquake suffered a major loss in terms of human lives and biodiversity.The mangrove forests of Nicobar Islands considered to be one of the pristine forests in India had been badly ravaged during tsunami.It was variously estimated that 62 -70% of mangrove forests of Nicobar Islands were destroyed (Ramachandran et al. 2005;Roy and Krishnan 2005;Sankaran 2005; Sridhar et al. 2006).
The resilience and dynamics of mangrove forests to human induced disturbances have been studied in many parts of the world (Ball 1980;Sherman et al. 2000;Giri et al. 2008), but the information available on the succession of mangroves after natural disasters like hurricane and tsunami are scanty (Roth 1992;Ross et al. 2000).The present study describes the species richness of mangrove re-colonizing habitats in the Central Nicobar group of Islands, which were completely destroyed by the tsunami, 2004.

Study site
Nicobar group of Islands consists of 24 Islands situated in the Bay of Bengal between Latitude 06°45' -9°15' N and Longitude 92°42' -93°50' E. Of the 24 islands, 12 are inhabited by the humans.The temperature ranges from 22°C -32°C and annual rainfall from 3,000 mm -3,800 mm (Sinha 1999).
The study was carried out in the Central Nicobar group of Islands (Figure 1).Tsunami ravaged mangrove forests of four islands namely Camorta, Nancowry, Katchall and Trinkat were studied for species diversity in the mangrove

Introduction
Mangrove habitats in the tropical coasts confined between latitudes 25° N and 30° S represent a unique ecosystem, which is vital for the maintenance of marine biodiversity (Valiela et al. 2001).Inhabiting the interface between land and sea at low latitudes, mangroves occupy a harsh environment, being subjected to daily tidal changes, temperature, salt exposure and varying degrees of anoxia.Therefore mangroves exhibit a high degree of ecological stability in the water-logged saline zone between sea and terrestrial environment (Tomlinson 1986;Alongi 2008;Giri et al. 2008).Mangrove habitat supports the survival of plant species that are adapted to live in the high salinity conditions.
Mangrove forests occupy 14,650,000 ha of coastline around the globe (Wilkie and Fortuna 2003).Bountiful information is available on the status of mangrove habitats, their extent and biodiversity (Sidhu 1963;Chapman 1976;Dagar et al. 1991;Duke 1992;Saenger and Bellan 1995;Li and Lee 1997;Spalding et al. 1997;Valiela et al. 2001;Jayatissa et al. 2002;Wilkie and Fortuna 2003).These tidal forests are often important nursery grounds and breeding sites for birds, mammals, fish, crustaceans, shellfish and reptiles; a renewable resource of wood; and sites for accumulation of sediment, nutrients, and contaminants (Sasekumar et al. 1992;Kathiresan and Bingham 2001).Mangrove forests protect land from hurricanes and tsunamis by acting as a physical barrier along the coast (Barbier 2006;Chang et al. 2006).
However, the mangrove forests have been declining at an alarming rate-perhaps even more rapidly than inland tropical forests and much of what remains is in degraded condition (Valiela et al. 2001;Wilkie et al. 2003).The remaining mangrove forests are under immense pressure from felling, encroachment, hydrological alterations, chemical spills, farm land conversion, storms and climate change (Menesveta 1997;Blasco et al. 2001;Valiela et al. 2001;McKee 2005;Giri et al. 2008).

Data Collection
The entire coastal lines of the above-mentioned islands were surveyed.Mangrove re-colonizing sites were selected for detailed study.Plant specimens were collected whenever identification was not possible in the field.The collected specimens were identified with the help  et al. 1991;Hajra et al. 1999;Sinha 1999).Nomenclatures of the identified species were checked with the International Plant Naming Index (IPNI).

Results and Discussion
A total of nine species of mangroves belonging to six families and 30 species of mangrove associates belonging to 21 families were enumerated (Table 1).The most common mangrove species in the re-colonizing vegetation are Rhizophora mucronata (Figure 2) and Bruguiera gymnorrhiza (Figure 3B).The common mangrove associates include Wedelia biflora, Dolichandrone spathacea, Cyperus javanicus, Ischaemum muticum and   Caesalpinia bonduc which were found in all the four Islands.Lumnitzera racemosa (Figure 3E) was recorded for the first time from the Central Nicobar Islands.The species richness of mangroves and mangrove associates are more or less similar among the three islands namely Camorta (6, 23), Katchall (6,22) and Trinkat (8,20).Lesser species richness was observed in Nancowry (4, 16).The lesser species richness could be due to the highest damage occurred in this island as reported by Ramachandran et al. (2005).
The observed species richness for the Central Nicobar Islands is lower than the earlier record of 17 and 85 for the mangrove and mangrove associates respectively (Dagar et al. 1991).Loss of mangrove habitats by the perennial submergence of the Nicobar Islands into the sea for about 1m due to the earth quake formed a major constraint for the re-colonizing species (Figure 4).However, the present study gives the first hand information on the plant diversity of re-colonizing mangrove habitats, which will eventually help in the long term monitoring of mangrove species in the Islands.

Table 1 .
List of Mangroves and Mangrove associates observed from the re-colonizing mangrove habitats of Central Nicobar Islands (Ca-Camorta, Ka-Katchall, Na-Nancowry and T-Trinkat).

Figure 1 .
Figure 1.Map showing the Central Nicobar Group of Islands.Figure 2. Saplings of Rhizophora mucronata observed in the regenerating habitat at Trinkat Island (Photo by P Nehru).

Figure 2 .
Figure 1.Map showing the Central Nicobar Group of Islands.Figure 2. Saplings of Rhizophora mucronata observed in the regenerating habitat at Trinkat Island (Photo by P Nehru).