The Sundarbans Mangroves [IM1406] ecoregion is the world's largest mangrove ecosystem. Named after the dominant mangrove species Heritiera fomes, locally known as sundri, this is the only mangrove ecoregion that harbors the Indo-Pacific region's largest predator, the tiger (Panthera tigris). Unlike in other habitats, here tigers live and swim among the mangrove islands, where they hunt scarce prey such as chital deer (Cervus axis), barking deer (Muntiacus muntjak), wild pig (Sus scrofa), and even macaques (Macaca mulatta). Quite frequently, the people who venture into these impenetrable forests to gather honey, to fish, and to cut mangrove trees to make charcoal also fall victim to the tigers.
But the ecoregion's importance is not based solely on its role as a priority tiger conservation area (Dinerstein et al. 1997). Mangroves are a transition from the marine to freshwater and terrestrial systems. They provide critical habitat for numerous species of fishes and crustaceans that are adapted to live, reproduce, and spend their juvenile lives among the tangled mass of roots, known as pneumatophores, that grow upward from the anaerobic mud to get the trees' supply of oxygen.
Location and General Description
The ecoregion lies in the vast delta formed by the confluence of the Ganges, Brahmaputra, and Meghna rivers. The maze of mangrove channels extends across southern Bangladesh and India's West Bengal State. The June to September monsoon brings heavy rains and frequent, devastating cyclones that cause widespread destruction. Annual rainfall can exceed 3,500 mm, and daytime temperatures can exceed a stifling 48(C during these monsoon months.
Mangroves are not diverse compared with most other terrestrial ecosystems. The undisturbed forests have an unstratified, dense canopy and an undergrowth made up of seedlings and saplings of the canopy trees (Tomlinson 1986). In the Sundarbans, the mangrove forests are characterized by Heritiera fomes, a species valued for its timber. Other species that make up the forest assemblage include Avicennia spp., Xylocarpus mekongensis, X. granatum, Sonneratia apetala, Bruguiera gymnorrhiza, Cereops decandra, Aegiceras corniculatum, Rhizophora mucronata, and the palm Nypa fruticans (Puri et al. 1989).
This vast mangrove ecosystem along the marine, freshwater, and terrestrial interfaces provides critical ecosystem functions. The tangled mass of roots from mangrove trees provides safe havens for juvenile stages of a gamut of species, from fish fry to shrimp naupleii.
The ecoregion harbors several mammals, but the most charismatic undoubtedly is the majestic Bengal tiger that swims from island to mangrove island searching for and hunting scarce prey. Because this ecoregion represents the only example of tigers ecologically adapted to a life in the mangroves, it has been designated a Level I TCU (Dinerstein et al. 1997). The tiger's reputation as a human-eater is greater here than anywhere else in its range; the people who venture into the Sundarbans must take great precautions to avoid being attacked. One fascinating deterrent is wearing a mask in the back of the head because tigers are believed to be less liable to attack a human looking directly at them.
Several other predators dwell in this labyrinth of channels. Two species of crocodiles (Crocodylus porosus and C. palustris), the Gangetic gavial (Gavialis gangeticus), and the water monitor lizard (Varanus salvator) use both land and water to hunt and bask in. Sharks and the Gangetic freshwater dolphins (Platanista gangetica) inhabit the waterways. And several birds of prey patrol the sky overhead. More cryptic but equally fascinating are the mudskippers, a gobioid fish that climbs out of the water into mudflats and even climbs trees. An abundance of crabs, hermit crabs, and shrimp scavenge among the roots.
More than 170 bird species are known to inhabit these mangrove forests, including a single endemic species (table 1). This brown-winged kingfisher is limited to the coastal habitats in this ecoregion.
Table 1. Endemic and Near-Endemic Bird Species.
Family Common Name Species
Alcedinidae Brown-winged kingfisher Pelargopsis amauropterus*
An asterisk signifies that the species' range is limited to this ecoregion.
The bird assemblage also includes the globally threatened lesser adjutant (Leptoptilos javanicus) and threatened masked finfoot (Heliopais personata) (IUCN 2000). There are twelve birds of prey that coexist here, including the osprey (Pandion haliaetus), white-bellied sea eagle (Haliaeetus leucogaster), and grey-headed fish-eagle (Ichthyophaga ichthyaetus). The mangrove ecosystem also is an important staging and wintering area for migratory birds, which include several species of shorebirds, gulls, and terns.
Bangladesh supports one of the world's highest human population densities. About half of this ecoregion's mangrove forests have been cut down to supply the fuelwood and other natural resources extracted from these forests by this large population. Despite the intense and large-scale exploitation, the ecoregion still is one of the largest contiguous areas of mangroves in the world.
There are seven protected areas that cover almost 2,700 km2, or 15 percent of the ecoregion (table 2). Despite the high proportion of the ecoregion being within the protected area system, only one of these, Sajnakhali, is large enough to support a space-dependent species such as the tiger. Many of the protected areas also lack trained and dedicated personnel and infrastructure to adequately manage them.
Table 2. WCMC (1997) Protected Areas That Overlap with the Ecoregion.
Protected Area Area (km2) IUCN Category
Sajnakhali 2,090 IV
Sundarbans East 210 IV
Char Kukri-Mukri 30 IV
Sundarbans South 200 IV
Sundarbans West 130 IV
Halliday Island 4 IV
Lothian Island 20 IV
Ecoregion numbers of protected areas that overlap with additional ecoregions are listed in brackets.
Types and Severity of Threats
The conservation status of the ecoregion was changed from vulnerable to endangered because of the projected human threats. The human population in the Sundarbans, now estimated at more than 2 million (Spalding et al. 1997), continues to increase very rapidly. Hunting and trapping wildlife, cutting and lopping trees for fuelwood and to make charcoal, and overexploiting the trees for timber by the forestry industry are some of the most severe current threats. Shrimp fry are being collected at unsustainable levels to supply the shrimp grow-out industry, and the mangrove forests are cut and cleared to build shrimp grow-out ponds, contributing to degradation and habitat loss. There is also the potential for harmful effluents to enter the mangrove waterways from a proposed fertilizer plant.
But some of the greatest threats to this ecoregion's biodiversity emanate from thousands of kilometers away. The rivers that feed and flush the mangroves bring down heavy silt loads as a result of deforestation and erosion in the Himalayan Range. This silt and water turbidity have profound effects on the sensitive mangrove ecosystem and its flora and fauna, especially on the juvenile stages that the mangroves support. The diversion of more than 30 percent of the Ganges River's dry season flow through the Farraka Barrage in India to provide irrigation for agriculture has drastically increased salinity levels and disrupted fish migration and breeding patterns. The delicately balanced community composition in these mangroves is determined to a large part by salinity levels.
Justification of Ecoregion Delineation
In keeping with our definition of representing habitat types of regional extent in separate ecoregions, we placed the large area of mangrove forests into the Sundarbans Mangroves [IM1406]. We used MacKinnon's (1997) digital map of the original vegetation, aided by Spalding et al. (1997) to define the ecoregion boundaries. This ecoregion falls within Udvardy's Bengalian rain forest biogeographic province.
References for this ecoregion are currently consolidated in one document for the entire Indo-Pacific realm.
Indo-Pacific Reference List
Prepared by: Gopal S. Rawat and Eric D. Wikramanayake