Sundarbans National Park

The world's total mangrove cover is around 15 million ha, of which Asia has the largest extent and Sundarbans being the largest single patch (Kumar et al., 2019). The Sundarbans mangroves provide a wide spectrum of ecosystem services, which include improvement of air quality, stabilizing temperature, reduction of ultraviolet radiation, oxygen generation, carbon sequestration, habitat of several flora and fauna (enhancement of biodiversity) and aesthetic beauty (Pal et al., 2019). Sundarban in India is a part of the Ganga-Brahmaputra Delta comprised of 102 islands, of which 54 are inhabited islands (540,000 ha) and 48 are forested (420,000 ha) islands (Mistri, 2019). Erosion across the Sundarban islands is more dominant than the accretion (Sarkar et al., 2019). Due to this, Sundarbans is now almost half the size of the area that existed in the late 1800s (Rahman & Asaduzzaman, 2010). The eastward migration of the Ganges River is causing a long-term ecological change in the entire Sundarbans forest. This process has intensified due to the increased diversion of freshwater sources to the Sundarbans forest that occurred in recent years (i.e. up to 40% of the dry season flow of the Ganges was diverted upstream after the creation of the Farraka Barrage in 1974; UNEP-WCMC & IUCN, 2013a). Reduced freshwater flow serves to increase salt-water intrusion and irregular inundation, and also leads to increased soil acidification (UNEP-WCMC & IUCN, 2013a). Siltation is also a problem within the site, and specific channels (i.e. the Matla River up to Canning Town) are becoming severely blocked and could soon be considered unnavigable (UNEP-WCMC & IUCN, 2013a). The wider Sundarbans forest is also experiencing extensive coastal erosion and retreat, which is considered to be largely attributable to increased frequency of storm surges and other extreme natural events, rise in sea-level and increased salinity (Cornforth et al., 2013). In the Indian Sundarbans, between 1969 and 2009, 21,025 ha has been lost, with 6,506 ha being lost in the past decade (Danda et al., 2011). Within the Indian Sundarbans, it is projected that the loss of area with sea-level rise for 12 of the most vulnerable islands is 3-32% (WWF-India, 2014). With sea-level rise, increased salinity will result in ecological succession to more salt-tolerant species, reducing mangrove biodiversity. Threatened aquatic species such as the Ganges river dolphin and Irrawaddy dolphin have both been found to exhibit dependency for habitat characteristics associated with high freshwater flow, and as such are highly vulnerable to habitat loss from this threat (Smith et al., 2009). This also represents a potentially great loss of carbon storage (Rahman et al., 2010), and loss of habitat for important biodiversity such as the Bengal tiger. The transboundary Sundarbans is also home to about 7.2 million people of which about 2.7 million live in Bangladesh and the rest in India (Danda et al., 2019). Half of the population in the vicinity of Sundarbans is landless and they are almost entirely dependent on rain-fed agriculture and subsistence fishing. High risk investment due to frequent cyclones and flooding makes any other industrial investment practically non-existent, especially in the forest adjoining sub-districts (Danda et al., 2019).

Eight globally threatened species have been locally extirpated over the last century: Javan rhinoceros Rhinoceros sondaicus (Critically Endangered, CR), Indian rhinoceros Rhinoceros univornis (Vulnerable, VU), water buffalo Bubalus bubalis (Endangered, EN), swamp deer Rucervus duvaucelii (Vulnerable, VU), barking deer Muntiacus muntjak (Endangered, EN), gaur Bos gaurus (Vulnerable, VU), hog deer Axis porcinus (Endangered, EN), and sambar Rusa unicolor (Vulnerable, VU) (IUCN, 1987; Gopal & Chauhan, 2006; UNEP-WCMC & IUCN, 2013a; IUCN, 2013).
The Sundarbans National Park supports a diverse assemblage of rare and threatened terrestrial and aquatic mammals (IUCN, 1987; UNEP-WCMC & IUCN, 2013a). However, populations of the Ganges river dolphin appear to have been extirpated from Indian Sundarbans (Mitra and Chowdhury, 2018) due to climatic and anthropogenic factors. For other aquatic animals, the threat from illegal poaching, habitat loss and degradation (both aquatic and terrestrial), pollution, altered hydrology and entanglement in hazardous fishing equipment remain significant, and are likely to increase in intensity in the future.
If the tiger population is to be considered as an indicator of the terrestrial health of the Sundarbans, then it appears that the biodiversity in the Sundarbans National Park is in a relatively stable and good state, for the moment (UNEP-WCMC & IUCN, 2013a, Jhala et al., 2016).

The Sundarbans has a varied bird life with a high level of avian diversity for a mangrove ecosystem (Khan, 2005). The area provides habitat for several globally threatened species (IUCN, 2013). However, little empirical data on population trends exists for the rare and threatened birds of the Sundarbans. One recent record of the Yellow-browed Bunting Emberiza chrysophrys (previously unreported in South Asia) (Kundu and Abhinav, 2018) has added to the number of 400 bird species reported in the area. One bird species is known to have become locally extinct from the Sundarbans over the last century: Swamp francolin Francolinus gularis (Vulnerable, VU) (Ministry of Environment and Forests, 2010; IUCN, 2013).
However, threats to the Sundarbans’ bird life (habitat loss and degradation from overexploitation, pollution, top-dying of Sundri trees and clearing for conversion to aquaculture) are significant and growing. Accordingly, it is likely that declines in avian abundance have occurred in the area and will increase in the future with the increasing intensity of these threats, as well as climate change, if appropriate management action is not taken.

Reptile diversity in the Sundarbans remains relatively high, and the area provides habitat for several globally threatened species: king cobra (Vulnerable, VU), olive ridley turtle (Vulnerable, VU), green turtle (Endangered, EN), loggerhead turtle (Endangered, EN), hawksbill turtle (Endangered, EN), Indian peacock softshell turtle (Vulnerable, VU), and the Northern river terrapin (Critically Endangered, CR) (IUCN, 1987; IUCN, 2013). Estuarine crocodiles are present, and while globally considered to be of Least Concern (LC), they occur in low numbers in the Indian Sundarbans (100 individuals) (UNEP-WCMC & IUCN, 2013a). The situation for many of these globally threatened/ locally rare reptiles remains promising, due to good management and breeding programmes in the area carried out by the Department of Forests. Estuarine crocodiles are captive-bred at the Bhagabatpur Crocodile Breeding Centre for re-introduction into the Sundarbans, and olive ridley turtles are also reared here and in captivity at Sajnakhali for release in the National Park (UNEP-WCMC & IUCN, 2013a). A breeding program is also being run by the Turtle Survival Alliance and the West Bengal Forest Department in Sajnakhali for the Northern river terrapin and this has seen the successful hatching of several juveniles of this species (UNEP-WCMC & IUCN, 2013a). The species is considered functionally extinct in the Sundarbans, but this program offers a chance of recovery for this globally Critically Endangered (CR) species (TSA, 2009; UNEP-WCMC & IUCN, 2013a). Overall, the situation for the herpetofauna of the Indian Sundarbans looks promising, through increased human-crocodile conflict is a cause of concern. Das and Jana (2018) stated that the increased instances of human-crocodile conflict, with 127 incidents between 2000-2013, is primarily due to increasing human pressures, habitat loss and degradation of the natural habitat of the saltwater crocodile (Crocodylus porosus).

Populations of Sundari trees (Heritiera fomes; EN) in the Sundarbans wider ecosystem are declining (IUCN, 2017) as a result of numerous impacts including “top-dying” and timber harvesting. The extent of pure H. fomes stands has decreased by 21% since 1926, and dominance of H. fomes in mixed stands is diminishing (Iftekhar & Saenger, 2008). Approximately 70% of H. fomes stems have been estimated to be moderately or severely affected by the “top-dying” disease (Islam & Wahab, 2005), which causes high mortality, threatening mangrove diversity, forest cover and ecosystem complexity. Due to declines in H. fomes, forest structure in the Sundarbans has become simpler and mean forest height is decreasing, reducing habitat for arboreal mammals and birds (Iftekhar & Saenger, 2008). In addition, as the dominant mangrove species in the back mangrove of the Sundarbans, loss of H. fomes trees poses a threat to the biodiversity, ecological and aesthetic values of the site (Rahman et al., 2010).