The species composition in the mangrove habitats of India is reviewed and discussed. The review shows that Indian mangroves consist of 46 true mangrove species belonging to 14 families and 22 genera, which includes 42 species and 4 natural hybrids. In other words, about 57% of the world's mangrove species are represented in India. The East coast has 40 mangrove species belonging to 14 families and 22 genera. The West coast has 27 species belonging to 11 families and 16 genera and the Andaman and Nicobar Islands (ANI) have 38 species belonging to 13 families and 19 genera. Among the 13 States/Union Territories having mangroves, diversity is highest in the ANI. Species namely Rhizophora × lamarckii, Lumnitzera littorea, Sonneratia ovata, S. lanceolata, S. × urama and S. × gulngai are restricted to ANI in India. In terms of mangrove diversity, India is the third richest country in the world (after Indonesia and Australia). However, a majority of the Indian mangrove habitats are now threatened by various factors such as reduction in freshwater flow, marine & coastal pollution, siltation, sedimentation and excessive salinity. Periodical floristic surveys are needed to monitor and preserve the rich mangrove plant diversity in India.

A review of the mangrove floristics of India

The mangroves of the Andaman and Nicobar Islands (ANI) are recognized as the best in the country in terms of density and growth. However, considerable disparities still exist in the species composition and classification of mangrove species in the ANI. Further, taxonomical identity and occurrence of certain mangrove species in the ANI remain imprecise. Considering these facts, the present study was carried out to provide an updated checklist of true mangrove species of the ANI based on the extensive field surveys and perusal of the existing literature. Surveys were carried out in eight regions of the ANI and in each region, specific sites were selected based on representativeness, importance, and accessibility. Altogether, 51 sites were surveyed during 2009–2014. The results show that the mangroves of the ANI consist of 38 true mangrove species belonging to 13 families and 19 genera, which account for about 50 % of the global true mangrove species. The Andaman Islands have 35 and the Nicobar Islands have 21 true mangrove species. Among the eight regions, the mangrove diversity is high in South Andaman, followed by Middle Andaman, Mayabunder, Diglipur, Nicobar, Little Andaman, Havelock, and Baratang. Given their limited geographical area, the high similarity index values between the regions (>0.6) indicate the homogenous nature of mangrove diversity in the ANI and absence of impassable dispersal barriers. The Nicobar region exhibited low similarity with other regions (0.61–0.69). The changes in species numbers compared to earlier studies could mainly be attributed to the lack of extensive field survey in the past. Thus, periodical information on the extent and status of mangroves in the islands is imperative not only to improve our understanding of phytogeography, but also for better management and conservation.

Mangrove floristics of the Andaman and Nicobar Islands: critical review and current scenario

Hybridization with natives augments the threats of introduced species in Sonneratia mangroves

Natural hybridization in mangroves – an overview

Mangrove ecosystems, which occupy intertidal environments across tropical and subtropical regions, provide crucial ecosystem services, such as protecting the coastal areas by reducing the impact of cyclones, storms, and tidal waves. Anthropogenic activities such as human settlements, deforestation, pollution, and climate change have increased the risk of biological invasions in mangrove habitats. Plant species can be introduced to mangrove habitats via anthropogenic means, such as trade and transportation, urbanisation, and agriculture, as well as through natural processes like wind, floods, cyclones, and animal-assisted seed dispersal. Additionally, some native plants can become invasive due to the changes in the mangrove ecosystem. Invasive species can significantly affect coastal ecosystems by out-competing native flora for resources, thereby altering fundamental properties, functions, and ecosystem services of the mangrove forests. The successful establishment of invasive species depends on a complex interplay of factors involving the biological attributes of the invading species and the ecological dynamics of the invaded habitat. This review focuses on exploring the mechanisms of invasion, strategies used by invasive plants, the effects of invasive plants in mangrove habitats and their possible management strategies. Based on the literature, managing invasive species is possible by biological, chemical, or physical treatments. Some non-native mangrove species introduced through restoration activities can often become more intrusive than native species. Therefore, restoration activities should prioritise avoiding the use of non-native plant species. 

The diversity, drivers, consequences and management of plant invasions in the mangrove ecosystems

In India, ＂Rhizophora＂ hybrids have been recorded in Pichavaram (Tamil Nadu) and in the Andaman and Nicobar Islands (ANI). ＂Rhizophora＂ hybrids in Pichavaram mangrove forest were identified as ＂Rhizophora＂ × ＂annamalayana＂, a cross between ＂R. apiculata＂ and ＂R. mucronata＂, whereas ＂Rhizophora＂ hybrids in the ANI were identified as ＂Rhizophora＂ × ＂lamarckii＂, a cross between ＂R. apiculata＂ and ＂R. stylosa＂. However taxonomical distinction between them was not much explored. A recent floristic survey revealed the occurrence both ＂R. × annamalayana＂ and ＂R. × lamarckii＂ in the ANI, India. Based on critical analysis of specimens and morphometric analysis, it was found that both ＂R. × lamarckii＂ and ＂R. × annamalayana＂ can be distinguished from ＂R. apiculata＂ by their smooth green bract and bracteoles and 2-4 flowered inflorescences within the leaf axils. ＂Rhizophora＂ × ＂annamalayana＂ is distinguished from ＂R. × lamarckii by its broader leaves (length: width ratio ＜ 1.8 vs ＞ 1.8), and shorter style (＜1.5 mm vs. ＞ 1.5 mm) and stamens in two whorls vs. usually in a single whorl. ＂Rhizophora＂ × ＂annamalayana＂ is the new record from the ANI. Higher relative density of ＂R. × annamalayana＂ than ＂R. × lamarckii＂ in the ANI is attributed to the abundance of its parental species (＂R. apiculata＂ and ＂R. mucronata＂) in the ANI.

https://tai2.ntu.edu.tw/taiwania/pdf/tai.2015.60.183.pdf

Taxonomical Identity of ＂Rhizophora＂ × ＂annamalayana＂ Kathir and ＂Rhizophora＂ × ＂lamarckii＂ Montrouz (Rhizophoraceae) in the Andaman and Nicobar Islands, India

Ragavan P, Ravichandran K, Mohan PM, Sxaena A, Prasanth RS, Jayaraj RSJ, Saravanan S. 2015. Note on Excoecariaindica (Willd.) Muell.-Arg, 1863 (Euphorbiaceae), from the Andaman and Nicobar Islands, India; a data deficient species. Biodiversitas 16: 22-26. Excoecaria indica (Wild.) Muell.-Arg was recorded from Middle Andaman and Great Nicobar Island representing a newaddition to the mangrove flora of the, Andaman and Nicobar islands. This species is characterized by its thorny trunk, crenulatelanceolateleaves and cherry-sized green fruits containing three seeds. Information about E. indica is inadequate, and it is recognized asdata deficient species. Further studies and conservation measures are imperative for managing the mangrove diversity of the islands withregards to this species.

/pdf/short-communication-note-on-excoecaria-indica-willd-muell-pri9rwi9rc.pdf

Short Communication: Note on Excoecaria indica (Willd.) Muell.-Arg, 1863 (Euphorbiaceae), from the Andaman and Nicobar Islands, India; a data deficient species

Ragavan P, Ravichandran K, Mohan PM, Sxaena A, Prasanth R S, Jayaraj RSC, Saravanan S. 2014. New distributional records of Sonneratia spp. from Andaman and Nicobar Islands, India. Biodiversitas 15: 251-260. Sonneratia lanceolata, Sonneratia x urama andSonneratia x gulngai was collected from Great Nicobar Island, which representing a new addition to the mangrove flora of India. S.lanceolata is distinguished from S. caseolaris by its drooping branches, lanceolate leaves and ovoidal bud without medial constriction.S. x urama and S. x gulngai are putative hybrids. S. x urama is putative hybrid between S. alba and S. lanceolata, whereas S. x gulngai is putative hybrid between S. alba and S. caseolaris. A detailed description along with colour plate and relevant notes is provided for further collection and identification of these species in the field.

/pdf/short-communication-new-distributional-records-of-sonneratia-gmsslnzijl.pdf

Short Communication: New distributional records of Sonneratia spp. from Andaman and Nicobar Islands, India

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Taxonomical Identity of ＂Rhizophora＂ × ＂annamalayana＂ Kathir and ＂Rhizophora＂ × ＂lamarckii＂ Montrouz (Rhizophoraceae) in the Andaman and Nicobar Islands, India

Short Communication: Note on Excoecaria indica (Willd.) Muell.-Arg, 1863 (Euphorbiaceae), from the Andaman and Nicobar Islands, India; a data deficient species

Short Communication: New distributional records of Sonneratia spp. from Andaman and Nicobar Islands, India