Introduction

Mangroves are swamp forests, found in sheltered estuaries along with river banks and lagoons in the tropical and sub-tropical countries (Kumar et al. 2016; Malik et al. 2017). They are renowned for their floral and faunal diversity (Iftekhar and Takama 2008), with some particular adaptive capacities to the coastal environment i.e., coastal tides, salinity, siltation and sedimentation (Paul et al. 2017). Mangroves have great role in countries national economy by their natural productivity and variety of products and services, in the form of renewable resources (Aheto et al. 2016). The economic potential of mangroves depends on three key sources; forest products, fisheries, and ecotourism (Paul et al. 2017). Moreover, mangroves play a vital role in coastal protection and preservation of habitats of endangered species (Mukhtar and Hannan 2012; Sandilyan and Kathiresan 2014).

The global pattern of biodiversity in mangroves showed that, over 41% of the world’s mangroves exist in South and Southeast Asia, where Indonesia alone occupied for 23% (Malik et al. 2017). A further 20% of the total mangrove areas lie in Australia, Brazil and Nigeria (Gopal and Chauhan 2006). However, the mangroves located in Ganges–Brahmaputra–Meghna (GBM) delta shared between Bangladesh and India are the largest mangrove forests in the world. It covers around 6017 km2 area and represents a unique ecosystem being habitat for the endangered Royal Bengal Tiger (Panthera tigris) (Iftekhar and Saenger 2008). Considering the conservational value of the Sundarbans, in 1997 United Nations Educational, Scientific and Cultural Organization (UNESCO) has declared 1397 km2 forest area as the World Heritage site (WCMC 2001). Besides, south-west part of this forest (Ramsar site) is also declared as world’s most important wetlands (Islam 2011). This forest is economically very significant to the marginal people of south-western Bangladesh, and is responsible for nearly 41% of the total forest revenue in Bangladesh (Roy and Hossain 2015). But, Sundarbans mangrove forests at one time covered nearly double the area, what exists at present (Das and Mandal 2016). In recent decades, mangroves have been increasingly threatened due to several human induced pressures i.e., changes of land use patterns, effluent discharges from industry and aquaculture ponds, decrease in freshwater flow, agricultural run-off, and oil spillage from sea ports; which have severe negative effects on biodiversity of Sundarbans (MacFarlane et al. 2007; Islam and Bhuiyan 2016; Chowdhury and Maiti 2016; Das and Mandal 2016). In addition, several diseases, natural disasters, rise in sea level and insufficient regeneration also affect the Sundarbans (Hossain et al. 2015). Degradation also occurred because of the development of Mongla export processing zone (EPZ) and sea ports in Khulna, for exports and imports (Hossain et al. 2015). The influence of anthropogenic pressure and contaminants from various sources may change the ecological stability of the mangroves in long run (Hossain and Bhuiyan 2016).

Owing to the concept of sustainable development and coastal environmental conservation in Southeast Asian countries, it is of great importance to understand the Sundarbans mangrove ecosystem, which is under threat due to natural and anthropogenic causes (Kumar and Ramanathan 2015; Neogi et al. 2016). To manage mangrove ecosystems sustainably within an integrated approach, it is essential to identify the information gaps and constraints. Although mangroves have been exploited for many centuries, the initiative for sustainable resources exploitation remained elusive. In these circumstances, this paper reviews the Sundarbans mangrove ecosystem, its services and role to the national economy of Bangladesh. This study also emphasizes to find out the causes of deteriorating Sundarbans mangrove, and finally propose an adequate future management guideline for sustainable forest management and conservation of this world heritage site.

Sundarbans mangrove and its role in national economy

The Sundarbans are the world’s largest mangrove forests, situated between India–Bangladesh border. These are located at the northern shoreline of the Bay of Bengal and the Bangladesh part of the Sundarbans lies between 89°00′ and 89°55′E and 21°30′–22°30′N (Fig. 1). It is known as one of the most diverse and productive ecosystems in the world (Borrell et al. 2016). Apart from being a unique ecosystem, Sundarbans provide extensive protection to coastal populations from cyclones, tidal flooding, erosion and other natural disasters (Das and Vincent 2009; Payo et al. 2016). For example, in case of recent cyclones Sidr (November 17, 2007) and Aila (May 25, 2009), Sundarbans acted as a protective barrier for the coastal communities (Islam 2013; Bhowmik and Cabral 2013). This forest consists of 200 islands, detached by 400 inter connected tidal rivers and canals (Banglapedia 2010). According to the recent estimation, the area of the Sundarbans in Bangladesh is 599,330 ha; which contains about 62% (Rahman et al. 1979) and the rest of the area (426,300 ha) is in West Bengal province of India (Sanyal 1983). It is a habitat for diverse floral and faunal species. In Sundarbans, nearly 334 species of plant, 50 species of mangrove, 35 legumes, 29 grasses, 19 sedges, and 18 euphorbias were identified by Chaffey et al. (1995). Among them, most significant and economically important plants are; Sundari (Heritiera fomes), Goran (Ceriops decandra), Garjan (Rhizophora mucronata), Gewa (Excoecaria agallocha), Golpata (Nypa fruticans), Keora (Sonneratia apetala), Passur (Xylocarpus mekongensis), Baen (Avicennia officinalis), Chanda lota (Dalbergia candenatensish), Amur (Amoora cucullata), Dhundul (Xylocarpus granatum), and Nol-hash (Eriochloa procera). However, Sundari is considered as threatened, due to the occurrence of top dying disease (Rahman et al. 2003; Sarwar 2015). The Bhat Kati (Bruguiera parviflora) and Kala Baen (Avicennia marina) are also rare plants found in Sundarbans (MoEF 2010). In addition, Sundarbans are habitat for variety of faunal species. Bangladesh part of Sundarbans is known to have 45, 42, 46 and 36% mammals, birds, reptiles and amphibian species respectively, amongst the total wild life species of Sundarbans including West Bengal (India) part (Table 1). The Sundarbans is host to about 50 species of mammals, 320 species of birds, 53 species of reptiles, 11 species of amphibians, 177 species of fish and 873 species of invertebrates (Table 1). However, 10 species of mammals, 11 species of birds, 16 species of reptiles and 1 species of amphibians are identified as endangered in Sundarbans (Khan 2013). Some of the endangered species are Royal Bengal Tiger (Panthera tigers), Jungle cat (Felis chaus), Rock python (Python molurus), Estuarine crocodile (Crocodilus porosus), Irrawaddy dolphin (Orcaella brevirostris), Northern river terrapins (Batagur baska), King crabs (Horse shoe), and Green frog (Euphlyctis hexadactylus) (Sarker 1993).

Fig. 1
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Sundarbans mangrove forest in coastal zone of Bangladesh

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Table 1 Comparison of wildlife species in Bangladesh Sundarbans with West Bengal (India) and around the world.
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Mangrove forests served various economic, social and environmental benefits. These forests provide livelihood for local people through fishing, honey and wax collection, tourism, wood and non-wood products (Islam and Islam 2011). Table 2 provides the information about shrimp, crab, honey, snails and bee wax production since the fiscal years 1999–2000 to 2012–2013 from Sundarbans. Sundarbans mangrove forest also meet the demand of wood for fuel, furniture, house posts, boats, bridges, construction and other things for household purpose. In 2003–2004 the estimated wood production by the Forest Department was 433,000 t (Hoq 2008). Table 3 provides the list of economically important trees and their uses, which have great role in national economy of Bangladesh. Former study showed that, only Sundari and Gewa trees of Sundarbans cover 40 and 21 m3/ha/year, respectively, with monetary value equivalent to 125 and 23 US$/m3 respectively (Uddin et al. 2013). The revenue income from forest is continuously increasing, due to increase in the tourists and visitors which enhanced by almost 25 times in 2012–2013 in comparison to 2000–2001 (Fig. 2a, b). Total revenue income from the forest products of Sundarbans are mentioned in Table 4. The increase of revenue collection from US$ 717,409 in 2008–2009 to US$ 879,181 in 2012–2013 fiscal year is shown.

Table 2 Products and services production from the Sundarbans since 1999–2000 to 2012–2013.
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Table 3 Economically important plant species of the Sundarbans and their uses.
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Fig. 2
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Sources: DoF (2014) and Roy and Hossain (2015)

a Number of tourists visited in Sundarbans, and b the revenue income (lac in BDT), since 2000–2001 to 2012–2013.

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Table 4 Forest products and revenue earned during 2008–2009 and 2012–2013 from the Bangladesh part of Sundarbans.
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Causes of degradation of Sundarbans mangrove ecosystem

Sundarbans mangrove forest is one of the most threatened wildlife habitats over the globe. The increasing trend of human population and their forest dependent livelihood is the main reason for mangrove degradation (FAO 2004; Ali et al. 2006). Besides overexploitation, carelessness of restocking, land reclamation, pollution, fishing and shrimp farming activities also create serious threats (Chowdhury and Maiti 2016). It has been witnessed that, 5% of forest cover was decreased within the period of 1989 and 2010 (Mahadevia and Vikas 2012). However, the major causes of the degradation of Sundarbans mangrove are described in the following sections.

Over exploitation and illegal forest cutting

To meet the rising demand of people, Sundarbans are facing over exploitation. Encroachment and illegal cutting of timber and firewood are the major problems (Paul et al. 2017). The main reasons of illegal forest cutting are; wide demand of wood, and unemployment in nearby areas. Sundarbans are situated near the Khulna news paper mill, which relies on nearby legal timber supplies (Rahman et al. 2010). Although, local people collected timber and fuel wood, animal fodder, medicines, fish, crabs, and honey for centuries, but recently significant exploitation occurred due to the increase of population pressure on Sundarbans (Rahman et al. 2010). Table 5 illustrates the scenario of forest cover change between 1989 and 2015, analyzed by remote sensing images. Due to illegal encroachment, biodiversity of Sundarbans is quickly disappearing. Table 6 shows the rare, threatened and endangered floral species; and Table 7 shows the threatened and extinct faunal species of Sundarbans. Moreover, fishing and collecting non-timber wood is the main livelihood for local people, which put the mangrove environment under incredible pressure. It has been also reported that, the density of fishes in shallow waters has declined (Mahadevia and Vikas 2012). Further, due to excessive collection of shrimp seed and artificial culture in forest areas, mangrove ecosystem is severely threatened (Islam and Bhuiyan 2016).

Table 5 Changes of vegetation density and health of Sundarbans since 1989–2015, determined by remote sensing image.
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Table 6 Rare, threatened and endangered floral species of Sundarbans mangrove forest.
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Table 7 Threatened and extinct fauna of Sundarbans mangrove forest.
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Change in coastal agricultural patterns and land use

Due to rapid population growth, present agriculture production is insufficient to meet the demand of the surrounding areas of Sundarbans. The main economic activity of the local people is to cultivate salinity tolerant rice species (Hussain 2013; Islam et al. 2016). Besides, rapidly expanding shrimp farming and salt production (evaporation of seawater to produce salt) take part in deterioration of the mangroves of Bangladesh (Islam and Bhuiyan 2016; Abdullah et al. 2017). The land cover of shrimp farm has increased from 22,000 to 276,000 ha between 1980 and 2013 (Islam and Bhuiyan 2016). Mangrove swamps are used for fishing, and shrimp farming mainly giant tiger prawn (Penaeus monodon) farming (Rahman et al. 2010). For example, the Chokoria Sundarban at the eastern coast of Bay of Bengal has completely been destroyed because of shrimp farming (Iftekhar and Islam 2004; Rahman and Hossain 2015). There are 14 different fishing methods and ghers (shrimp farming ponds) used by the fisherman inside the Sundarbans (Islam and Bhuiyan 2016). Immense clearance of mangroves is taking place to build shrimp culture ponds, which also results in reduction of the mangrove habitat (Islam and Bhuiyan 2016). Shrimp farms are extensively grown-up at the south-western coastal region of Bangladesh. Huge amounts of agrochemicals and hormones are used in shrimp farms to control the disease and pests (Hossain et al. 2013; Islam and Bhuiyan 2016), which ultimately degrade the coastal ecosystems. Moreover, about 4520 t of pesticides are being used for agricultural activities in the coastal districts of Bangladesh (Rahman et al. 2009). Among them, almost 25% (1130 t) of pesticides residue may go to the nearby water bodies (FRI 1994; Rahman et al. 2009). This indiscriminate and excessive use of pesticides poses great threat for aquatic environment of mangroves, and contaminate the food chain.

Coastal pollution

To improve the living standards in coastal areas, recently various industrial activities have come up along with aquaculture farming which discharged huge amount of garbage, wastewater and others effluents (Rahman et al. 2010). Increased movement of tourist ships and boats causes noise pollution and wastes released from them also degrade the wild environment (Hussain and Acharya 1994). Sundarbans mangrove ecosystem also became vulnerable due to pollution by different trace elements and agrochemical residues, which may change the biogeochemistry of mangroves (Rahman et al. 2009). Pesticides and high concentration of trace elements e.g., Cd, Cr, Cu, Pb, and Zn were identified in mangrove sediments (Chowdhury and Maiti 2016; Kumar et al. 2016). Table 8 shows the trace metals concentrations of Sundarbans, compared with other mangroves in the world. It was found that, toxic Zn accumulation capacity of shrimp is much higher, in comparison to other fish species (Ghosal et al. 1997). Ahmed and Ali (2004) found that trace metal concentration in some benthic animals e.g., hermit, king crab, mudskipper, and gastropod exceeds the tolerant limit in Pasur river of Sundarbans. Table 9 describes the concentrations of trace elements uptake by some species of Sundarbans. Due to excessive trace metal contamination, mangrove plants may initiate a variety of sub-cellular responses, which can change the cellular structure (Arora et al. 2010). Rahman et al. (2009) reported high concentration of trace elements in mangroves is changing the physiology of species, causing death of plants and animals, contaminating the food chain and rising the risk to future re-generations.

Table 8 Comparison of trace elements concentrations (µg/g) in surface sediment of Sundarbans with mangrove sediments around the world.
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Table 9 Concentrations (mg/kg dry weight) of trace elements uptake by some species of Sundarbans
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Oil spillage

Contamination by oil spillage is a severe threat for mangroves, and harmful for aquatic species and sea birds. Sources of oil pollution are potentially due to the Mongla sea port situated just 3 km away from the Sundarbans. This port annually deals with approximately 1500–1600 vessels and 12,000–13,000 cargo ships (Rahman et al. 2009). Frequently, shipping vessels pass through the Sundarbans using north-east route (Scott 1989). Many mechanized river crafts and fishing boats also enter the Sundarbans frequently. These vessels discharge waste oil and cause oil spillages, and release others wastes (UNEP 2001). Crude oil and its byproducts also create serious contamination, during the shipment through the rivers of Sundarbans (Rahman et al. 2010). Rahman et al. (2009) gave an estimation about the crude oil spills in Bay of Bengal, and it is reported to be around 400,000 t per year, where the contribution of Bangladesh is nearly 6000 t. Bangladesh imports approximately 1.20 and 0.5 million tons of crude oil and refined oil annually, which is shipped via small tankers (PMA 2005). Contamination mainly occurred from these vessels and sometimes from accidental spillages. There are many examples of such accidental spillage. Since 1990, huge oil spill was noticed in Chandpai Range of Sundarbans from unknown source. Again, near the Dhangmari forest station an oil spill occurred in 1994 due to ship wreckage, which spread almost 15 km (Rahman et al. 2009). Recently (in 2014), a vast oil spill occurred at the Shela River in Sundarbans. The spill occurred when an oil-tanker carrying 350,000 l of furnace oil, collided with a cargo vessel and sank in the river and the oil spread over 350 km2 area (The Daily Star 2014; SOSA 2014). Such spills threatened trees, plankton, small fish, dolphins and birds. The water soluble fractions of oils like; benzene (C6H6), toluene (C6H5CH3), and phenols (C6H5OH) are very toxic to floral and faunal species of Sundarbans (Rahman et al. 2010). Shigenaka (2002) found that, aromatic hydrocarbons having lower molecular weight are the main components of oil mixtures, which can damage the cellular membranes of mangrove roots. Moreover, oil attached on the leaf surface can block stomata and affect photosynthesis, respiration, and metabolism of the mangrove plants (Rahman et al. 2010).

Upstream water flow reduction and salinity

Stream flow of Ganges, Bahmaputra and Meghna (GBM) rivers are the sources of freshwater in Bangladesh (Rahman et al. 2016; Bhuiyan et al. 2017). Sundarbans mangrove forests receive freshwater discharge mainly from the flow of Ganges River (Rahman and Rahman 2017). Annual peak discharge of the river varies from 31,600 to 76,000 m3/s and minimum discharges were found to be 657–858 m3/s near the Hardinge Bridge, located about 185 km upstream of Sundarbans (Aziz and Paul 2015). The Gorai River originates from the Ganges and the freshwater carried by it is distributed to Passur (85%) and Baleswar (15%) rivers. But the pattern has changed since 1975, due to the diversion of water at Farakka Barage in India, just 17 km upstream from Bangladesh (Mirza 1998). It has been found that, since the period of 1997–2015, the maximum, average and minimum discharges have reduced by almost 23, 43 and 65%, respectively, in dry season (January–May), comparing to the pre-barrage period i.e., 1935–1975 (Rahman and Rahman 2017). The Sundarbans ecosystem depends on the availability of fresh water. But, due to reduction of water flow from upstream, salinity rate increased from 10 to 25 dS/m in between 1968 and 2003 (Islam and Gnauck 2009). As a result, the regeneration of Sundari, the dominant timber species of Sundarbans has reduced in the south-western part (Paul et al. 2017), which are the high salinity affected areas of Sundarbans (Fig. 3). Salinity is an important physiological factor, used as one of the influencing parameters to assess the potential for plant species and brackish water animals (Nandy et al. 2007). Optimum level of salinity is essential for plant growth (Clifford 1985). Balanced water flow maintains the optimum environmental and ecological conditions for mangroves development. But due to fresh water withdrawal from the Ganges River in upstream, coastal mangrove ecosystem is now severely threatened (Rahman et al. 2016, 2017). It effects on the river morphology, groundwater, salinity, fisheries, forestry, ecology and navigation (Rahman and Rahman 2017). As a result, the major environmental agents are affected which are illustrated in Fig. 4.

Fig. 3
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Source: Hossain and Bhuiyan (2016)

Distribution of salinity in Sundarbans mangrove.

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Fig. 4
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Impacts of fresh water flow reduction on coastal environment

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Plant diseases

“Top dying” is one of the main causes for deteriorating dominant Sundari trees of Sundarbans (Sarwar 2015). Almost 5–6% of the total H. fomes across the Sundarbans is now suffering from this disease and half of them have been affected in high saline zones (Rahman et al. 2010). According to the Forest Department, “top dying” disease break outs occurred at large scale since 1980. Former study also indicates about 70% steams of H. fomes are affected moderately or severely by “top dying” disease (Islam and Wahab 2005). In 1994–1996, Forest Department survey results found that, average 134,291.70 m3 of H. fomes died every year due to this disease and thus posing a significant loss in terms of biodiversity and economy. Besides, increasing salinity levels in Sundarbans are another significant threats to the trees. However, several diseases i.e., leaf blight (Alternaria alternate, Botrydiploida theobromae, Gliocladium versoeseni), leaf spot (Cercospora rhizosphorae), dieback (Phytophthora nicotianae), seed rot (Fusarium spp.), stump and collar rot (Fomes spp.), anthracnose (Glomerella cingulata), lethal yellowing (Mycoposma like bodies), bleeding disease (Ceratocystis ulmi), and powdery mildew (Oidium spp.) are also responsible for decline in the plant species of Sundarbans (Rahman et al. 2010).

Climate change and natural disasters

Climate change has serious impacts on coastal environment and livelihood, which threaten to damage mangroves (Dasgupta et al. 2017). UNESCO (2007) predicted that, due to anthropogenic stress and increase of 45 cm [Intergovernmental Panel on Climate Change (IPCC) predicts likely to happen at the end of the twenty-first century] in sea level, 75% of the Sundarbans forest will be destroyed. Natural resources of the Sundarbans, especially various plant species, are now in threatened condition due to sea level rise. It is predicted that, rising sea levels may already have flooded about 7500 ha areas of Sundarbans (WWF 2007). Stable sea levels are needed for long-term existence of mangroves. But, due to the global climate change rise in sea level and increased intensity and frequency of cyclones, storms, coastal floods and erosion (Islam and Uddin 2015), are disrupting the mangroves ecosystem (Neogi et al. 2016). Recent cyclones Sidr, Reshmi and Aila had devastating impacts on Sundarbans as reported by many authors (Islam 2013; Islam et al. 2015; Dutta et al. 2015; Deb and Ferreira 2017). It was estimated that, the monetary value of the damage caused by cyclone Sidr is $142.9 million (Saadi 2010). Prolonged inundation by saline water flooding during cyclone devastated the mangroves (Paul et al. 2017). Furthermore, sediment deposition due to flooding causes harm to mangrove root system and regeneration (Deb and Ferreira 2017). Coastal erosion has also significant effects on the mangrove swamps (Pahlowan and Hossain 2015). Moreover, increase in CO2 and ocean acidity changes the marine biogeochemistry which has long term effects on coastal mangrove ecosystem, by reducing the pH of water and soil (Vivekanandan et al. 2016).

Occurrence of fires

Fire is the major threat and recently caused serious damage to the Sundarbans mangrove. Forests in an area around 1.0 km2 at Napitkhali under Chandpai range of Sundarbans got burnt, posing a threat to natural habitat for many rare species including the Royal Bengal Tiger (Rahman et al. 2010). In March 10, 2010 about 250 ha forest land was destroyed by fire; it was the habitat for many rare plants and animals. In the last 3 years, at least 12 fire incidents occurred in different parts of Sundarbans (MAP 2010). Sundarbans host various plant species which produce plenty of nectar collected by honey bees. Though mangroves are the key source of honey and wax in Bangladesh, but the collection practice is very traditional (Islam and Wahab 2005). Generally, honey collectors (mowal) use fire to remove the honey bees from nest, which sometimes spreads in Sundarbans due to lack of proper safety measures (Rahman et al. 2010).

Inadequate knowledge and high dependency of local people

The local people of Satkhira, Bagerhat and Khulna districts are highly dependent on Sundarbans for their livelihoods. These people are not well educated and have inadequate knowledge regarding the values of mangroves protection. They have lack of knowledge about forest conservation, thus destroy the forests resources (Rahman et al. 2010), although, numerous laws, regulations and policies exist for the protection of Sundarban mangroves. Moreover, global climate change affects the development of mangroves (Islam et al. 2015). So, appropriate measures should be taken immediately to ease the effects of climate change. However, despite the protection measures, there have been large scale human encroachments into the Sundarbans turning forest land to agricultural land. Such anthropogenic activities have put tremendous pressure on the Sundarbans.

Tourism

Tourism sector has developed successfully in Sundarbans, with the increase of visitors from 50,000 to 117,000 from the years 2002 to 2010 (Khanom and Buckley 2015). The main attraction for tourists of Sundarbans are Royal Bengal Tiger and spotted deer. Although tourism has significant role in local and national economy of Bangladesh, but it has detrimental effects on Sundarbans because of habitat destruction, noise pollution, poor sanitation, waste disposal and contamination (Khanom et al. 2011). Due to lack of ecotourism and proper monitoring, Sundarbans ecosystem is affected day by day (Das and Bandyopadhyay 2013).

Poor planning and management

Rahman et al. (2010) identified three basic reasons for disappearing mangrove forest in Bangladesh; (i) lack of well-trained forest security and officials, and institutional capacity (ii) lack of proper planning and sufficient knowledge about coastal zone management, and (iii) lack of application of the development plans. Recently examples of poor planning are the decision to establish 1320 MW coal fired power plant in Rampal, just 14 km away from the Sundarbans violating the rules of project implementation in eco-sensitive zone (Kumar 2013). Annually this plant will burn about 4.75 million tons of coal and generate roughly 0.3 and 0.5 million ton’s ashes and other wastes respectively, including harmful effluents. It is also predicted that, from the coal burning huge amount of CO2, CO, SOx, NOx, hydrocarbons, volatile organic compounds (VOC) and some other airborne particles will be emitted (The Daily New Age 2016). From experiences, it is known that, a typical coal based power plant uses only 33–35% coal heat to generate electricity (Chowdhury 2012). Major portion of this heat is absorbed by cooling water, which may discharge into the nearby Poshur River (Hance 2016). Furthermore, the source of bituminous coal is from West Bengal, India, using the channel and rivers of Sundarbans, which may hamper the sound environment of the forest and increase the threat.

Increased man–animal conflict

Due to global climate change, recent devastating cyclones and increased deforestation; wildlife habitat is badly affected. Moreover, huge forest land is lost due to submersion in water, which increase the threat of wild animals coming into human localities for food and shelter (Rahman et al. 2010). A conflict situation is increasing by the day among man and wild animals, which is harmful for both. This is particularly threatening for the existence of critically endangered Royal Bengal Tigers in Sundarbans region, that are historically known to prey on humans. In fact, it is well known that several tigers that entered into human habitations have been killed due to fear of man eating (Rahman et al. 2010).

Existing management scenarios of Sundarbans

The history of the Sundarbans mangrove management is very conventional. To manage Sundarbans mangrove, detailed work plan was first prepared in the year 1893–1894 (Chowdhury and Ahmed 1994). After that, a working plan was prepared to manage the forests based on the inventory of Sundarbans in period of 1960–1980 (Choudhury 1997). In 1993, forest master plan was prepared, which recommended two sanctuaries. In first sanctuary, yearly plantation target of around 18,000 ha areas during 1993–2002, and increase up to 21,000 ha during 2003–2012 was planned (MoEF 1993). But, due to financial and legal constraints planted trees could not be protected from grazing, illegal removal and encroachment (GoB 1993). Besides, the negligence and dishonesty of the personnel of forest department and local political leaders this initiative almost failed (Akhtaruzzaman 2000). The second one paid attention to the development of wildlife sanctuaries; and Sundarbans was divided into three divisions and declared wildlife sanctuaries. However, the areas of these sanctuaries are not sufficient to provide protection to wildlife in the long run (Islam and Wahab 2005; Rahman et al. 2010). Recently, a management plan for the protection of Sundarbans is being prepared by Food and Agricultural Organizations (FAO). The aim of this project is to achieve sustainable management of mangroves in order to utilize forest resources (Choudhury 1997). In recent times, forest cover, species diversity and ecosystem function have declined rapidly, although a number of guidelines (Table 10) exist to protect the forest resources. The main cause is the ineffectiveness of the plans and policies, due to lack of implementation capacity (Iftekhar and Islam 2004). Although, Department of Forest (DoF) tried to implement certain conservation plans i.e., national conservation strategy, environmental policy, and national environmental management plan to protect the Sundarbans. However, management approaches based on logical and scientific basis have not been developed yet (Rahman et al. 2010; Roya and Gowb 2015). Moreover, to find out sustainable and effective forest management plan, sufficient research efforts have not so far been under taken (Islam and Wahab 2005).

Table 10 Relevant policies, laws, rules, acts and treaties of Sundarbans mangroves forest.
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Sustainable forest management approach

Sustainable forest management depends on numerous factors i.e., integrated management of forest products and goods, active participation of key stakeholders, adequate planning and management, and effective monitoring (Uta et al. 1999). Conservation of forest ecosystems at local levels strengthen resource allocation, which also contribute to regional and global scale conservation (Aheto et al. 2016). Sundarbans mangrove forest in Bangladesh is now extensively degraded and poorly stocked. At this crucial moment it is now essential to develop and implement a sustainable forest management approach for survival of this unique ecosystem. To protect and conserve the Sundarbans mangrove forest of Bangladesh, following integrated approaches are proposed (Fig. 5).

Fig. 5
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Sustainable management approach for Sundarbans mangrove forest

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  1. i.

    Participation of key stakeholders: It involves local communities, forest officials, governmental agencies, researchers, national and international experts and organizations, donor agencies, policy makers, and participation and empowerment of women groups. Local people are the main and key stakeholders (Aheto et al. 2016), so they should be included in forest management plan.

  2. ii.

    Public awareness and campaign: Public awareness about the values of forests and forest protection can be build up by seminars, workshops, training, meetings, videos and slide shows. Besides, publications (e.g., book, magazine, journal) on the value of mangrove forests can be helpful.

  3. iii.

    Reduction of forest dependency: Most of the local residents are dependent on the forest resources for their livelihood. Due to rapid population growth and over exploitation, forest ecosystem is degrading. So, regional planning and relocation of people, set up of alternative livelihood, initiation of technical education, agriculture rehabilitation such as; saline tolerant species, cash grants and microcredit can be helpful for reduction on forest dependency.

  4. iv.

    Forest monitoring and evaluation: Zoning of Sundarbans according to vulnerability, estimation of forested areas and inventory of forest resources using satellite based monitoring systems, ecosystem health and liveliness, ensuring security and management actions, monitoring of timber including fuel wood, pollution control, survey of forest regeneration should be done. Besides, unrestricted use of agrochemicals, and random discharge of industrial and sewage effluent should be controlled.

  5. v.

    Ecological restoration and regeneration: Ecological restoration is very essential to protect the threatened species. It can be evolved by improving and promoting effective regeneration, protecting threatened species, ensure freshwater flow, develop ecotourism, development of forest area and forest cover. It is very much important to monitor rivers and marine water quality to develop healthy mangrove ecosystem. Proper guidelines should be introduced, for the treatment and removal of oil and oily substances discharged from the Mongla port and Khulna municipal area. Moreover, cost effective and eco-friendly techniques should be introduced for pollution control and remediation.

  6. vi.

    Adequate policy, legal and institutional framework: Formulation of proper rules and regulations, capacity building and training for respective institutions and personnels, attempts to reduce the threats to forests, ensure mechanism for protection, management and benefit sharing among the stakeholders, accountability and financial transparency are very much important for forest management. Despite many laws and regulations (which already exist), but due to lack of proper implementation of these, forest sectors are highly vulnerable. Therefore, it is urgently required to implement the guidelines by the concerned authorities. It is also required that time to time the policies and laws should be updated for better management according to the demands.

  7. vii.

    Disaster management program: Natural disasters are one of the main reasons hindering the Sundarbans forest management. Proper forecasting, warning and evacuation, pre-disaster planning, inter communication among the relevant organizations, media communication, and disaster aid for the local people may be helpful to protect the forest from adverse effects of natural disasters.

  8. viii.

    Scientific research and planning: For long term sustenance of the Sundarbans mangrove, adequate scientific research is essential (Fig. 5). Without proper scientific understanding it will not be possible to recover the degraded areas of the forest. Eco-friendly and green technologies for pollution and disease control should be emphasized upon.

Furthermore, it is necessary to protect the forest from illegal encroachment and poaching of wildlife. It is also essential to recruit honest and enthusiastic forestry professionals. Moreover, widespread education to promote the importance of mangroves and its sustenance should be the priority of government and non-government organizations.

Conclusion

The Sundarbans are unique mangrove ecosystems in the world with considerable ecological and economic values. They provide different resources and services to the mankind since long time. But nowadays, the ecosystem of Sundarbans is under threat, due to various natural and man-made pressures. Among the identified causes, overexploitation, coastal land use change, pollution, increased water salinity and natural disasters are the main threat to the ecosystem of Sundarbans. The deterioration of mangroves can lead to serious impacts including decline in biodiversity, genetic erosion, extinction, and changes in the natural biogeochemistry. Although government has made several forest policies, laws and management plans to protect the Sundarbans mangroves, but due to lack of proper monitoring and integration, these are not very successful. To conserve and protect this vital ecosystem, it is essential to develop and implement suitable management strategies for sustainable future. In these circumstances, proposed management approach by refining the existing strategies and combining updated information may be helpful for sustenance of Sundarbans. This has to include an extensive combination of management systems that may effectively be practiced in Sundarbans, and similar ecosystems around the globe.