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Biodiversity and Its Conservation in the Sundarban Mangrove Ecosystem (Gopal and Chauhan, 2006)

Aquat. Sci. 68 (2006) 338–354
1015-1621/06/030338-17
                                                 Aquatic Sciences
DOI 10.1007/s00027-006-0868-8
© Eawag, Dübendorf, 2006




Overview Article


Biodiversity and its conservation in the Sundarban
Mangrove Ecosystem
Brij Gopal* and Malavika Chauhan

School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India

Received: 15 December 2005; revised manuscript accepted: 9 March 2006


Abstract. The Sundarban, covering about one million ha    darban has been extensively exploited for timber, fish,
in the delta of the rivers Ganga, Brahmaputra and Megh-   prawns and fodder. The regulation of river flows by a se-
na is shared between Bangladesh (~60 %) and India      ries of dams, barrages and embankments for diverting
(~40 %), and is the world’s largest coastal wetland. The   water upstream for various human needs and for flood
area experiences a subtropical monsoonal climate with    control has caused large reduction in freshwater inflow
an annual rainfall of 1,600–1,800 mm and severe cy-     and seriously affected the biodiversity because of an in-
clonic storms. Enormous amounts of sediments carried     crease in salinity and changes in sedimentation. Heritiera
by the rivers contribute to its expansion and dynamics.   fomes (locally called Sundari, from which Sundarban
Salinity gradients change over a wide range of spatial and  derives its name), Nypa fruticans and Phoenix paludosa
temporal scales. The biodiversity includes about 350 spe-  are declining rapidly. During the past three decades, large
cies of vascular plants, 250 fishes and 300 birds, besides  parts of the remaining Sundarban have been protected for
numerous species of phytoplankton, fungi, bacteria, zoo-   wildlife, particularly tiger, through the creation of several
plankton, benthic invertebrates, molluscs, reptiles, am-   sanctuaries and a biosphere reserve. Parts of the Sundar-
phibians and mammals. Species composition and com-      ban in both India and Bangladesh have been declared
munity structure vary east to west, and along the      World Heritage sites. However, its biodiversity continues
hydrological and salinity gradients. Sundarban is the    to be threatened by a growing human population that not
habitat of many rare and endangered animals (Batagur     only places pressure on its biological resources, but also
baska, Pelochelys bibroni, Chelonia mydas), especially    impacts on the freshwater inflows from upstream areas.
the Royal Bengal tiger (Panthera tigris). Javan rhino,    Oil exploration in coastal areas is also emerging as a new
wild buffalo, hog deer, and barking deer are now extinct   threat. Further threats arise from global climate change,
from the area. Large areas of the Sundarban mangroves    especially sea level rise. The future of the Sundarban will
have been converted into paddy fields over the past two    depend upon the management of freshwater resources as
centuries, and more recently into shrimp farms. The Sun-   much as on the conservation of its biological resources.


Key words. Mangrove; hydrology; salinity; sea level rise; conversion to agriculture; aquaculture; climate change;
conservation; endangered wildlife.




* Corresponding author e-mail: brij@nieindia.org
Published Online First: September 4, 2006
                                                             Overview Article
Aquat. Sci. Vol. 68, 2006                                                                339


Introduction                                  area lies in Brazil, Australia and Nigeria (Spalding et al.,
                                        1997). While practically all mangroves occur in small
Mangroves1 are intertidal forested wetlands confined to             patches that develop in deltaic habitats, the mangroves in
the tropical and subtropical regions (Tomlinson, 1986).            the Ganga-Brahmaputra-Meghna Delta, shared between
The total global area of the mangroves is estimated at             India and Bangladesh, are the only contiguous and larg-
only 18.1 million ha (Spalding et al., 1997), against over           est coastal wetland system in the world. Popularly known
                                        as Sundarban2, they currently cover about one million ha
570 million ha of freshwater wetlands including peat-
lands globally (but excluding paddy fields; Spiers, 1999).           area, greater than the combined area of Wadden Sea wet-
Although mangroves have been exploited for many cen-              lands that are shared between Denmark, Germany and
turies, our scientific understanding of these wetland for-           the Netherlands.
ests remained poor until the 1970s (Lugo and Snedaker,               Exploration of the Sundarban mangroves dates back
                                        to the 16th century (Rollet, 1981). A large bulk of pub-
1974; Blasco, 1975; Chapman, 1976). During the past
three decades or so, these wetland forests have received            lished literature exists on the Sundarbans of both India
increasingly greater attention which is reflected in an             (Naskar and Guha Bakshi, 1987; Chaudhuri and Choud-
exponential increase in the number of publications (Elli-           hury, 1994; Guha Bakshi et al., 1999) and Bangladesh
son, 2002). Several recent publications have examined             (Seidensticker et al., 1991; Hussain and Acharya, 1994;
issues concerning ecology, management and conserva-              IUCN-BD 2002, Islam and Wahab, 2005) covering many
tion of mangroves (Robertson and Alongi, 1992; Ricklefs            aspects of their habitat characteristics, flora, fauna (par-
and Latham, 1993; Ellison et al., 1999; Kathiresan and             ticularly fisheries), utilization and management, yet very
Bingham, 2001; Macintosh and Ashton, 2002; Ellison,              little is known about the functional aspects of this ecosys-
2002; Linneweber and de Lacerda, 2002; Vannucci,                tem. In this review, we examine the current state of our
2003; Saenger, 2003).                             knowledge of Sundarban’s biodiversity, various factors
  The biodiversity of mangroves has also been of in-             threatening it, and the recent conservation measures. This
creasingly greater interest, firstly because of the Conven-           is preceded by a brief overview of the geology, climate,
tion on Biological Diversity, and secondly, because the            soils and hydrology which are the major driving variables
mangrove ecosystems are among the most threatened by              of mangrove biodiversity in this region. It may be noted
the global climate changes, particularly the sea level rise          here that since 1947 the Sundarban mangroves are di-
(Macintosh and Ashton, 2002, 2004). Mangroves are               vided between India and Bangladesh (formerly East Pa-
relatively well known for their floral diversity which is            kistan), and the two parts differ considerably in the nature
comprised of only 65–69 species of vascular plants               and extent of investigations, conservation and manage-
which have several specific adaptations to the dynamic             ment. They also differ substantially in the level of human
coastal environment (see Kathiresan and Bingham, 2001)             exploitation over more than a century. This makes it dif-
and among the fauna, their fisheries (both fishes and crus-           ficult to integrate the information of the entire Sundar-
taceans) resources are better understood. The global pat-           ban. Therefore, we frequently refer to the Indian and
terns of biodiversity in mangroves also present an inter-           Bangladesh parts of the Sundarban separately.
esting picture. Whereas the latitudinal pattern of
mangrove flora is normal in as much as the highest spe-
                                        The Sundarban mangroves
cies richness of plants occurs around the Equator and
declines at higher latitudes – both north and south (Duke
et al., 1998; Ellison et al., 1999), the longitudinal distri-         The Sundarban (21º 30′ to 22º 40′ N, 88º 05′ to 89º 55′ E)
bution is quite ‘anomalous’ with high concentrations in            comprises essentially of numerous islands formed by the
the Eastern Hemisphere between 90º E and 135º E (Rick-             sediments deposited by three major rivers, the Ganga,
lefs and Latham, 1993; Ellison and Farnsworth, 2001).             Brahmaputra and the Meghna, and a dense network of
Interestingly, the mangrove-inhabiting molluscs follow a            smaller rivers, channels and creeks. The maximum eleva-
similar pattern (Ellison et al., 1999).                    tion within the Sundarban is only 10 m above the mean
  More than 41 % of the world’s mangroves occur in              sea level. The western and eastern limits of the Sundar-
South and Southeast Asia of which Indonesia alone ac-             ban are defined by the course of the River Hooghly (a
counts for 23 %. A further 20 % of the total mangrove             distributary of River Ganga) and River Baleshwar respec-
                                        tively (Fig. 1). The River Harinbhanga (known as
                                        Ichamati or Raimongal in Bangladesh) demarcates the
1
  The term ‘mangrove’ is used variously referring to plants (Macnae, 1968;
  Unesco, 1973), their communities (Blasco, 1975; Hamilton and Snedaker,
  1984) and the whole ecosystem. Macnae (1968) reserved the term
                                        2
  ‘mangal’ to describe the mangrove plant communities. In this paper, we     Often referred to in plural as ‘Sundarbans’ because the forest comprises
  follow Arroyo (1979, 1997) who used the term mangrove for the entire      of hundreds of islands separated by rivers and creeks, or to mean both the
  ecosystem, its plant communities as well as the plants inhabiting them.    Indian and Bangladesh parts together.
340    B. Gopal and M. Chauhan                         Biodiversity of Sundarban Mangrove Ecosystem


border between India and Bangladesh. About 60 % of the      ban in the two countries often differ considerably. Ac-
mangrove forests lie in the Khulna District of Bangla-      cording to recent estimates, the area of the Sundarban in
desh and the rest in the 24-Paragnas District of West       Bangladesh is 599,330 ha (1978 Landsat data; Rahman et
Bengal (India). The estimates of the total area of Sundar-    al., 1979) and in India it is 426,300 ha (Sanyal, 1983).


                                 Geology
                                 The geological formation of the Sundarban is of com-
                                 paratively recent origin. Several geomorphological
                                 changes since the Tertiary period that included tectonic
                                 movements in northwestern Punjab and the southeastern
                                 flow of the River Ganga, resulted in the deposition of
                                 sediments in the Bengal Basin and development of the
                                 Sundarban Delta (Wadia, 1961). It is noteworthy that the
                                 Ganga and Brahmaputra together carry the world’s larg-
                                 est sediment load to the oceans (Coleman, 1969; Milli-
                                 man and Meade, 1983; Milliman et al., 1995). Neotec-
                                 tonic movements in the Bengal Basin between the 12th
                                 and 15th century AD further resulted in an easterly tilt
                                 (Morgan and McIntire, 1959). During the 16th century,
                                 the R. Ganga changed its course to shift eastwards and
                                 join the Brahmaputra (Deb, 1956; Blasco, 1975; Snedak-
                                 er, 1991). Later, in the mid 18th century, the combined
                                 Ganga (now called Padma) and Brahmaputra again tilted
                                 eastwards to empty into the R. Meghna (Williams, 1919;
                                 Snedaker, 1991). This continuing tectonic activity greatly
                                 influenced the hydrology of the deltaic region because of
Figure 1a. Location of the Sundarban in the Ganga Brahmaputra
                                 changes in the sedimentation patterns and the reduction
Delta.




Figure 1b. The Sundarban mangroves are shared between India and Bangladesh. Mangroves once extended up to the Dampier-Hodges line
– the inward limit of tidal influence. Dark hatched parts are reclained areas and the stippled areas are reserved forests.
                                                Overview Article
Aquat. Sci. Vol. 68, 2006                                                341


in freshwater inflows. Most rivers (distributaries) other    sands, which form sand dunes, occur mainly along the
than the Hooghly, that contributed to the formation of the   coast.
Ganga Delta (from west to east: Muriganga, Saptamukhi,       Similarly in the Bangladesh Sundarban, the soil is a
Thakuran, Matla, Gosaba and Bidya), have lost their      silty clay loam with alternate layers of clay, silt and sand.
original connections with the Ganga because of siltation    The surface is clay except on the seaward side of islands
(Fig. 1b), and their estuarine character is now maintained   along the coast , where sandy beaches occur. In the east-
by the monsoonal runoff alone (Cole and Vaidyaraman,      ern part, the surface soil is soft and fertile whereas it is
1966). Thus the delta-building process has nearly ceased    harder and less suitable for tree growth in the west
in the west, but has accelerated in the eastern part. The   (Choudhury, 1968). The soil pH averages 8.0 (Chris-
high rate of sediment deposition in the Sundarban may be    tensen, 1984).
understood from the estimated increase in the land area
by more than 800 km2 in 80 years between 1793 and 1870
(Richards, 1990). Recent studies by Chakrabarti (1995),    Climate
however, show that the mangroves are the dominant geo-     The climate of the area is characterized by relatively high
morphic agent in the evolution of tidal shoals and their    temperature and humidity (>80 %) throughout the year,
accretion to the main landmass.                and well distributed rainfall during the monsoon season.
                                Temperatures rise from a daily minima of 2–4 °C in win-
                                ter to a maximum of about 43 °C in March and may ex-
Soils                             ceed 32 °C during the monsoon. Recent reports suggest
The land is moulded predominantly by tidal action. An     that the air temperature over the Sundarban and adjacent
intricate network of waterways, of which the larger chan-   parts of the Bay of Bengal are gradually increasing (Huq
nels (often 1.5 to 2.0 km wide), run in a generally north-   et al., 1999; Agrawala et al., 2003). The cold season lasts
south direction, intersect the whole area. Innumerable     from about the middle of November to the end of Febru-
small khals (= creeks) drain the land at each ebb tide.    ary and is followed by the summer from March to May.
Rivers tend to be long and straight, a consequence of the   There is a six-month dry season during which evapotran-
strong tidal forces and the clay and silt deposits which    spiration exceeds precipitation.
resist erosion. Easily eroded sands collect at the river      The rainfall over the Ganga-Brahmaputra deltaic re-
mouths and form banks and chars, which are blown into     gion decreases from east to west and from south to the
dunes above the high-water mark by the strong southwest    north. In the Bangladesh region, mean annual rainfall
monsoon. Finer silts are washed out into the Bay of Ben-    varies from about 1,800 mm in Khulna, north of the Sun-
gal, but mud flats are formed on the leeward side of the    darban, to 2,790 mm on the coast. The average annual
dunes where they are protected from wave action. These     rainfall in the Indian region is only 1,661.6 mm. It de-
mudflats become overlain with sand from the dunes, and     creases from 1,805 mm in the south on Sagar Island to-
develop into grassy meadows. This process of island-      wards Kolkata in the north. Most of the rainfall (about
building continues for as long as the area on the wind-    74 % of the total) occurs during the southwest monsoon
ward side is exposed to wave action. With the formation    period (June–September). Some precipitation is received
of the next island further out, silt begins to accumulate   in the latter half of the hot season and in October.
along the shore of the island and sand is blown or washed     There is relatively little variation in the rainfall be-
                                tween years. During the first half of the 20th century, the
away (Seidensticker and Hai, 1983).
  Most of the soils derived from alluvial deposits are    highest and lowest annual rainfalls were only 142 % (in
azonal with little or no profile development (Chaudhuri     1933) and 62 % (in 1935) of the normal respectively.
and Choudhury, 1994; Sarkar et al., 1999). Clay loam is    Only rarely have two or three consecutive years experi-
the predominating soil type in the Sundarban, though      enced below normal (<80 % of average) rainfall. On an
silty and sandy loams also occur in many areas. Clays     average there are 80 rainy days (>2.5 mm rainfall) in a
with or without muck occur in swamps and alluvial lakes.    year.
Alluvial soils along the coast, and especially in the Sun-     Winds are generally light to moderate with a slight in-
darban area, show a white efflorescence of sodium chlo-     crease in force during the summer and monsoons, but in
ride, as they are impregnated with salts by tidal estuaries.  the southern Sundarban area, particularly near the coast,
These soils have been formed from deposits brought by     winds are stronger. Winds blow mostly from directions
tidal currents. Numerous tidal flats have been formed af-    between the south-east and south-west during May to Sep-
ter the headwater flow through the deltaic distributaries    tember. In October, winds vary in direction. During the
of the R. Ganga were silted up. The parent deposits are    winter, winds blow mainly from the north-west. In March
either rich in calcium or magnesium, or consist of half-    and April they blow from the south and south-west.
decomposed organic matter. The coastal soils are usually      Thunderstorms are common during summer after-
classified as saline, non-saline and alkali soils. Pure     noons. These may be in association with severe squalls
342    B. Gopal and M. Chauhan                        Biodiversity of Sundarban Mangrove Ecosystem


and occasional hail. These are commonly known as
nor’westers (because the associated squalls usually come
from the north-west) or Kalbaisakhi (the disastrous
winds of Baisakh, the first month of the Bengali calen-
dar). Storms result in heavy rain and a sharp drop in
temperature.
  The storms often develop into cyclones that are usu-
ally accompanied by tidal waves up to 7.5 m high (Sei-
densticker and Hai, 1983). Available long-term records
show that cyclones over the Bay of Bengal adjoining the
Sundarban are increasing in their intensity, but decreas-
ing in their frequency of occurrence. This correlates with
the rising trend in temperatures mentioned earlier and has
a significant bearing on the extent of coastal flooding,
erosion and saline water intrusion due to storm surges
(Huq et al., 1999; Agrawala et al., 2003).


Hydrology and salinity regimes
The hydrology of the Sundarban is dominated by the
freshwater flows from Rivers Ganga, Brahmaputra and
Meghna, which exhibit very high seasonal variation in
                                Figure 2. Salinity zones in the Bangladesh Sundarban.
their discharge, and the tides which range in height from
2 to 5.94 m. Tidal influence extends to more than 50 km
inland from the shoreline and surges increase considera-
bly during the cyclonic storms.                blanks which may check the penetration of high tides to
   The freshwater flows from the rivers and the tidal in-   the interior of the islands. Irregular flooding of these
gress result in a gradient of salinity that varies both spa-  blanks by high tides, coupled with the capillary action of
tially and temporally. In general, the salinity is higher   the clayey soil and excessive heat during dry periods,
nearer the coast and the water is nearly fresh on the inland  results in the deposition of a salt crust on the soil surface,
side limit of the Sundarban. Similarly, the salinity de-    converting them into saline blanks. Lands on the sea
creases from west to east. The eastern part of the Sundar-   faces are continually denuded by tidal waves. Towards
ban in Bangladesh is oligohaline (<5 ‰ salinity) whereas    the west, new land formation occurs because of heavy
most of the Indian Sundarban is polyhaline (Fig. 2).      silt deposition from the Hooghly River and its distribu-
   During the past few decades, however, the sources of   tary, the Muriganga at their confluence with the Bay of
all rivers in the western part of the Indian Sundarban     Bengal. Thus, the habitat is characterized by a continu-
have progressively silted up, disconnecting the inflow of    ing erosion of banks, the formation of new islands, and
fresh water into the mangrove delta. Freshwater flows      ever-changing soil texture and salinity of water under the
are much larger from Brahmaputra and Meghna rivers       influence of tides.
on the Bangladesh side particularly in the Baleshwar        In recent years, the Farakka Barrage, built in 1974 on
River on the eastern side of the Sunarban (Seidensticker    the River Ganga within India, has also affected freshwa-
and Hai, 1983). The reduced freshwater flows in western     ter flows into the Bangladesh part of the Sundarban.
parts of the Sundarban have resulted in increased salinity   Similarly, the construction of several dams and barrages
of the river waters, and has made the rivers shallower     in the Damodar River catchment area and on the Ganga
over the years. At the same time, during ebb tides, the    have resulted in a decreased silt load and less deposition
receding water level causes scouring of top soil and cre-   of detritus in the estuaries downstream.
ates an innumerable number of small creeks, which
normally originate from the centre of the islands. The
                                Biodiversity
ebb tide eroding action is stronger in some islands than
others within the Sundarban. The receding water carries
large volumes of silt which is deposited along the banks    The large spatial and temporal variability in hydrological
of rivers and creeks during high tides. This results in in-  regimes (both freshwater inflows and the tides), topogra-
creasing the height of the banks as compared to the inte-   phy and texture of the substratum, the salinity, and their
riors of the islands. Over time, such eroded channels     interactions, result in very high habitat heterogeneity in
extend further inwards into the islands and form muddy     the mangrove ecosystems, and thereby ensure an equally
                                                 Overview Article
Aquat. Sci. Vol. 68, 2006                                                   343


                                Table 1. Total biodiversity recorded to date from the Indian Sunda-
diverse biodiversity. There has been some discussion
                                rban mangroves.
about the characteristic mangrove plants and a distinction
is usually made between the “true mangrove” and “man-     Group of Organisms     No. of   Reference
grove associate” species. However, following the defini-                  species
tion of wetland species by Gopal and Junk (2000, 2001),
                                Flowering Plants      105    Naskar and Mandal (1999)
here we consider all those species which depend directly    Pteridophytes
or indirectly upon the mangrove habitats, or on any other    (Ferns & Fern allies)
organism living in the mangrove as mangrove organ-       Algae            150    Sen et al. (2000)
                                Lichens           32    Santra (1999)
isms.
  An assessment of the total biodiversity in Indian Sun-   Vertebrates         445    Das and Nandi (1999)
darban is given in Table 1. Overall, there are relatively    Chondrichthyes       22
                                 Osteichthyes (Fishes)   154
small differences between the species composition of the
                                  Amphibians        8
Indian and Bangladesh parts, largely because the deve-
                                  Reptiles        58
lopment of the mangrove system has been dictated by         Birds         163
similar processes. However, the overarching gradients of      Mammals         40
salinity and freshwater that occur across the Sundarban    Phylum-                Das and Nandi (1999)
from west to the east, are clearly reflected in the distribu-   Sarcomastigophora     45
                                 Aplicom plexa       24
tion of biota. The Sundarban in the two countries differs
                                 Myxozoa           4
also in the extent of management interventions. It must     Ciliophora         31
also be emphasised that the Sundarban mangrove in India
                                Invertebrates
has been explored frequently and in greater detail, par-
                                 Porifera          1    Mukherjee (1975)
ticularly floristically, whereas the mangroves within       Cnidaria          33    Mandal and Nandi (1969)
Bangladesh were poorly investigated until recently        Ctenophora         2    Nandi et al. (1993)
                                 Platyhelminthes      41    Chaudhuri and Chaudhury
(Chaffey et al., 1985). Recent surveys provide consider-
                                                   (1994)
able information on several groups of plants, fishes, rep-     Turbellaria        1    Anon (1995)
tiles, amphibians and birds (Ismail, 1990; Hussain and       Monogenera        21
Acharya, 1994; Seidensticker, 1991; IUCN-BD, 2002a,        Trematoda        13
                                  Cestoda          6
b, c). Differences in the nomenclature followed in the
                                 Nemathelminthes      68
two countries also sometimes make comparisons diffi-       Acanthocephala       3
cult. The following account is based largely on the Indian    Nemertinea         2
                                 Rotifera          4
part of the Sundarban, with frequent comparisons with
                                 Mollusca         143
the information available from Bangladesh.
                                 Sipuncula          2
                                 Echiura           3
                                 Annelida          78
                                  Polychaeta        69
Species diversity
                                  Oligochaeta        6
Floristic diversity. Mangrove plants are usually divided      Hirudinea         3
into ‘true mangrove’ and ‘mangrove associate’ species.      Arthropoda        476
                                  Crustacea        240
Globally, Duke (1992) recognised 69 species (belonging
                                  Insecta         201
to 26 genera and 20 families) of true mangroves (major
                                  Arachnida        33
and minor, sensu Tomlinson, 1986) though recently         Merostomata        2
Kathiresan and Bingham (2001) recognized only 65 spe-      Entoprocta         1
                                 Bryozoa           3
cies (22 genera and 16 families). Of these, at least 30 true
                                 Brachiopoda         1
mangroves occur in the Indian Sundarban. Debnath and       Chaetognatha        4
Naskar (1999) identified 36 species as true mangroves.      Echinodermata       20
The Bangladesh part of the Sundarban differs mainly in      Hemichordata        1
the relative abundance of various species. Whereas mem-
bers of the Rhizophoraceae and Avicenniaceae generally
dominate most other mangrove areas, the Bangladesh
part of the Sundarban has the greatest abundance of Ster-   only one species of Bruguiera (B. parviflora), whereas
culiaceae (Heritiera) and Euphorbiaceae (Excoecaria).     Lumnitzera racemosa and Barringtonia sp. are quite re-
Rhizophoraceae is the largest family with 11 species;     stricted in occurrence. Interestingly, the Sundarban sup-
four genera (Rhizophora, Bruguiera, Avicennia and Son-     ports fewer species than other mangrove areas in India
neratia) are represented by four species each and 5 gen-    and Southeast Asia. For example, there are 45 mangrove
era (Xylocarpus, Excoecaria, Thespesia, Derris and       species recorded from the Andaman and Nicobar Islands
Tamarix) have three species each. In Bangladesh there is    in the Bay of Bengal (Deshmukh et al., 1991a) and 55
344    B. Gopal and M. Chauhan                        Biodiversity of Sundarban Mangrove Ecosystem


                               Table 2. Rare, threatened and endangered flora of the Indian Sunda-
species from Bhitarkanika in the delta of the Brahmani
                               rbans.
and Baitarni rivers in Orissa on the east coast of India
(WWF-I, 2001).                        Family       Species             Status
  The total flora of the Sundarban has also been esti-
                               Rhizophoraceae   1. Rhizophora apiculata     Occasional
mated differently in India and Bangladesh. In India, the
                                         2. Bruguiera parviflora     Occasional
total vascular flora (including mangrove associates) is                              Occasional
                                         3. Ceriops decandra
estimated at 100 species representing 34 families and 57                             Occasional
                                         4. Kandelia candel
genera. comprises of 30 species of trees, 32 shrubs, and   Meliaceae      5. Aglaia cucullata       Rare
the rest are herbs, grasses, sedges and two ferns. Many              6. Xylocarpus mekongensis    Threatened
                                         7. Xylocarpus granatum     Threatened
terrestrial upland plants within the Sundarban area have
                               Sterculiaceae    8. Heritiera fomes       Threatened
apparently not been included because Seidensticker and
                               Rubiaceae      9. Scyphiphora hydrphyllacea  Very Rare
Hai (1983) recorded 334 plant species representing 245
                                         10. Hydrophyllax maritime    Very Rare
genera from the Bangladesh Sundarban. This record lists
                               Tiliaceae     11. Brownlowia lanceolata    Occasional
numerous epiphytes and climbers, which include 13 or-
                               Arecaceae     12. Nypa fruticans        Occasional
chids and several ferns. Several species in the Indian
                               Acanthaceae    13. Acanthus volubilis      Very Rare
Sundarban do not occur in Bangladesh part, whereas
                               Papilionaceae   14. Cynometra ramiflora      Rare
many other species have been reported from Bangladesh
                                         15. Dalbergia spinosa      Rare
alone (Choudhury, 1968; Ismail, 1990). Interestingly,
                               Sapotaceae     16. Manilkara hexandra      Rare
some species listed earlier from Bangladesh by Prain
                               Rutaceae      17. Atalantia correa       Very Rare
(1903) and Choudhury (1968), such as Bruguiera sexan-
gula, Rhizophora apiculata and Sonneratia alba, were
not recorded by Chaffey et al. (1985) and it is feared that
these species may have disappeared from the eastern part   Botrychia, Catenella and Caloglossa form a turfy cover-
of the Sundarban. Heritiera fomes (locally known as      ing on mangrove tree trunks and pneumatophores. At a
Sundari, the most important timber species from which     lower level on the same trees, species of Caloglossa occur
Sundarban derives its name),which is abundant on the     along with Chaetomorpha and Rhizoclonium. Species of
Bangladesh side, is not common on the Indian side where    Lyngbya, Oscillatoria and Microcoleus are common on
it is considered endangered. Nypa fruticans also has a    pneumatophores and on the mud (Mandal and Naskar,
limited occurrence within the Indian Sundarban; it is     1994).
rapidly disappearing because of extensive exploitation.
Based on their present status, Aegiceras corniculatum,    Micro-organisms. The diversity of bacteria and fungi in
Kandelia candel, Rhizophora sp., Sonneratia acida, Son-    the Sundarban has not been examined in detail, although
neratia apetala and Sonneratia caseolaris also require    some reports mention various microorganisms in the
conservation measures (Table 2).               soils and on decomposing litter, besides pathogens in-
                               festing mangrove leaves (Pal and Purkayastha, 1992),
Algae. The algal flora of the Sundarban is very poorly     fish, prawns and mammals (Das, 1999). Bacteria play a
known, but the available information suggests that the    significant role in the mangrove environments and the
Sundarban has a highly diverse algal flora comprised of    Sundarban is expected to harbour a diverse bacterial
both benthic and planktonic forms ranging from the fresh-   community which is yet to be explored, unlike other In-
water to marine environments. In the only study in Bang-   dian mangroves that are better known (Balasubramanian
ladesh, Islam (1973) reported 34 species of planktonic    and Ajmal Khan, 2002). Furthermore, 32 species of li-
and benthic algae. Various published reports on the algal   chens have also been recorded from the Indian Sundar-
flora provide only a patchy picture as they are based on    ban (Santra, 1998).
short-term surveys of small isolated areas (see Pal et al.,
1988; Santra et al., 1988, 1991). Sen et al. (1999) record-  Faunal diversity. The fauna of the Sundarban has at-
ed 80 species of algae (32 Cyanophyceae and 27 Chloro-    tracted much attention because of the huge economic
phyceae) from different parts of the Indian Sundarban.    importance of many species. Crustaceans account for the
These included only 7 species of diatoms, whereas Baner-   largest proportion of animal biomass, with an estimated
jee and Santra (1999) listed 48 species of diatoms from    annual harvest of 40 million kilograms of fiddler crabs
the Hooghly-Matla estuary alone. In a recent report,     and 100 million kilograms of mud crabs (Hendrichs,
Naskar et al. (2004), however, list 150 species including   1975), besides a considerable harvest of shrimps, prawns
15 species of Rhodophyceae and 2 species of Phaeophyc-    and lobsters. The Sundarban supports very rich estuarine
eae. Another study recorded 12 genera and 16 species of    and coastal marine fisheries. Among the diversity of in-
benthic algae belonging to the Cyanophyceae, Chloro-     sects, honeybees hold an important position for the pro-
phyceae, Chrysophyceae and Rhodophyceae. Species of      duction of large quantities of honey and beeswax. Also,
                                                  Overview Article
Aquat. Sci. Vol. 68, 2006                                                    345


                                 Table 3. Threatened fauna of Sundarbans
the Sundarban is the only mangrove forest with a species
of the tiger – the Royal Bengal Tiger (Panthera tigris).     Scientific Name          Common Name
The total faunal diversity of the Sundarban in listed in
                                 Mammalia
Table 1.
                                 Muntjanus muntjack        Barking Deer
                                 Panthera tigris tigris      Royal Bengal Tiger
Fish. The fish fauna of the Bangladesh Sundarban in-        Felis bengalensis         Leopard Cat
cludes 53 pelagic and 124 demersal species (Sarker,        Felis viverrina          Fishing Cat
                                 Platinista gangetica       Gangetic Dolphin
1989; Rainboth, 1991). Of these, over 120 species have
                                 Orcaela brevirostris       Irrawady Dolphin
been recorded in commercial catches (Seidensticker and      Neophocaena phocaenoides     Little Indian Porpoise
Hai, 1983). The Indian Sundarban supports a similar        Manis pentadactyla        Chinese Pangolin
number of species (165 species; Sanyal, 1999). The fish      Reptilia
diversity is directly related to the salinity gradients in dif-  Crocodylus porosus        Estuarine Crocodile
                                 Lepidochelys olivacea       Olive Ridley Turtle
ferent parts of the Sundarban. Whereas Mukherjee
                                 Batagur baska           Batagur Turtle
(1975a) considered only brackish water and marine spe-
                                 Lissemys punctata         Indian Flapshelled Turtle
cies of fish in the Indian Sundarban, Sanyal (1999) ex-      Trionyx gangeticus        Indian Softshelled Turtle
panded the list by including 31 freshwater species. Re-      Kachuga tecta           Indian Tent Turtle
                                 Varanus bengalensis        Common Indian Monitor
cently, Islam and Haque (2004), in their detailed review
                                 Varanus flavescens         Yellow Monitor
of the fisheries resources of Bangladesh Sundarban, have      Varanus salvator         Water Monitor
described the distribution of fish species along the salinity   Python morulus          Indian Rock Python
gradients. Fishes such as Harpodon nehereus, Trichiurus      Aves
savala, Setipinna sp., Pampus sp., Sardinella sp. and       Ardea goliath           Great Goliath Heron
                                 Pelecanus phillippensis      Dalmatian Pelican
Salar sp. occur in areas with salinity, whereas Pangasius
                                 Leptotilos duius         Lesser Adjutant Stork
pangasius and Lates calcarifer occur in freshwater areas
or those with very low salinity. The dominant fishes in
brackish water zones (moderate salinity) are Hilsa (Tenu-
alosa) ilisha, Pomadasys hasta, Polynemus sp. and Coilia     1985a,b). Several species of kingfishers (including
sp. Several marine fishes also often occur within the man-     brown-winged and stork-billed kingfishers, Pelargopsis
groves.                              amauropterus and P. capensis, respectively) and the
                                 magnificent white-bellied sea-eagle (Haliaeetus leu-
Reptiles and amphibia. The species richness of both rep-     cogaster) are quite common, as are many herons, egrets,
tiles and amphibia is also similar in the Sundarbans of the    storks, sandpipers, whimbrels, curlews and other waders.
two countries: 8 and 7 amphibians and 53 and 59 reptiles     There are also many species of gulls and terns, especially
in Bangladesh and India, respectively (Hussain and Ach-      along the coast and the larger waterways. The grey-
arya, 1994; Sanyal, 1999; Naskar et al., 2004). Among       headed fish eagle (Ichthyophaga ichthyaetus) and Pal-
snakes, the Family Boidae is the largest with 13 species,     las’s fish-eagle (Haliaeetus leucoryphus) are quite rare.
followed by Hydrophidae (7 species), Elapidae (4 spe-       Apart from the species particularly associated with the
cies) and Typhlopidae and Viperidae (2 species each).       sea and wetlands, there is also a considerable variety of
Noteworthy species are the king cobra (Ophiophagus        forest birds, such as woodpeckers, barbets, shrikes,
Hannah), Indian spectacled cobra (Naja naja) and Indian      drongos, mynahs, minivets, babblers and many others
python (Python molurus). Among 14 turtles and tortois-      (Salter, 1984).
es, six species are nearly extinct or threatened (Table 3).      The Indian side of the Sundarban has a similar avian
Of the four marine turtles, the olive ridley turtle (Lepido-   diversity (Law, 1953, 1954, 1956, 1959; Mukherjee,
chelys olivacea), though endangered, is the most abun-      1972, 1975b; Naskar and Mandal, 1999). Though only
dant. The green turtle (Chelonia mydas) is rare due to      about 150 species were listed by Naskar (1999), Chaud-
excessive fishing, while the loggerhead (Caretta caretta)     huri and Choudhury (1994) report more than 300 species
and hawksbill (Eretmochelys imbricata) are not common       of which about 100 are migratory. The list includes a
(Hussain and Acharya, 1994). The river terrapin (Bat-       large number of species that remain in the reclaimed ar-
agur baska) and three of the 14 species of lizards and      eas adjoining the mangroves.
monitors are also endangered (Table 3).
                                 Mammals. Of significant interest is the fact that the
Birds. The avifauna of the Sundarban is highly varied       mammalian diversity exhibits significant differences
and very rich in species. In Bangladesh, 315 species have     between the Indian and Bangladesh sides of the Sun-
been recorded (Sarker and Sarker, 1986; Hussain and        darban: the former has only 31 species of mammals
Acharya, 1994). These include about 95 species of water-     (Chaudhuri and Choudhury, 1994, Sanyal, 1999) against
fowl (Scott, 1989) and 38 species of raptors (Sarker,       49 species in Bangladesh (Hussain and Acharya, 1994).
346    B. Gopal and M. Chauhan                       Biodiversity of Sundarban Mangrove Ecosystem


At least five species, namely the Javan rhinoceros (Rhi-    and erosional processes, create a large diversity of habi-
noceros sondaicus), water buffalo (Bubalus bubalis),      tats which are occupied by different plant communities.
swamp deer (Cervus duvauceli), gaur (Bos frontalis)      The distribution of different communities along the flood-
and probably the hog deer (Axis porcinus) have disap-     ing gradient is often interpreted in terms of a zonation or
peared locally during the past century (Seidensticker     successional sequence (Untawale 1987; Naskar and Gu-
and Hai, 1983). The one-horned rhino (Rhinoceros uni-     habakshi, 1987). Heritiera species require more freshwa-
cornis), Indian bison (Bos gaurus) and Sambhar (Cer-      ter and occur together with Phoenix paludosa and Nypa
vus unicolor), which were once common here, are also      fruticans, generally above the tide level. These species
now locally extinct. The only primate is the rhesus      may occur separately, or in association with each other, or
macaque (Macaca mulatta) which still occurs in good      with Excoecaria agallocha or Ceriops decandra. The lat-
numbers, but its population is declining gradually       ter species join Rhizophora and Bruguiera in relatively
(Blower, 1985; Gittins, 1981). The Sundarbans of        saline and frequently inundated intertidal habitats, where-
Bangladesh and India support one of the largest popula-    as Sonneratia and Avicennia species occur below the
tions of tiger, Panthera tigris (estimated 350-400).      mean tide level. Recent studies on mangroves (Bunt,
Spotted deer (Cervus axis) and wild boar (Sus scrofa)     1999), including those in the Bangladesh Sundarban (El-
occur in large numbers and form the principal prey of     lison et al., 2000), based on large scale sampling and de-
the tiger. Besides three species of otters, there are wild   tailed multivariate analysis, have however failed to detect
cats (Felis bengalensis, F. chaus and F. viverrina) and    any zonation pattern with respect to edaphic variables that
the Ganges River Dolphin (Platanista gangetica).        co-vary with elevation. Champion (1936) recognized the
  Here, the notoriety of the Sundarban for its man-eat-   Sundarban as a seral moist tropical forest, comprised of a
ing tigers must be mentioned. Every year a significant     mosaic of beach forest and tidal forests. The latter were
number of people (an average of 23 annually ) are killed    divided into four types: low mangrove forests, tree man-
by tigers, particularly in the southern and western parts of  grove forests, salt-water Heritiera forests and freshwater
the Sundarban. Hendrichs (1975) hypothesised that such     Heritiera forests. Later Champion and Seth (1968) classi-
killings were linked with the high salinity levels and the   fied the wetland forests into littoral forests (freshwater)
absence of freshwater for drinking purposes. However,     and tidal swamp forests (mangroves). Mandal (2003) dis-
recent analyses suggest that the killing of humans is at    tinguished further between the vegetation of the active
least partly related to the frequency of tiger-human inter-  delta (southern part of the Sundarban with dense swamp
action which makes humans available as easy prey        forest), and freshwater areas (rivers, creeks, ponds) and
(Salter, 1984; Siddiqi and Choudhury, 1987).          agricultural fields of the mature delta.
                                  The Indian part of the Sundarban with higher salinity
Invertebrates. In the Indian Sundarban, arthropods are     supports sparse Excoecaria agallocha, a dense understo-
the most abundant invertebrates (476 species) comprised    ry of Ceriops, and dense patches of the hantal palm
almost entirely of crustaceans (240) and insects (201     (Phoenix paludosa) on drier soils. Xylocarpus sp. and
species). Molluscs (143 aspecies), annelids (78 species)    Bruguiera sp. occur sporadically throughout the area.
and nematodes (68 species) are the next most common      Oryza coarctata, Nypa fruticans and Imperata cylindrica
groups of invertebrates, other than the protopzoans (104    are prevalent on mud flats (Khan, 1986). Large stands of
species) (Chaudhuri and Choudhury, 1994). As men-       Sonneratia apetala occur on newly accreted mud banks.
tioned earlier,the crustaceans are of great economic im-    Sand dunes bordering the sea are primarily colonized by
portance and a large number of species of prawns and      grasses such as Paspalum vaginatum, Panicum repens,
shrimps are exploited commercially. A substantial pro-     Aeluropus lagopoides and Phragmites karka, which are
portion of species are planktonic. Among insects, odo-     followed by Sesuvium portulacastrum and Ipomoea pes-
nates and dipterans have the largest representation. At    caprae (= I. biloba) which constitute the climax dune
least 15 other phyla are represented in the invertebrate    vegetation. Salicornia brachiata, Hygrophyllus asiatica
fauna of the Sundarban. Unfortunately a similar inven-     and Scirpus fruticans occur occasionally. Within Bangla-
tory is not available for the Bangladesh Sundarban.      desh, the salinity of the Sundarban decreases eastwards
Available reports suggest the occurrence of at least 24    and, accordingly, a polyhaline, a mesohaline and an oli-
species of shrimp, 7 species of crabs, 8 species of lob-    gohaline zone are recognized. Within each of these
sters and 8 species of molluscs that are commercially     zones, vegetation associations change with elevation
exploitable.                          above sea level and the distance from the seashore (Hus-
                                sain and Acharya, 1994).
Zonation and community diversity                  The successional sequence of mangrove communities
The hydrological (depth and frequency of flooding) and     follows a similar pattern throughout the Sundarban.
salinity gradients that develop in a geo-morphologically    Small variations occur according to the salinity regimes
dynamic environment influenced by both depositional       (Karim, 1988). Grasses such as Leersia, Porteresia and
                                                 Overview Article
Aquat. Sci. Vol. 68, 2006                                                  347


Phragmites and trees such as the species of Avicennia
and Sonneratia are the pioneers on newly accreted land.
With an increase in elevation of the land mass relative to
the tidal height, the succession proceeds to a stage where
Heritiera fomes, Ceriops decandra and Excoecaria agal-
locha constitute the climax community.

Threats to biodiversity
The biodiversity of wetlands is governed by a large vari-
ety of factors ranging from the evolutionary history and
paleoclimate to the current biophysical and anthropo-
genic factors, besides the nature and intensity of interac-
tions with adjacent systems. Furthermore, various socio-   Figure 3. Factors affecting biodiversity in mangroves (adapted from
                               Gopal, 2006).
cultural, economic and political influences also affect the
biodiversity directly as well as indirectly. These factors
have been discussed elsewhere in the context of inland
freshwater wetlands by Gopal (2005, 2006) and a con-     leasing, while timber extraction was allowed in the re-
                               serve forests. It is estimated that 1,570 km2 mangroves
ceptual framework is shown in Figure 3. The same fac-
tors influence the biodiversity of mangrove wetlands      were reclaimed in the three decades since independence
such as the Sundarban, where the interplay of hydrology,   in 1947 (Richards, 1990).
geomorphology and chemistry is most pronounced. Man-       Thus major threats to biodiversity have come mainly
groves differ significantly from the freshwater wetlands    from the growing human population, and consequently,
in their hydrology as they interact daily with both the sea  overexploitation of both timber and fauna, and conver-
and inland areas.                       sion of the cleared land to agriculture and aquaculture.
  The biodiversity of the Sundarban mangroves has,     Today, the area around the Sundarban (in both India and
however, been affected by centuries of human exploita-    Bangladesh) is densely populated. Approximately 2.5
tion of the forests, their conversion to paddy fields, and   million people live in hundreds of small villages very
reclamation of land for various uses. Eaton (1991) de-    close to or within parts of the Sundarban, on which very
scribes in detail the history of human colonisation of the  large numbers of people depend for their livelihood at
Sundarban from 1200 to 1750 A.D. and Richard and Flint    certain times of the year.
(1991) provide details of the transformation of the Sun-     Numerous people are engaged in the commercial ex-
darban thereafter under British rule, whereas Presler     ploitation of sundari and other tree species, while the lo-
(1991) discusses the management of the Sundarban until    cal people depend on the forest for firewood, timber for
1952. It is estimated that the Sundarban originally cov-   boats, poles for house-posts and rafters, golpatta (Nipa
ered more than 40,000 km2 in coastal Bengal. Clearing of   fruticans) leaf for roofing, grass for matting and fodder,
the forest and rice agriculture was first actively promoted  reeds for fencing, and fish for their own consumption.
by the then Turk sultan rulers of Bengal. After the British  Honey and wax are collected during the summer season
colonised Bengal, the area of the Sundarban still included  (April to June) (Krishnamurty, 1990). Tens of thousand
14,627 km2 of land and 4881 km2 of waterways in 1793     of fishermen harvest fish and shrimps for most of the year
(Richard, 1990). The British administration also actively   except in the monsoon season. In recent years, collection
promoted deliberate conversion to agriculture. Hunter     of shrimp juveniles has increased manifold, particularly
(1875) describes the state of the Sundarban at the end of   for aquaculture in reclaimed areas (see, Haque, 2003,
the 18th century when the then Governor of Bengal im-     Hoq et al., 2001, 2006). It is estimated that up to 60 % of
posed a rule bringing the forest outside permanent settle-  the shrimp post-larvae (PL) collected from nature die
ments under Government control. The Commissioner for     during sorting, transportation and stocking. This forces
the Sundarban was charged with the task of “regulating    further harvests from the mangroves. Unfortunately the
and managing the waterlogged forests and swamps of the    PL harvest of shrimp and prawns is probably the most
lower delta” and “to ensure that private landowners      lucrative local economic activity available to a large pro-
cleared, settled and reclaimed Sundarban forests and     portion of the Sundarban population. PL harvesting earns
swamps for rice cultivation” (Richards, 1990). By 1870,    approximately five times more gross revenue per capita
2,790 km2 of mangroves had been cleared. Another       than rice farming, and probably proportionately even
2,750 km2 forests had been reclaimed within the next     more on a net revenue basis as inputs other than labor are
sixty years even though about 10,000 km2 of the Sundar-    negligible compared to agriculture.
ban mangroves were declared a protected or reserve for-      Although the ecological impacts of this activity are
est. The protected forests were available for clearance on  uncertain because of the lack of detailed information, the
348    B. Gopal and M. Chauhan                        Biodiversity of Sundarban Mangrove Ecosystem


by-catch which is discarded and usually dies, is a sub-    1990). Similarly, Nypa fruticans and Phoenix paludosa
stantial proportion of the Sundarban fisheries and is most   are also declining rapidly. As a long-term consequence
likely a major negative influence on the aquatic biodiver-   Heritiera is being replaced by Excoecaria (Christensen,
sity within the protected mangrove areas. While the re-    1984). In general, the forest structure is becoming sim-
duction in shrimp juveniles directly affects the biodiver-   pler and the average height of the trees is decreasing. It is
sity, shrimp farming is also causing rapid deterioration in  estimated that in the Bangladesh part of the Sundarban,
the Sundarbans similar to that in other mangrove areas     0.4 % of the forest area is replaced by dwarf species every
(Thornton et al., 2003; Barbier and Sathirathai, 2004; Is-   year. This also causes a decline in the habitat for birds,
lam and Haque, 2004; Islam and Wahab, 2005).arge-       monkeys and other tree-dwelling species.
scale collection of shrimp from estuaries directly affects     While the deterioration in vegetation is already well
the species that depend upon them for food.          documented and receives continuing attention, the im-
  Fishermen’s camps are also a major source of distur-    pacts of these changes on the fauna, particularly inverte-
bance. Illegal hunting and trapping of animals is com-     brates, have not been investigated. Reports suggest that
mon, not only by fishermen and woodcutters, but also      the changes in herbaceous vegetation are affecting the
occasionally by military defence personnel, e.g. in Hiron   population of spotted deer which now has a much lower
Point in the Sundarban South Wildlife Sanctuary (Blow-     population in the western areas where salinity is the high-
er, 1985). Habib (1989) reported that over 3,300 m of     est. However, an increase in Sonneratia sp. favours the
deer nets were removed between 1981 and 1987, but such     deer because the leaves of this mangrove tree constitute
poaching is now rare. Adult marine turtles and Batagur     its staple diet. There are several studies in other man-
which get caught in fishing nets are often killed and mar-   grove areas showing the effects of salinity levels, and
keted for food.                        their seasonal variation, on the recruitment and growth of
  Among hydrological factors, the freshwater flows are    various animals and the effect on their predators (see
the most significant as they influence the salinity as well   Hussain and Acharya, 1994).
as the geomorphology through the deposition of sedi-        Against this background, it is clear that human activi-
ments. The tidal frequency is a near constant factor, but   ties continue to significantly impact biodiversity in the
the tidal height has a significant bearing on the extent of   Sundarbans. While some freshwater flows are diverted
inundation and also the erosion of freshly deposited sedi-   upstream for irrigation, or are regulated for flood control
ments. The salinity regimes are also influenced by tidal    and hydropower, major concerns have been voiced about
height. Human activities, such as forest clearing and sub-   the effects of the diversion of water into the Hooghly
sequent conversion to other land uses, directly influence    River by constructing a barrage at Farakka on the Indian
the freshwater flow and sediment accretion. The nature of    side of the border with Bangladesh. The water is required
the sediments (clay to sand) also has a great bearing on    primarily to keep the channel open for navigation from
the mangrove vegetation as well as the fauna. The Sunda-    the Bay of Bengal up to Calcutta Port. Regular dredging
rban mangroves lie on a delta that is relatively young     of the main channel is needed to maintain the depths re-
geologically and has been undergoing drastic changes.     quired by large, seafaring ships. In the absence of ade-
These changes are caused by neotectonic activity that is    quate information on Sundarban biodiversity before the
causing the tilting of the delta towards the east, and by   Farraka barrage was installed, it is not possible to assess
the enormous amounts of sediments transported by the      the extent of qualitative or quantitative changes since
rivers originating in the Himalaya. The accretion of sedi-   then. However, the hilsa (Hilsa ilisha) fisheries have de-
ments in the western part and the tilting to the east causes  clined considerably within the Indian part of the Sundar-
the river to migrate eastwards. It was pointed out earlier   ban, largely because the barrage is a barrier to t upstream
that most of the distributaries of the River Ganga on the   migration to the breeding sites (Sinha and Khan, 2001;
Indian side have silted up and do not carry freshwater.    Payne et al., 2004).
Thus, increased levels of salinity, particularly during the    Whereas much emphasis has been placed on the al-
dry season (low flow period) affect biodiversity, with the   terations in freshwater flow regimes in recent years, the
salinity-tolerant species gradually overtaking species de-   impact of embankments and polders needs to be assessed
pendent upon regular freshwater inputs. Recent studies     in detail. . During the past century, all streams and creeks
by Hoq et al. (2006) have clearly demonstrated the strong   outside the protected areas, have been extensively em-
influence of salinity, temperature and conductivity on the   banked to protect the cultivated land from inundation
seasonal abundance and distribution of shellfishes and     with saline/brackish water. About 3,700 km of embank-
finfishes within the Bangladesh Sundarban. The palyno-      ments were constructed in Bangladesh between 1960 and
logical evidence clearly shows that Heritiera fomes was    1970 (Hussain and Acharya, 1994). These embankments
very abundant in the western parts of the Sundarban 5000    and polders have altered the flooding regimes, increased
years ago, and has declined relatively recently as the sa-   salinity intrusion, promoted erosion, accelerated silta-
linity has increased (Blasco, 1975; see also Rahman,      tion, and reduced the nutrient exchanges, and thereby af-
                                                Overview Article
Aquat. Sci. Vol. 68, 2006                                                349


fected the biodiversity (Rahman et al., 1992; Bhattach-    Sundarban East (31,226 ha); and Sundarban South
arya, 1999).                          (36,970 ha), protecting about 23.5 % of the remaining
  Further threats to biodiversity because of pollution    Sundarban under the Bangladesh Wildlife (Preservation)
have arisen on both the landward and seaward sides of the   (Amendment) Act, 1974. These sancturies (IUCN, 1997)
mangrove. The agrochemicals (fertilizers and pesticides)    lie on disjunct deltaic islands in the Sundarban Forest
used extensively in the catchments of the Ganga and      Division of Khulna District, close to the border with In-
Brahmaputra rivers and their numerous tributaries, as     dia and just west of the main outflow of the Ganges,
well in the fields close to the mangroves, pollute both the   Brahmaputra and Meghna rivers (Fig. 4b). In 1987, the
waterways and the landmass, and affect the aquatic vege-    Sundarban National Park in India, and in 1997, parts of
tation and fauna directly. Growing industrialisation of the  the Sundarban in Bangladesh, were inscribed on the
area around Calcutta, particularly the Haldia industrial    World Heritage list (IUCN, 1997).
complex, and the industries situated on the western side      The entire Indian Sundarban area south of the Damp-
of the Hooghly River, contribute significantly to the pol-   ier-Hodges line (that demarcates the inward limit of tidal
                                influence), including 5,366 km2 of reclaimed lands, has
lution load and hence, to the degradation of the Sundar-
ban mangroves. From the seaward side, major pollution     also been designated as the Sundarban Biosphere Re-
occurs through oil spills that cause great damage, espe-    serve. Within the Biosphere Reserve several distinct
cially to the aquatic fauna and seabirds (Blower, 1985).    zones have been recognised: a Core Zone comprised of
An oil spill from the wreckage of a Panamanian cargo      the National Park and the Tiger Reserve, a Manipulation
                                Zone (2,400 km2 of mangrove forests), a Restoration
ship in August 1994 near the Dangmari Forest Station,
affected about 15 km2 area including a considerable part    Zone that covers 240 km2 of degraded forest and saline
of the Sundarban. Large scale mortality was recorded in    mud flats, and a Development Zone which includes
the seedlings of Heritiera and Excoecaria, grasses, fishes,   mostly the reclaimed areas. Only the Core Zone is under
shrimps and other aquatic animals (Hussain and Acharya,    strict conservation measures. Income generating activi-
1994). Recently, the off-shore exploration for oil and gas   ties such as the collection of seeds of black tiger prawn
by both Bangladesh and India has raised strong protests    (Penaeus monodon), the culturing of oysters and crabs,
concerning fears about its likely impacts on the Sundar-    mushroom cultivation and bee-keeping for honey pro-
ban’s biota (Anonymous, 2004; Saha et al., 2005).       duction are allowed in the Manipulation Zone. Efforts are
                                being made, however, to also rehabilitate certain degrad-
                                ed areas through afforestation. Among faunal species, the
Conservation                          estuarine crocodile and the Olive Ridley turtle are receiv-
                                ing some attention by way of captive breeding. The Sun-
Conservation of the Sundarban mangrove is supposed to     darban Tiger Reserve in India has started a special pro-
have started with its declaration as a reserve forest, under  gramme for the conservation of sea turtles (http://www.
the Indian Forest Act in 1878, after Schlich (1875) raised   wildbengal.com/sanctuaries/sunderban_main.htm).
concern over its conservation (Presler, 1991). However,      The approaches to conservation differ considerably
as described earlier, this did not help conservation per se,  between the Indian and Bangladesh Sundarbans and this
but only regulated the exploitation and conversion by the   has been discussed at length by Seidensticker (1991).
government. Soon after independence, India declared      The Bangladesh Sundarban is managed as a refuge
Lothian Island (3,800 ha) as a Wildlife Sanctury, and lat-   where wildlife is protected in small sanctuaries located
er, in 1960, another 35,240 ha were brought under the     in the larger forest tract by providing protection to the re-
Sajnakhali Wildlife Sanctury. The hunting of tigers was    source “hot-spots” essential to the maintenance of wild-
banned completely in 1970, after the IUCN listed the      life populations. This approach assumes that inviolate
Bengal tiger (Panthera tigris tigris) as an endangered     core areas surrounded by restricted-use buffer zones can
species. Later, under Project Tiger, the Government of     ensure the survival of communities and species such as
India established a Tiger Reserve in the Sundarban cover-   large mammals and birds which depend upon the re-
ing 2,585 km2 in 1973 (Fig. 4a). Another 241 km2 area     sources such as food or nesting and roosting sites over
was demarcated as a subsidiary wilderness area. The core    larger areas.
area of 1,330 km2 was later designated as a National Park.     In India however, the strategy for Sundarbans conser-
Another wildlife sanctuary was established in 1976 on     vation involves the setting aside of areas where the entire
Haliday Island (595 ha) to protect the spotted deer (Axis   life-cycle needs of a community can be met and the eco-
axis), wild boar (Sus scrofa) and rhesus macaque        logical needs of wildlife can be linked into the overall
(Macaca mullata), which are dominant animals in a for-     management of the system for the Sundarbans. Further-
est type consisting mainly of Ceriops decandra.        more, in so doing, the ecological processes upon which
  Soon thereafter, in 1977, Bangladesh created three     wildlife depend become integral values in the manage-
wildlife sanctuaries: the Sundarban West (71,502 ha),     ment matrix.
350    B. Gopal and M. Chauhan                          Biodiversity of Sundarban Mangrove Ecosystem


                                   Future of the Sundarban

                                   Two major factors will determine the future of the Sunda-
                                   rban mangroves and their biological diversity. First, the
                                   demand on freshwater resources is bound to grow as the
                                   human population in both countries continues to in-
                                   crease, resulting in the restriction of freshwater flows to
                                   the monsoon season when extensive floods occur in the
                                   eastern parts of India and in Bangladesh. The resultant
                                   increase in salinisation and accretion of sediments may
                                   alter vegetation composition. Impacts on animal commu-
                                   nities may also occur due to both, the direct effects of
                                   salinity and indirectly through food chain modifications
                                   caused by the alterations in the nature and amount of de-
                                   tritus available in the mangrove ecosystem.
                                     Therefore, it is the human response to the spatial and
                                   temporal variability in precipitation, and hence the fresh-
                                   water availability, which will determine the water avail-
                                   ability for sustaining the functions and values of the
                                   mangroves. The proposed plan of India for the inter-basin
                                   transfer of water through a river-linking program in India
                                   has already raised concern for the fate of the Sundarban
                                   and its rich biodiversity, as well as for the millions of
                                   people dependent upon it (Ahmed et al., 2004).
                                     Secondly, global climate change is expected to in-
Figure 4a. Protected areas of the Indian Sundarban.
                                   crease the average temperature and spatio-temporal vari-
                                   ability in precipitation, as well as cause a rise in sea level
                                   (Ellison, 1994). The increase in temperature and variabil-
                                   ity in rainfall will put further pressure on freshwater re-
                                   sources and hence, alter the freshwater inflows to the
                                   mangroves. If precipitation declines in the Ganga-Brah-
                                   maputra basins it may lead to a further reduction in the
                                   availability of freshwater in the deltaic region. Some
                                   models of climate change also present an increased fre-
                                   quency of tropical cyclones and storm surges, which may
                                   cause further changes in freshwater-seawater interactions
                                   and hence affect the mangroves (Ali, 1995, 2003; Ali et
                                   al., 1997). Substantial areas of the Sundarban along the
                                   coast are expected to be inundated by seawater in this
                                   case, and the increased landward salinity intrusion would
                                   affect the biotic composition.
                                     Ultimately, the future of the Sundarban mangroves
                                   hinges upon the efficiency of managing the limited fresh-
                                   water resources for meeting both human and environ-
                                   mental needs, coupled with effective adaptive responses
                                   to the added threats from climate change.


                                   Acknowledgments
Figure 4b. Wildlife sanctuaries (dark shaded areas) in the Bangla-
                                   We are thankful to many researchers whose published
desh Sundarban.
                                   studies have been used freely in preparing this overview.
                                   Thanks are also due to Wolfgang Junk for inviting us to
                                   prepare this review and to the organisers of the IN-
                                   TECOL’s International Wetlands Conference for partial
                                                       Overview Article
Aquat. Sci. Vol. 68, 2006                                                         351


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                                       ies of the Bengal Plain, India. Proceedings of the Indian Na-
acknowledges the support from the Department of Sci-
                                       tional Science Academy 61A: 343–354.
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by way of a Project under the Fast Track Scheme during            of India and Burma. Indian Forest Record (New Series) 1:1–
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