In the wake of a natural disaster, there is an increased recognition of the role that “natural barriers” such as coral reefs, mangroves and sand dunes play in protecting coastlines across the world. Mangroves are unique ecological communities found along tropical and sub-tropical shorelines or estuaries that help in safeguarding the resilience of coastal areas to the threats posed by tropical storms and tsunamis. While anthropogenic disturbances have been regarded as one of the principal causes of mangrove loss, natural disasters can be equally fatal.
The 2004 tsunami affected large parts of south-east Asia, including the Indian coastline and the Andaman and Nicobar Islands, and was accompanied by a tectonic subsidence, a large-scale sinking of the land surface. In a recent study, scientists led by Nehru Prabakaran from Salim Ali Centre for Ornithology and Natural History, Wildlife Institute of India and Leibniz Center for Tropical Marine Research (Germany) examined the diversity and composition of mangrove species in the Nicobar archipelago post the 2004 tsunami and tectonic subsidence and demonstrated significant impacts of this large scale disturbance on the coastal vegetation.
The scientists sampled 34 locations across the Nicobar Islands to assess the diversity of mangrove species and compared it with the diversity reported in the past. Of the 34 locations, 22 were sampled for species abundance and composition. The researchers also sampled new inter-tidal habitats that are formed on the erstwhile terrestrial forests after the sinking of the land to understand the natural colonization of mangroves.Trees devastated by the tsunami and land subsidence (Photo: Nehru Prabakaran)
While the study did report the presence of new successional habitats for mangroves, the estimated loss of mangrove areas was an astounding 97%, a loss much higher than reported in previous studies. The researchers found only three sites with surviving patches of pre-subsidence mangroves for the entire group of islands.
“The studies that used satellite data less than six months from the tsunami probably considered the dead standing trees as a mangrove vegetation. This is evident from the fact that most of the survived mangrove patches indicated by the earlier studies were actually habitats with vast stretches of dead trees,” said Prabakaran.
The authors reported 20 surviving mangrove species in the Nicobar Islands, with the highest diversity in the Central Islands. Four species recorded pre-disturbance were not reported in the current study indicating local extinctions. However, some species were recorded in areas where they were never reported earlier. Successional habitats showed a dominance of two mangrove species (mainly Rhizophora mucronata and Bruguiera gymnorhiza) which contributed to the bulk of their abundance.
Considering the fact that there has been a large-scale loss of mangrove habitats, the study has important implications for mangrove restoration. “Restoration activities must initially focus on facilitating the growth of well-adapted species so as to ensure a functioning mangrove cover,” said Prabakaran, asserting that the zone preferences of mangrove species should be kept in mind during these restoration activities. “Further, some of the areas can be left alone from human interventions to understand the natural recovery process, providing a comparative evidence to the effectiveness of the restoration activities in restoring such habitats,” he suggested.
“There are very few studies that document mangrove succession post natural disturbances. Species such as Nypa fruticans once common in southern Nicobar pre-tsunami, has been stripped away by the tsunami from almost every creek and inter-tidal zone where it used to occur,” said Vardhan Patankar, scientist at Centre for Wildlife Studies, and National Centre for Biological Sciences, with several years of research experience in the Nicobar Islands. “Traditional housing on Little and Great Nicobar Islands used these leaves as thatch, thereby stressing the importance of Nypa rejuvenation as a management intervention,” said Patankar.
Mangrove forests are a crucial part of the coastal ecosystems and form the first line of defense for any natural disturbances. The loss of such habitats by both anthropogenic and natural causes has significant impacts on the stability and functionality of these systems.
This is a companion discussion topic for the original entry at https://indiabioscience.org/news/2018/surviving-natures-wrath-mangroves-of-nicobar