About 71% of India’s mangroves are found in the east coast (Forest Survey of India 2013) and the largest areas are those of the Indian part of the Sundarbans and the Andaman and Nicobar Islands. Mangroves of the Andaman and Nicobar Islands account for 13% of the total Indian mangrove cover (Forest Survey of India 2013), the diversity of which is similar to the Southeast Asian mangroves (Spalding et al. 1997). With a relative mangrove density of 76.5% (Mandal and Naskar 2008), the mangroves of Andaman and Nicobar Islands are recognized as the best in the country in terms of density and growth (Dagar et al. 1991).
Despite extensive studies of biogeography, ecology, and forestry of the mangrove ecosystems of the Andaman and Nicobar Islands since the 1870s, considerable disparities in species composition still exist, and there is a lack of comprehensive locality data. The number of true mangrove species (plants restricted to intertidal and adjacent communities) reported from the Islands ranges from 17 (Parkinson 1923) to 36 (Debnath 2004). Notwithstanding the geographical isolation of different islands, many biogeographic studies (for example, Debnath 2004) have given imprecise distributional data (Andaman Islands; Nicobar Islands) for the mangrove species reported in their study. Further, mere inclusion of associated littoral vegetation and of species unlikely to be present by several authors based on earlier reports also has contributed to erroneous additions to the mangrove flora of the Andaman and Nicobar Islands, which emphasizes the importance of extending our knowledge of mangrove species diversity and distribution in the Islands.
Among the 25 countries that have mangrove vegetation in Asia, five viz., Indonesia, Malaysia, Myanmar, Bangladesh, and India, experience intensive logging or conversion to other land uses (FAO 2007). Mangrove forest area in the Andaman and Nicobar Islands continues to decline rapidly, and approximately 369 km2 of mangroves were lost over a period of 24 years from 1989 to 2013 (Forest Survey of India 2013). Though not subject to human-driven land-use changes and urbanization threats, recent reports indicate that there is a dearth of data relating to the detrimental factors responsible for such continued loss. Hence, there is an urgent need to conserve the remaining mangrove area in the Islands.
The knowledge of exact species composition is imperative not only for understanding the structure and function of mangroves and their biogeographical affinities but also to plan strategies for their conservation and management (Jayatissa et al. 2002, Wang et al. 2003). The present study aimed at assessing the species diversity and distribution of mangroves in the Islands, supplemented with a critical review of the published literature. The probable reasons for the significant disparities in numbers of mangrove species reported in the earlier studies are also discussed vis-à-vis our field surveys.
Materials and methods
Study area and methods of survey
Mangrove communities across the entire stretch of the Andaman and Nicobar Islands were surveyed for a period of 5 years at different seasons during 2009–2013. Preliminary surveys for identifying specific survey sites were carried out in seven regions (four from the Andaman group of Islands and three from the Nicobar group). The survey sites were selected based on representativeness, importance, and accessibility. The 51 sites selected for conducting comprehensive mangrove surveys are mapped in Figure 1, and the GPS coordinates are given in Table S1. Sites were accessed using both road and boat transport in order to survey the extensive range of tidal reaches. Each of the survey sites was visited at least twice, and care was taken to observe both floral and vegetative characters. The mangrove species were identified in the field following Tomlinson (1986) and Giesen et al. (2006), and locations were recorded. Herbarium specimens for all the true mangrove species were prepared. Because of the conspicuous similarities in floristic characteristics between the Andaman and Nicobar Islands and Southeast Asian countries, the classification scheme adopted by Giesen et al. (2006) for classifying the Southeast Asian mangroves was followed in the present study.
Review of the published literature
Mangrove species composition in the published literature was compared with the present study to help determine the validity of earlier records and to extend our knowledge of the current distribution of mangrove species in the Islands. Generally, mangroves are categorized into two viz., exclusive species (referred as strict mangrove, obligate mangrove, or true mangrove) and non-exclusive species (referred as semi-mangrove, back mangrove, or mangrove associate) based on their habitat and morphological specialization (Tomlinson 1986). The list given in Table 1 is not exhaustive, and publications in which no categorization was followed (i.e., true mangroves vs. mangrove associates) were not considered when reviewing mangrove species composition. However, these are briefly discussed with regard to the topics studied separately.
The floristic similarities between the seven regions were examined based on the species presence/absence data using the Sørensen (1948) similarity index. It was calculated as,
where a=the number of shared species; b=the number of species only in collection 1; c=the number of species only in collection 2.
Results and discussion
Mangrove species reported in earlier studies
The numbers of true mangrove species recorded in 18 publications in which mangroves were categorized into true mangroves and mangrove associates are summarized in Table 1. There is significant disparity in species numbers, and these have increased from 17 (Parkinson 1923) to 36 (Debnath 2004) in a period of about 80 years. A similar increase in mangrove species diversity over the years has been reported from Australia (Duke 1992) and from Sri Lanka (Jayatissa et al. 2002).
In addition to the literature listed in Table 1, mangrove species composition including halophytes and associated littoral vegetation of the Andaman and Nicobar Islands has also been investigated by a few authors, although without classifying them into true mangroves and mangrove associates. According to Blasco (1977), the principal halophilous nonparasitic species of the Andaman and Nicobar Islands amount to a total of 35 species distributed among 22 families and 26 genera. Rajagopalan (1987) reported 42 species of typical mangroves and the closely associated littoral vegetation distributed among 23 families and 32 genera. Singh et al. (1987a) studied the pattern and process in mangrove forests of the Islands and reported 26 species of exclusive mangroves, though without giving a species list. Blasco and Aizpuru (1997) have reported 37 species of halophytes including grassy types and ferns distributed among 22 families and 26 genera, whereas Mandal and Naskar (2008) reported 61 species of mangroves and mangrove associates distributed in 30 families and 39 genera in the Islands. Apart from these, Chengappa (1934) investigated the regeneration of mangrove forests in Andaman, and Gopinathan and Rajagopalan (1983) discussed the important mangrove species of the Islands. Ellis (1987) studied the floral composition of nearshore vegetation of the Islands, and Jagtap (1992), in his account of the marine flora of the Nicobar Islands, reported 10 species of mangroves. In a recent study of re-colonizing mangrove species in tsunami-devastated habitats, Nehru and Balasubramanian (2011) reported nine mangroves and 30 mangrove associates from the Nicobar Islands.
New distributional records
A total of five species (four from the Rhizophoraceae and one from the Lythraceae sensu lato) have been reported as new distributional records for the Andaman and Nicobar Islands between 1987 and 2012 (Table 2), of which Sonneratia ovata (Lythraceae sensu lato) was unknown from elsewhere in India (Kathiresan and Rajendran 2005, FAO 2007, Mandal and Naskar 2008).
Composition of true mangrove species
The true mangrove species composition of the Andaman and Nicobar Islands recorded by 16 papers published from 1923 to 2014 is summarized in Table 3. As two of the 18 references listed in Table 1 viz., Naskar and Mandal (1999) and Devraj (2001) have provided identical checklists to Dagar et al. (1991) and Balachandra (1988), respectively, they were not considered in this review of the species composition. Altogether, 51 species have been reported as true mangroves from the Andaman and Nicobar Islands, of which only 11 were considered as true mangroves by all authors.
Parkinson (1923) reported a total of 17 true mangrove species from the Islands. It is pertinent to note that two species widely recognized as true mangroves, viz. Excoecaria agallocha and Heritiera littoralis were considered by him to be terrestrial plants, and two other species, viz., Aegialitis rotundifolia (observed by Kurz 1870 and Brandis 1907) and Kandelia candel (observed by Brandis 1907), were not encountered in the Islands during his surveys. In his account of mangrove forests of the Islands, Sahni (1958) categorized true mangroves into two viz., principal and subsidiary species. Cerbera manghas, an occasional element of coastal forest and back mangrove communities (Tomlinson 1986), was included in his list of true mangroves. Mall et al. (1985) surveyed the mangrove forests of Middle and South Andaman and reported 23 species of true mangroves. Dagar (1987) reported the presence of A. rotundifolia and K. candel from the Islands after a lapse of 80 years and, in addition, listed three species viz., Ceriops decandra, Sonneratia griffithi, and Xylocarpus meckongensis as “unobserved species”. His subsequent publication (Dagar et al. 1991), however, did not confirm the presence of A. rotundifolia or K. candel, but two of the three species claimed “unobserved” in his preceding publication (C. decandra and X. meckongensis) were reported to be present. Nevertheless, the total number of true mangrove species was 34 in both publications. Another publication by Dagar and Singh (1999), however, included all the doubtful species (A. rotundifolia, Avicennia alba, C. decandra, K. candel) in the list of true mangroves. Singh et al. (1986) investigated the mangrove forests of Middle and South Andaman and reported Bruguiera sexangula, Rhizophora lamarckii, and Sonneratia apetala as rare. Balachandra (1988) included Ardisia solanacea (actually Ardisia elliptica), Carallia brachiata, and Dolichandrone spathacea as true mangroves. Das and Dev Roy (1989) also included A. alba and K. candel in their list of true mangroves from the Islands.
Like Dagar et al. (1991), Singh and Garge (1993) also reported X. meckongensis from the Islands. It is important to note that no distributional data were given in either report, despite inclusion of a rare species. Jagtap (1994) also included A. alba, but A. marina, a common species reported in all literature except Parkinson (1923), was not recorded in his study. Dam Roy (2003) surveyed only the Andaman group of Islands and considered Flagellaria indica to be a true mangrove. However, it was reported as an associate in his subsequent publication (Dam Roy et al. 2009).
Debnath (2004) reported the highest number of mangrove species (36). Brownlowia tersa, Derris trifoliata, and Dolichandrone spathacea were considered as true mangroves according to his classification. A few associate species, which occasionally occur within mangroves, were included as true mangroves by both Balachandra (1988) and Debnath (2004). Yao et al. (2011), in their comparative study of the mangrove flora in China and India, included five species whose presence remains uncertain, on the basis of records in Dagar (1987) and Dagar et al. (1991). In a recent account of the floral composition and taxonomy of mangroves, Ragavan et al. (2014) claimed that 33 species of true mangroves were present, but they described only 31 species (excluding Rhizophora hybrids).
The adverse impact of including mangrove associates or beach vegetation and species unlikely to be present when reporting the total number of mangrove species in a given area have been discussed in detail by Jayatissa et al. (2002). The inclusion of doubtful species in the mangrove flora of the Islands not only made a few of the past reports extremely unreliable (for example, Dagar et al. 1991, Dagar and Singh 1999, Debnath 2004) but also meant that the exact mangrove species composition of the Islands was uncertain.
Diversity and distribution of mangroves in the Andaman and Nicobar Islands
A total of 25 true mangrove species distributed among 10 families and 14 genera were recorded from the Andaman and Nicobar Islands during the present study (Table 4), of which 23 were recorded from South Andaman, and 20 each from North, Middle, and Little Andaman. Among the survey sites in the Nicobar Island group, 10 species were recorded from Nancowrie and Great Nicobar. Mangrove species diversity was poor at Car Nicobar, where only five species were found. Figure 2 shows the number of species of true mangroves in different families recorded from different regions of the Andaman and Nicobar Islands. As South Andaman was the most intensively studied region, the differences in species diversity among the survey regions may have been influenced by the differences in research effort (Nguyen et al. 2013).
The present distributional status of all the reported true mangrove species in the Andaman and Nicobar Islands was assessed and is summarized below.
Acanthus and Avicennia are the only two genera of Acanthaceae that has representatives in mangrove communities. The genus Acanthus was represented by two species in the Islands, viz., Acanthus ilicifolius and Acanthus ebracteatus, of which the former was found commonly distributed, while the latter was rare and observed only from Sippighat and Shoal Bay of South Andaman. In addition, a few Acanthus specimens with unarmed leaves and trailing stems closely resembling Acanthus volubilis were observed from Carbyn’s Cove in South Andaman during 2009. Neither fruits nor flowers were observed throughout our surveys despite repeated visits. A year later, encroachment of the site by the local communities has rendered further examination of the species impossible. Acanthius volubilis was recorded nearly three decades ago as a rare species (Dagar 1987, Dagar et al. 1991, Dagar and Singh 1999) from Middle and South Andaman. It is important to note that Dam Roy et al. (2009) identified the Acanthus species mentioned above as A. volubilis based on spineless leaves and trailing stems. However, A. ilicifolius with entirely spineless leaves was also recorded from Sippighat of South Andaman, indicating the significant variation in leaf forms in relation to the degree of spininess (Tomlinson 1986). It is important to note that recently Ragavan et al. (2014) reported A. volubilis from Shoal Bay.
Avicennia was found only in the Andaman group of Islands and was represented by two species viz., Avicennia marina and Avicennia officinalis. Its absence from the Nicobar Islands could not be explained. However, Gopinathan and Rajagopalan (1983) reported A. marina three decades ago from Spiteful Bay of the Nancowrie group of Islands. Das and Dev Roy (1989), Jagtap (1994), Dagar and Singh (1999) have added Avicennia alba to the mangrove flora of the Andaman and Nicobar Islands, although it has not been reported in any other studies including the current one. As no photographs or locality data were provided, this species could not be verified. However, Mandal and Naskar (2008) also included A. alba (a “+” sign indicating its presence in table 2, p 140) in the mangrove flora of the Andaman and Nicobar Islands in their review.
Arecaceae was represented by two species viz., Nypa fruticans and Phoenix paludosa in the Islands. These two species were commonly distributed in all survey regions from both the Island groups except Car Nicobar.
Two species viz., Lumnitzera littorea and Lumnitzera racemosa representing Combretaceae were recorded in our surveys of the Islands. The latter occurs widely in Gobind Nagar of Havelock and Lakshmanpur of Neil Island (South Andaman). In none of the survey sites were L. littorea and L. racemosa found to co-exist (for example, Neil Island), which could be due to their ecological niches excluding each other in a given habitat (Giesen et al. 2006).
Excoecaria agallocha (Euphorbiaceae) was common in the Islands and found to inhabit the landward edges of mangrove forests along with terrestrial vegetation.
Lythraceae sensu lato was represented by four species from the genus Sonneratia viz., Sonneratia alba, Sonneratia caseolaris, Sonneratia griffithi, and Sonneratia ovata, of which only S. alba was reported by all the reviewed literature. While Parkinson (1923) had reported S. alba as a species with restricted distribution at Betapur of Middle Andaman, it was found to be commonly distributed in all the regions across various habitats, either alone (Burmanallah and Shoal Bay of South Andaman) or together with stands of Avicennia and Rhizophora (Dugong Creek of Little Andaman) in our surveys. Further, the Sonneratia species reported as Sonneratia acida (actually S. caseolaris) by Parkinson (1923) had white petals in contrast to their characteristic dark red petals. Nehru and Balasubramanian (2011) reported S. caseolaris from four re-colonizing mangrove habitats viz., Kamorta, Katchall, Nancowrie, and Trinket of Nicobar Islands, although the corresponding photograph (figure 3D, p 255) in the article suggests that S. alba has been misidentified as S. caseolaris for the following reasons: 1) mucro with no re-curved point, 2) white staminal filaments, 3) dull pericarp, and 4) cup-shaped hypanthium. Sonneratia apetala has not been recorded during the present study, although it has previously been reported from the Islands by several authors (Table 3). A few trees of Sonneratia griffithi were observed at Betapur (Middle Andaman) during our surveys, as recorded by Ragavan et al. (2013). Subsequent to the new record of S. ovata from Radhanagar of Havelock in the Andaman group of Islands (Dam Roy et al. 2009, Goutham-Bharathi et al. 2012) and from tsunami-impacted mangrove habitats of the Nicobar Islands (Nehru and Balasubramanian 2012), our surveys revealed its presence at Wandoor (South Andaman), thus, indicating that regional endemics may have a wider distribution in the Islands.
Tomlinson (1986) recognized three species of Xylocarpus (Meliaceae) viz., Xylocarpus granatum, Xylocarpus meckongensis (=Xylocarpus gangeticus), and Xylocarpus moluccensis on the basis of habitat, trunk, fruit size, and root elaboration, of which only the former two are regarded as true mangrove species. Parkinson (1934), too, recognized three different species of Xylocarpus and highlighted the problems in differentiating X. granatum from X. meckongensis. However, there is a great deal of ambiguity in Xylocarpus species due to deficient taxonomic descriptions (Tomlinson 1986, Jayatissa et al. 2002). Two species of Xylocarpus viz., X. granatum and X. moluccensis (=X. gangeticus and X. meckongensis), were found in our surveys. Of these, the former was commonly distributed, whereas the latter was rare and found only at Baratang of Middle Andaman. However, X. moluccensis was also observed at Havelock (South Andaman) and at Karmatang of Mayabunder (North Andaman) by the investigating team during a pre-tsunami period (2003).
One species representing Myrsinaceae, Aegiceras corniculatum, was commonly encountered in our surveys.
Three species of the genus Bruguiera viz., Bruguiera cylindrica, Bruguiera gymnorrhiza, and Bruguiera parviflora were reported in our surveys, among which B. gymnorrhiza was found to be the most common in the middle and upper intertidal zones. Singh et al. (1987b) were the first to record Bruguiera sexangula in the Andaman and Nicobar Islands, and it was reported in all subsequent studies. However, it was not encountered during our surveys across the Islands. It is important to note that the Bruguiera species reported as B. sexangula by Dam Roy et al. (2009) at Burmanallah (South Andaman) was confirmed to be a variegated form of B. gymnorrhiza during our surveys. As B. gymnorrhiza and B. sexangula are morphologically similar, the number of colleters (finger-like glandular structures inside the base of stipules) could be a useful diagnostic character (Sheue et al. 2005).
Although two species of Ceriops viz., Ceriops tagal and Ceriops decandra have previously been reported from the Islands, only the former was found in the present study. Dagar et al. (1991) reported C. decandra from Chidiyatapu (South Andaman) two decades ago as a very rare species. Dagar and Singh (1999), Debnath (2004), and Yao et al. (2011) have also reported C. decandra from the Andaman and Nicobar Islands but without giving a description or locality data.
Of the four previously reported Rhizophora species, only three viz., Rhizophora apiculata, Rhizophora mucronata, and Rhizophora stylosa were found in our studies. Identification of mangrove hybrids is problematic (Duke et al. 2002). Singh et al. (1987b) was the first to record Rhizophora lamarkii (a sterile hybrid between R. stylosa x R. apiculata) from Havelock amidst R. apiculata and R. mucronata stands (Table 2). It was subsequently added to the mangrove flora by several authors (Table 3). However, our present study revealed the presence of Rhizophora hybrids at Havelock (South Andaman) and Kimious Bay (Car Nicobar), which stresses the importance of periodic surveys in mangrove stands where parental species of Rhizophora taxa typically co-occur (Duke 2007) as there are major taxonomic problems with Rhizophora (Duke et al. 1998) despite its ubiquitous occurrence throughout the tropical world (Tomlinson 1986, Duke et al. 2002). Further, it is important to note that hybrids of Rhizophora were previously reported by Ragavan et al. (2011) from Havelock.
The inclusion of Kandelia candel in the mangrove flora of the Islands by several authors (Table 3) could not be explained as it has not been observed since Brandis (1907).
Scyphiphora hydrophyllaceae was common in the Andaman Islands, but it could not be located in the Nicobar Islands in our surveys.
Sterculiaceae was represented by Heritiera littoralis along the landward edge of mangrove forests, often with terrestrial plants (for example, at Burmanallah and Sippighat of South Andaman, Kalipur of North Andaman) in the Islands.
Similarity in floristic composition between survey regions
The Sørensen similarity indices were low (CS<0.5; Table 5), indicating distant floristic affinities between the survey regions. However, given the limited geographical area, the low similarity index values between the Andaman and the Nicobar Island groups (≤0.4) suggests the need for periodical surveys, as mangrove biodiversity is usually homogenous in regions where there are no impassable dispersal barriers (Spalding et al. 2007, Nguyen et al. 2013).
Until recently, the mangroves of the Andaman and Nicobar Islands had remained intact and untouched by humans, but this situation is now changing quite rapidly (Spalding et al. 2010), and the current incomplete knowledge of exact species composition and lack of implementation of adaption-centric conservation strategies could be damaging. The present study was designed to review the existing literature and to provide an updated checklist of the true mangrove species of the Islands. At present, 25 true mangrove species distributed among 10 families and 14 genera could be identified vis-à-vis 17 to 36 species reported in earlier studies. While the discernible changes in species numbers in earlier studies could be attributed to numerous reasons, the disappearance of a few rare species reported in earlier studies remains unexplained, which emphasizes our limited knowledge of their location and the urgent need for conservation.
Five species viz., Acanthus volubilis, Sonneratia caseolaris, Sonneratia griffithi, Sonneratia ovata, and Xylocarpus mekongensis are found to be rare in the Islands with restricted distribution. As the rural population in the Islands has traditionally used mangroves as a source of wood and non-wood forest products, they should be informed about rare species and the importance of their conservation.
The shortfalls of the present study are the inadequate floristic exploration of western coasts of the Islands and of some of the crocodile-infested areas (for example, Dugong creek of Little Andaman, Campbell Bay of Great Nicobar), which should be addressed in the future for precise documentation of species diversity. Though these Islands are part of the Indian subcontinent, the mangrove flora present here exhibits greater affinities to the Southeast Asian flora (for example, Sonneratia ovata) due to their geographical proximity. Hence, a detailed study of the diversity and distribution of mangroves in the Islands would not only improve our understanding of phytogeography but may also lead to additions to the mangrove flora of India. Further, this study stresses the significance of regular updating of information on the extent and status of mangroves of the Andaman and Nicobar Islands for better management and conservation.
The authors thank Space Applications Centre, Ahmedabad, India, for funding the research project on Coastal Zone Studies, as a part of which this study was carried out.
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