The Sundarbans freshwater swamp forests are a tropical moist broadleaf forest ecoregion of India and Bangladesh. It represents the brackish swamp forests that lie behind the Sundarbans Mangroves where the salinity is more pronounced. The freshwater ecoregion is an area where the water is only slightly brackish and becomes quite fresh during the rainy season, when the freshwater plumes from the Ganges and Brahmaputra rivers push the intruding salt water out and also bring a deposit of silt. It covers an area of 14,600 square kilometers (5,600 square miles) of the vast Ganges-Brahmaputra Delta, extending from India's West Bengal state into western Bangladesh. The Sundarbans freshwater swamp forests lie between the upland Lower Gangetic plains moist deciduous forests and the brackish-water Sundarbans mangroves bordering the Bay of Bengal.[7]
The fertile soils of the delta have been subject to intensive human use for centuries, and the ecoregion has been mostly converted to intensive agriculture, with few enclaves of forest remaining. The remaining forests, together with the Sundarbans mangroves, are important habitat for the endangered Bengal Tiger (Panthera tigris). In addition to the endangered tiger, there are several other threatened mammal species, such as the capped langur (Semnopithecus pileatus), smooth-coated otter (Lutrogale perspicillata), Oriental small-clawed otter (Aonyx cinerea), and great Indian civet (Viverra zibetha). The ecoregion also contains the leopard (Panthera pardus) and several smaller predators such as the jungle cat (Felis chaus), fishing cat (Prionailurus viverrinus), and leopard cat (Prionailurus bengalensis).[7]
This ecoregion is nearly extinct, the victim of large-scale clearing and settlement to support one of the densest human populations in Asia. Hundreds of years of habitation and exploitation by one of the world's densest human populations have exacted a heavy toll of this ecoregion's habitat and biodiversity. There are two protected areas — Narendrapur (110 km2) and Ata Danga Baor (20 km2) that cover a mere 130 km2 of the ecoregion. Habitat loss in this ecoregion is so extensive, and the remaining habitat is so fragmented, that it is difficult to ascertain the composition of the original vegetation of this ecoregion. According to Champion and Seth (1968), the freshwater swamp forests are characterized by Heritiera minor, Xylocarpus molluccensis, Bruguiera conjugata, Sonneratia apetala, Avicennia officinalis, and Sonneratia caseolaris, with Pandanus tectorius, Hibiscus tiliaceus, and Nipa fruticans along the fringing banks.[7]
The Sundarbans Mangroves ecoregion is the world's largest mangrove ecosystem, with 20,400 square kilometers (7,900 square miles) of area covered. Named after the dominant mangrove species Heritiera fomes, locally known as sundri, it lies in the vast delta formed by the confluence of the Ganges, Brahmaputra, and Meghna rivers across southern Bangladesh and Indian state of West Bengal. This is the only mangrove ecoregion that harbors the Indo-Pacific region's largest predator, the Bengal Tiger. 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). 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.[8]
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. In the Sundarbans, the mangrove forests are characterized by Sundari, a species valued for its timber. Other species that make up the forest assemblage include Avicennia spp., Xylocarpus mekongensis, Xylocarpus granatum, Sonneratia apetala, Bruguiera gymnorrhiza, Cereops decandra, Aegiceras corniculatum, Rhizophora mucronata, and Nypa fruticans palms. Several predators dwell in this labyrinth of channels. Two species of crocodiles — Crocodylus porosus and Crocodylus palustris — dwell here, as well as 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.[8]
As Bangladesh supports one of the world's highest human population densities, the population pressure resulted in half of this ecoregion's mangrove forests 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 — Sajnakhali (2,090 km2), Sundarbans East (210 km2), Char Kukri-Mukri (30 km2), Sundarbans South (200 km2), Sundarbans West (130 km2), Halliday Island (4 km2), Lothian Island (20 km2) — that cover almost 2,700 km2, or 15 percent of the ecoregion. Among these only Sajnakhali is large enough to support a space-dependent species such as the tiger.[8]
