Mangrove flora The study of the floral diversity of the Sundarbans is limited to a group of commercially important species and few others. The forest supports a diverse plant genetic stock including timber species, wild relatives of cultivated plants and a number of endemic species. While most of the mangroves in other parts of the world are characterized by members of the Rhizophoraceae, Avicenniaceae, and Laganculariaceae, the mangroves of Bangladesh are dominated by the Streculiaceae and Euphorbiaceae. The mangroves of the Sundarbans are unique when compared to non-deltaic coastal mangrove forest; the reason for this difference is the large freshwater influence in the north-eastern part and the elevated level of the ground surface. Sunduri (Heritiera fomes) and Gewa (Excoecaria agallocha) are the dominant species, occurring in suitable localities along the coast. Unlike former times, Rhizophoraceae are now of minor importance. Inventory of the Sundarbans shows that there has been a change in favour of mixed Heritiera fomes and Excoecaria agallocha at the expense of pure Heritiera fomes forest. A further important change that the forest has undergone is a reduction in canopy density. In 1959, 78% of the forest had a canopy closure of 75% or more; now only 65% of the forest has canopy closure of 70% or more. Current inventory also shows that approximately 70% of Heritiera fomes stems are moderately or severely affected by topdying.
Mangrove fauna The mangrove forest provides a unique ecosystem and extensive habitats for a wide variety of faunal species. The Sundarbans is a very high quality wildlife conservation area of regional and international importance. Many authors have studied and given lists of the wildlife assemblages of the Sundarbans (Hendrichs, 1975; Mukherjee, 1975; Khan, 1982; Hussain et al., 1983; Sanyal, 1983, 1987; Khan, 1986, 1987; Seidensticker & Hai, 1983). The international importance of the Sundarbans as an essentially long-term wildlife habitat has also been documented (Gittins & Akonda, 1982; Seidensticker, 1986, 1987). Several studies (Seidenstiker & Hai, 1983; Blower, 1985a, b; Khan, 1986) reported that the mangroves of Bangladesh support at least 120 species of commercially important fishes, 290 species of birds, 42 species of mammals, 35 reptiles, and 8 species of amphibians. The mangroves also support a large number of species, which are now extinct elsewhere in the country. These include 2 amphibians, 14 reptiles, 25 birds, and 5 mammals.
Fish fauna The fishery resources are one of the primary concerns in the Sundarbans mangrove ecosystem. The fishery resources are characterized by a very high degree of endemism and diversity. The number of fish species available in the Sundarbans and their taxonomic status has been described among others by Rahman (1975, 1989) with varying degrees of accuracy. According to Hussain and Acharya (1994) the mangrove waters support 53 species of pelagic fish belonging to 27 families and 124 species under 49 families of demersal fish. However, this number is expected to be as high as 400 species using the mangrove swamps in various ways. Diadromous species such as Pangasius pangasius, Hilsa ilisha, Lates calcarifer are common in the low saline zone. The moderately saltwater zones are dominated by Hilsa ilisha; Pomadasys hasta, Polynemus spp., Coilia spp., Johnius spp. etc. The highly saline zones are likely to support species such as Harpodon nehereus; Trichiurus savala, Setipinna spp., Pampus spp., Sardinella spp., Salar spp. etc. Considerable seasonal changes also occur in the quality and quantity of fish fauna in the region. Over 120 species of fish are reported to be commonly caught by commercial fishermen in the Sundarbans. According to Mukherjee (1975) only brackish water species and marine forms are found in the Indian Sundarbans, freshwater species being totally absent. This may be assumed to apply also to the Bangladesh Sundarbans, except possibly in the eastern portion where there is freshwater in Baleswar River. Mention should also be made of mud-skippers or gobies which occur in large numbers and are a characteristic feature of mangrove swamps.