Mangroves anchor the edges of the world, but they are slipping away, thanks to coastal development, pollution, over-harvesting, nutrient loading, overuse of freshwater, and climate change.
The world’s largest intact halophytic (salt-tolerating) mangrove forest is the Sundarbans, a UNESCO World Heritage Site that straddles India and Bangladesh. It forms the transition zone between the Ganges and the Bay of Bengal, and is a stronghold for the endangered Bengal tiger, as well as many other species, from monkeys to crocodiles.
The Sundarbans are dominated by Sterculiaceae and Euphorbiaceae mangroves, which are less common in most of the rest of the world. These include Sundari (Heritiera fomes) and looking-glass (Heritiera littoralis) mangroves. The hard wood of the latter was long used in boat building.
However, as a recent report by Dr. Md. Mizanur Rahman warns, these mangroves are in trouble. They face rising temperature, rising seas, silt and pollution washing down from deforested areas in the Himalaya, and pressures from aquaculture activities around the Sundarbans.
They are also being assaulted by rising salinity, brought by the formerly fresh rivers and streams that feed them. As agriculture increases in the region, water levels drop, minerals accumulate, and salinity rises. Brackish water is also expanding underground.
“Predictions from Sundarbans territory show that salinity may be double over the next few decades posing risks for survival of flora in Sundarbans,” writes Rahman.
He continued, “Natural vegetations of such areas are being destructed causing major changes in landscapes and biodiversity. Destruction of remaining natural habitats in core areas, buffer zones and corridors are also occurring. Most of the coastal districts already face severe salinity problems, with saline water pushing up to 250 km inward during the dry season.”
According to Rahman, Sundari trees and nypa palms are declining, changing the makeup of the ecosystem.
He added, “A salt concentration of 20-40% is suitable for mangrove ecosystems, while 40-80% diminishes the number of species and their size. Only a few species can exist and grow in 90% salt concentration. Sundari, Bain, Kakra, Passur and Dhondul tree species are being quickly replaced by Gewa and Keora.”
The fate of the Sundarbans mangroves lies both in how they can be protected locally, and in the health of the whole Ganges system. What happens upstream affects what comes down the pike.