Mangrove Forests of Coastal India  Cite

Mangroves of India: globally unique

Mangroves of India are globally unique. Sundarbans has the largest mangrove cover, occupying 42.3%, and Gujarat has the second largest cover, with 23.5% of total cover in India (SFR, 2021). These two areas alone occupy 65.8% of the mangrove cover, but thriving in adverse conditions of high energy tidal coast, and experiencing extreme situations. For example, Sundarbans is in humid and wet conditions with high biodiversity, whereas the mangrove forest of Gujarat is in arid and dry conditions with low biodiversity. Interestingly, the Andaman & Nicobar Islands has India's third largest mangrove forest, occupying 12.3% of the total cover, and are located in low energy tidal coast in humid and wet conditions with rich biodiversity (Kathiresan, 2018,2021).

India is the second richest country for mangrove biodiversity in the world, after Indonesia (Ragavan, 2015; Kathiresan, 2018, 2019). Our country harbours 54% of true mangroves in the world, that is 43 out of 80 global species. It has the highest world record of 85 species of mangrove associates. Bhitarkanika in the Odisha State is regarded as the 'mangrove genetic paradise' globally, similar to the one in Baimaru of Papua, New Guinea. In the Bhitarkanika, there is a small island called "Kalibhanj dia" with an area of about 900 ha. This is alone endowed with 96 species of mangroves and mangrove associates and larger populations of birds and crocodiles. The mangrove diversity is recorded to be the highest in Odisha followed by Andaman & Nicobar, Sundarbans, and the lowest in Gujarat. A list of true mangroves in India is given in Table 1. India has two globally- threatened mangrove species viz., Heritiera fomes and Sonneratia griffithii.

Fig. 1: A panoramic view of mangrove forest in Pichavaram, Tamil Nadu

Table 1: Mangroves occurring along the east and west coasts of India and in Lakshadweep, Andaman & Nicobar Islands

Abundant (a) = Species present in 81-100% of sampling points; Frequent (f) = Species present in 61-80% of sampling points; Common (c) = Species present in 31-60% of sampling points; Rare (r) = Species present in 1-30% of sampling points

Sundarbans in India and Bangladesh put together is the largest mangrove forest in the world, and it is also the only mangroves in the world colonized with the Royal Bengal Tigers. It is situated in the Gangetic Delta, which is the largest wetland with the highest sedimentation in the world. The dense mangrove forest is unique in extending 100 kilometers inland from the seafront, and flora and fauna residing here are amazingly adjusted to the rigorously fluctuating tidal environment. The Sundarbans is a genetic paradise for mangrove-associated animals and it provides a "home" for globally threatened species such as tigers, fishing cats, Gangetic dolphins, estuarine crocodiles, horseshoe crabs, water monitor lizards, and river terrapins. It is internationally recognized as a world heritage site by UNESCO, and it is the only mangrove-tiger kingdom of the world (Kathiresan, 2018, 2018a).

Indian mangrove forests have the highest record of biodiversity, with a total of 5,746 species. Of which, 84% are fauna and 16% are flora, and thus animals are 5.3 fold higher than plant species. No other countries in the world have recorded so many species to be present in the mangrove ecosystems. Twelve groups of organisms are dominant by exceeding 100 species, and they are mangrove species (true mangroves + mangrove associates), marine algae (phytoplankton + seaweeds), fungi, protozoa, nematodes, polychaets, arachnids, crustaceans, mollusks, insects, birds, and finfish (Kathiresan, 2017, 2018; Chandra et al., 2019; Kathiresan, 2019) (Table 2).

Table 2: Total numbers of floral and faunal species reported to exist in mangrove ecosystems of India

Major group	Sub-group	No. of species
Floral groups:	Mangroves	43
	Mangrove associate plants	86
	Seagrass vegetation	11
	Marine algae (Phytoplankton + seaweeds)	557
	Bacteria	69
	Fungi	103
	Actinomycetes	23
	Lichens	32
	Total of floral species	924
Faunal groups:	Protozoa (Protista)	349
	Cnidaria	73
	Rotifera	53
	Nematoda (free-living)	125
	polychaeta	244
	oligochaeta	21
	Arachnida	309
	Crustacea	624
	Insecta (Insects)	1422
	Mollusks	173
	Other minor groups	84
	Tunicata (Urochordata)	6
	Finfish (Pisces)	659
	Amphibians	14
	Reptilia (Reptiles)s	57
	Birds (Aves)	523
	Mammals	86
	Total faunal species	4822
	Total number of species	5746

Mangroves harbour most of the groups of animal communities with 30 different groups under 21 phyla from the lowest phylum of Protozoa to the advanced phylum Mammalia. There are a total of 4,822 species of animals, which contributes to 4.76% of Indian fauna (Kathiresan, 2018; Chandra et al., 2019).

Other natural treasures along the mangroves of India are: (i) the world's largest nesting site for the Olive Ridley turtle in Gahirmatha coast of Odisha; (ii) seagrass meadows associated with the seacow (Dugong); (iii) coral reefs associated with most beautiful ornamental fishes; and, (iv) intertidal mudflats teeming with the migratory and residential birds.

Mangroves are of spiritual value in India. A mangrove species (Excoecaria agallocha) has been worshipped as the temple tree for a long period of 17 centuries in Chidambaram, Tamil Nadu. In Sundarbans, the goddess bonabibi (mother of the forests) and her consort Dakshin Rai are worshipped as protectors by both Hindus and Muslims. In Karnataka, a sacred shrine "Kalika Mandir" is surrounded by dense mangroves in Kali Nadi.

Mangrove forests of India

India has a mangrove forest cover of 4,992 sq. km, occupying 3.6% of the global mangroves and 0.15% of total geographical area of India (SFR 2021). The mangroves are dense, healthy and floristically diverse along the east coast of India and Andaman and Nicobar Islands, compared to the west coast of India. The east coast has 56% of total mangrove cover, with 89% of mangrove species in India, whereas the west coast has 32% of cover, with 63% of mangrove species Kathiresan et al., 2013). The mangrove-richness of east coast can be attributed to the mighty rivers (e.g. Ganga, Brahmaputra, Mahanadhi, Krishna, Godavari and Cauvery) along the east coast that form deltas, rich in sedimentation, upstream water discharge, nutrient-rich alluvial soil, in addition to the smooth topography, which increases the intertidal areas for mangrove colonization along the east coast. On the contrary, the west coast has narrow intertidal areas due to steep coast, and absence of deltas as a result of funnel-shaped estuaries. The Andaman and Nicobar Islands have 12.3% of the total mangrove cover, endowed with 70% of mangrove species, colonizing in low energy tidal coast with accumulation of peat and calcareous materials in coastal fringes, tidal estuaries, small rivers, neritic inlets, and lagoons (Bhatt et al., 2013; Kathiresan, 2018a).

Forest Survey of India has been assessing the area and extent of mangrove cover using remote sensing techniques since 1987. There is a general trend of increasing mangrove forest cover in India (Fig.2). For example, 17 km² of mangrove cover increased between 2 years (2019-2021) at the annual increase rate of 0.2% in India, in contrast to the loss of global mangroves. The mangrove cover increased significantly in Odisha, Maharashtra,Karnataka and West Bengal, but declined in Gujarat during the period. However, 41% of mangrove forests in India are poor with less canopy density of <40% (SRF, 2021).