Indian Ocean tsunami: Nicobar Islands lost 97 percent of mangrove cover, uncovered unknown species

The 2004 Sumatra-Andaman earthquake followed by the catastrophic tsunami gobbled up landmass and stripped the coast of trees in the Nicobar Islands in the Indian Ocean. Mangroves, which flourish where land and water meet, bore the brunt of the natural disaster.

Superseding initial reports that documented 60 to 70 percent mangrove cover loss, a new study reveals that in fact 97 percent of mangrove cover was razed due to the event.

But the field study also points to a silver lining: the natural disturbance triggered the emergence of potential habitats for regrowing mangroves and uncovered hitherto unrecorded mangrove species in the Nicobar islands.

“A large number of trees were uprooted and most of the inter-tidal habitats were permanently submerged causing large scale tree mortality. However, the land drowning also created new inter-tidal habitats with potential for mangrove re-establishment at the erstwhile coastal terrestrial habitats and human settlements,” Nehru Prabakaran of Wildlife Institute of India, lead author of the study, told Mongabay-India.

The intertidal zone is the area between the highest-tide marks and lowest-tide marks. This habitat is covered with water at high tide and exposed to air at low tide.

A massive series of tsunamis set-off by the third largest earthquake recorded globally (the 2004 Sumatra Andaman earthquake) battered the immediate coastal areas of western Indonesia and spread out across the Indian Ocean to impact communities in Australia, Myanmar, India, Kenya, Malaysia, Maldives, Seychelles, Somalia, Sri Lanka, Tanzania and Thailand.

It severely damaged the coastal habitats of the Nicobar Islands – the landmass closest to the earthquake epicentre.

Almost no pre-tsunami mangroves left

The Nicobar Islands, a part of the Andaman and Nicobar Islands chain, stretch in an arc southward between Myanmar and the island of Sumatra in Indonesia.

At present, 21 islands (spread in three groups) form the Nicobar archipelago. 

The Northern group consists of two islands (Car Nicobar and Batimalve); Central group consists of nine islands (Kamorta, Katchall, Nancowrie, Trinket, Teresa, Bombuka, Chowra, Tillanchong and Isle of Man); and the Southern group consists of ten islands including Great Nicobar and Little Nicobar.

During the tsunami, giant waves slammed into the coastline. The entire stretch of the Nicobar Islands was drowned into the sea to varying levels from 1.1 metres to 3 metres.

This sudden land drowning (due to tectonic subsidence) coupled with the tsunami altered the structure of mangrove forests, said Prabakaran of WII.

The severity of the subsidence can be gleaned from the complete submergence of Megapode Island – 0.2 square km in size and largely covered by mangroves at the time – located to the west of Great Nicobar island.

The 2009 Forest Survey of India report noted a decline in mangrove cover in the Andaman and Nicobar Islands by 20 square km attributing the loss to “after effects of tsunami.” The mangrove cover in 2007 (data for the period October-December 2006) is recorded at 615 square km while that for 2005 (data for the period October-December 2004) shows a cover of 635 square km. The 2017 report documented 617 square km of mangrove cover in the Andaman and Nicobar Islands.

“We found that mangrove cover in the Nicobar islands declined by around 97 percent, which is higher than the earlier reports. Except for one site in Car Nicobar island, there is no pre-tsunami mangrove vegetation surviving in Nicobar Islands in the aftermath of the tsunami,” Prabakaran said.

The study was carried out between 2010 and 2013 and was facilitated by Salim Ali Centre for Ornithology and Natural History (SACON). 

Out of the 21 islands in Nicobar, 20 islands (except Batimalve island) were included in the study. The entire coastline of the islands was surveyed for the presence of mangroves and species diversity was elucidated.    

The researchers documented 20 mangrove species from 34 sites across the Nicobar Islands, of which eight species were recorded for the first time from the island group.

“We think there could be two major reasons for the new reports of eight mangrove species. These islands are often under explored and therefore the species were already there but not reported,” Prabakaran said.

The second factor could be that the tsunami may have carried the seed material of the unreported species from nearby sources (such as Malaysia, Indonesia and Singapore) along with the huge pile of debris dumped on the Nicobar coasts after the tsunami.

Prabakaran elaborated on the larger picture based on the Nicobar mangrove mapping data: planning for mangrove conservation against the backdrop of natural disasters.

“Sundarbans is also facing a great threat from land drowning (due to tectonic subsidence). Evidence suggests that the sea level at some sites in the Sundarbans has increased nearly one metre in the last century. Therefore, to preserve these mangroves we need a plan in place,” he said.

“The observation of mangrove species (Rhizophora mucronata, R. apiculata and R. stylosa) in Nicobar islands surviving even after one metre sudden land drowning, provides some insights into how we can proceed in mangrove conservation during such scenarios,” Prabakaran said.

Daniel Friess of National University of Singapore, who was not associated with the mapping, found the study interesting because it shows that “mangroves can also be heavily impacted by natural processes such as subsidence.”

“We tend to focus on the impacts of humans on mangroves, through deforestation and pollution. Mangroves can only grow where the physical conditions are right and events such as the tsunami can change the local coastal environment dramatically,” Friess told Mongabay-India.

How did the tsunami wreak havoc on the Nicobar mangroves?

The earthquake-tsunami combo dealt a double blow to the mangroves in Nicobar.

While the force of the tsunami uprooted a huge number of trees, submergence of land made the habitat unsuitable for mangroves.

“The soil layer of the inter-tidal areas where mangroves grow are usually flooded during high-tide and the water gets drained leaving the soil exposed during the low-tide (two cycles of high and low tide happens everyday). But after the land drowning, the mangrove habitats in Nicobar Islands were permanently inundated with seawater. This hampered soil respiration and almost all the mangrove vegetation that survived tsunami started wilting and were dead within two months after the disaster,” Prabakaran explained.     

During their survey, the researchers discovered that four mangrove species that were common in Nicobar Islands before the tsunami were not found and believed to be locally extinct in the event’s aftermath.

The commonality among these four species was that they preferred to grow on the landward margins (towards the land as opposed to the sea) of the mangrove forest.

“When there is an increase in sea level (by land subsidence here), the mangrove species that grow towards the seaward margin can retreat towards land but the landward mangrove movement will be restricted because of the often unsuitable geomorphology,” Prabakaran said.

For example, at many sites in Nicobar, mountains or elevated areas start just after the mangrove habitat. This may be the reason why there is more potential local extinctions among the landward mangroves, he said.

As for the discrepancy of the figures on mangrove forest cover loss and potential influence of land drowning on mangrove decline, Prabakaran said the earlier studies based on satellite data were conducted immediately after the tsunami and ground-truthing data was not yet available.

“However, information was yet unavailable on the ground reality in Nicobar Islands, largely due to its remoteness and lack of basic amenities to facilitate fieldwork-based studies. Therefore, we initiated this field-based study to know what is the ground reality in Nicobar Islands? How different mangrove species are responding to varying levels of land drowning? What mangrove species are re-establishing in the new inter-tidal habitats,” he said.

Bringing science to mangrove restoration

Based on their mangrove species mapping, the authors zeroed-in on specific sites across the Nicobar islands that can be taken up for the restoration of mangrove forest.

Moreover, they suggest specific species that can be given priority during the restoration and that the habitat specificity of each species needs to be carefully considered during the planting.

“Often during mangrove restoration, one species is planted everywhere. Each mangrove species has special requirement. If you plant species that are suited for the seaward margin on the landward side, they will not survive and vice versa. The restoration needs to be scientific otherwise it will be a failure,” Prabakaran said.

“For example, if you plant Rhizophora species only on the landward side, it may not survive as it grows along the sea-facing side,” he said.

Prabakaran also stressed on harvesting seed materials within the Nicobar group itself rather than collecting and transporting them from the neighbouring Andaman chain.

“Initially, seed materials were being brought in from the Andaman Islands. There were no seed materials in Nicobar after the tsunami. This process takes a lot of time and expense and the seeds may not survive by the time they are brought to the Nicobar sites. So we have we have identified sites and recommended collection of seeds within the Nicobar islands,” he said.

The study also highlighted that some sites need to be left alone from any human assisted mangrove restoration to understand how these forests re-establish naturally and to derive comparative measures on the effectiveness of restoration projects on bringing back the coastal biodiversity.

CITATION:

Nehru, P., & Balasubramanian, P. (2018). Mangrove species diversity and composition in the successional habitats of Nicobar Islands, India: A post-tsunami and subsidence scenario. Forest Ecology and Management, 427, 70-77.