www.cafetinnova.org Indexed in Directory of Open Access Journals (DOAJ) USA ISSN 0976-1519, Volume 06, No. 01 March 2015, P.P. 01-06 Degradation and Deterioration of Mangrove Biodiversity at Indian Sundarbans: A Case Study from Jharkhali Island, WB, India ABHIROOP CHOWDHURY AND SUBODH KUMAR MAITI Department of Chemical Engineering, National Institute of Technology, Karnataka, Surathkal, Mangalore, India. Email: abhiroop.chowdhury@gmail.com Abstract: Mangroves are halophytic plants that fringe the estuaries and coasts of tropical and sub-tropical region of Earth. Sundarban, located in the southern reaches of Ganga-Brahmaputra-Meghna delta is the world s largest contiguous mangrove forest. It is designated as an UNESCO world heritage site and is shared by Bangladesh (60%) and India (40%). But this conserved ecosystem is under serious threat due to natural disasters and anthropogenic stress like land conversion for aquaculture and paddy cultivation. True mangroves and Rhizophoraceae family members like Heritiera fomes (Malvaceae) and Kandelia candel (Rhizophoraceae) are getting excluded from Indian part of sundarbans, by stress tolerant mangroves namely Avicennia marina, A. alba, A. officinalis and associated mangrove flora, Phoenix padulosa (Arecaceae). The study area is located in central part of Indian sundarbans (Jharkhali Island) that was affected by anthropogenic stress and had borne the brunt of numerous natural disasters like AILA at 2009. The main objective of the work is to understand the changes in mangrove biodiversity over a period of four years and to find out the differences in species diversity, species dominance and changes in succession pattern over the years. The study reveals a reduction of rhizophoraceae family members from 41% in 2008 to 14.2% in 2011. It is observed that the lowest abundance of Rhizophoraceae at 2009 and a possible reason is the intrusion of salt water due to AILA at May, 2009. The excessive salt may be responsible for reduction of biodiversity and abundance of rhizophoraceae members to 11% at 2009. The consecutive years have seen the proliferation of invasive species like Derris trifoliata (Fabaceae) and salt resistant Avicennia sp. Environmental stress are known to encourage the proliferation of invasive species, which is evident from raise in abundance of D. trifoliata from 2% to 11% between 2008 and 2009. Avicennia sp have a recorded increase of 6% in 2008 to 18% in 2011, indicating a possible raise in soil and water salinity over the years. Increase in mangrove associates like Phoenix padulosa is also observed from 2% in 2008 to 8% in 2011, also indicating raise in salinity levels. Keywords: Mangroves, Sundarbans, Biodiversity, Normalized Shannon s Index, Simpsons Index of Dominance 1. Introduction "Mangrove" is an ecological term referring to a taxonomically diverse floral assemblage, which forms the dominant plant communities in tidal, saline wetlands along sheltered tropical and subtropical coasts [1] and have adaptations to propagate and survive in saline and anoxic soil conditions. Mangroves act as a fragile link between marine and fresh water ecosystems, pollution sink and source of nutrient flux into marine ecosystem. It also renders ecological services by sequestering maximum amount of CO2 than any other terrestrial which accounts to 1,023 Mg carbon per hectare [2] Apart from that coastal and estuarine mangrove are responsible for stabilizing soil particles hence arrest soil erosion, and can also effectively act as a first line of defense against natural disasters like storm surges and tropical cyclones. Mangrove ecosystem is facing serious anthropogenic exploitation along with changes in geological and geophysical parameters [3]. Land conversation for agriculture, aquaculture is responsible for destruction and degradation of mangrove habitats [3]. Recent assessments on extent of mangroves worldwide suggests that between 1990 and 2010 there is a reduction of 3% of mangroves cover throughout the world and reasons are primarily land conversions for coastal development, rice production and aqua cultural projects [4] However during 1980-2005, the aerial extent of mangrove forest loss is 30-50%, as a result of coastal development, aquaculture expansion, and over harvesting, which accounts for 3.6 million Ha [5]. Sundarbans is the largest contiguous mangrove ecosystem of the world, situated in the Ganga- Brahmaputra-Meghna delta covering an area of 9,630 Km2which is designated as the UNESCO World Heritage Site and Ramsar Wetland Site [6].Anthropogenic interventions like rapidly expanding aquaculture industry, illegal exploitation of forest resources, encroachment of forest areas and illegal poaching of wildlife is responsible for rapid loss of mangrove biodiversity of Sundarbans. Recent census in #03060101 Copyright 2015 CAFET-INNOVA TECHNICAL SOCIETY. All rights reserved.
2 Degradation and Deterioration of Mangrove Biodiversity at Indian Sundarbans: A Case Study from Jharkhali Island, WB, India 2000 estimated a totalpopulation of 4.2 million resides in Sundarban Biosphere Reserve in abject poverty [7]. People inhabitthe reclaimed area of the Sundarbans, but the saline water is unsuitable for both domestic and agricultural uses[7].natural threats like sea level arise due to global climate change is also aggravating the ordeal of this sensitive ecosystem [8]. This study mainly focus on changes in mangrove biodiversity of Jharkhali island in Indian Sundarbans to access the changes in biodiversity in natural mangrove patches from 2008-2011. 2. Materials and Methods 2.1. Study Area Study site is Jharkhali Island, which enjoys almost a central position in Indian Sundarban. Here the environmental parameters are optimum. Overlooking it is Herobhanga Reserve Forest in South-East and Sajnekhali Sanctuary in West. Three natural mangrove patches A, B and C) are present in that island which is elucidated in Fig 1. Figure 1: Quadrat Sample Sites Later remote sensing LISS III (23rd November, 2009) image was procured. The mangrove patches are also identified from the standard false color classified image produced from Near Infra-red, Red and Green bands using supervised classification (maximum likelihood method) in TNTmips 2013 environment, depicted in Fig 2. Figure 1: False colour classification showing Natural mangrove patches The color classification was based on the fact that green vegetation emits maximum in near infra-red band, which is sensed during satellite photography and can be isolated from LISS III imagery along with red, green and blue bands. This area is under acute anthropogenic stress namely land conversion for agriculture and aquaculture. 2.2. Study Period The study was conducted after monsoon during the period of September October from 2008 to 2011. Monsoon rains helps to rejuvenate the vegetation by decreasing surface water and soil salinity, so sampling time is selected to be after Monsoon. 2.3. Methodology Random 10m x 10 m quadrat plots [6, 9] are selected from three natural mangrove patches present in Jharkhali Island named as A, B, and C. 12 random quadrats are selected in each patch and their average value is considered for analysis. Diversity, Abundance is calculated by using Simpson s Index of Dominance, Simpson s index of Diversity, Shannon s Index of Diversity and Normalized Shannon s Index, to access the changes in biodiversity and community structure over the period of four years. 3. Result and Discussions The commonly used metrics to quantify landscape composition are Shannon s index that emphasize the richness component of diversity, and Simpson s index, highlight the evenness component [10]. These indices show considerable variation in response to changes in landscape richness and evenness [10]. The study shows a variation of diversity of mangrove flora at Jharkhali from 2008-2011. The percentage of
ABHIROOP CHOWDHURY AND SUBODH KUMAR MAITI 3 the different species and their relative abundance is given in table 1 Table 1: Percentage cover of Mangrove species, along with their Family, at Jharkhali island from 2008-2011 Sl Name of Percentage Cover Family No. Flora 2008 2009 2010 2011 1 Acanthus illicifolius Acanthaceae 6.736 12.584 7.44 7.988 2 Aegialitis Plumbagenace 8.549 12.808 8.155 6.912 rotundiflia ae 3 Aegiceras corniculatum Myrsinaceae 2.85 5.168 5.436 6.298 4 Aglaia cuculata Meliaceae 1.425 0.225 0.429 0.460 5 Avicennia alba Acanthaceae 1.684 3.820 4.721 5.837 6 Avicennia marina Acanthaceae 4.145 6.742 7.725 9.062 7 Avicennia officinalis Acanthaceae 0.907 4.27 2.861 3.686 8 Brownlowia tersa Malvaceae 2.591 0.225 0.429 0.153 9 Bruguiera Rhizophorace cylindrica ae 5.44 1.348 2.861 0.768 10 B. Rhizophorace 7.513 0.899 5.150 5.376 gymnorrhiza ae 11 Ceriops Rhizophorace 8.679 5.169 5.722 3.840 decandra ae 12 Ceriops tagal Rhizophorace 9.456 4.494 5.150 4.301 ae 13 Clerodendron inerme Lamiaceae 4.663 4.944 5.865 5.837 14 Derris trifoliate Fabaceae 2.202 11.91 8.87 7.834 15 Excoecaria agallocha Euphorbiaceae 6.088 6.292 3.577 3.225 16 Heliotropium Boraginaceae 5.181 3.82 10.014 12.596 currasavicum 17 Heritiera fomes Malvaceae 1.295 0 0 0 18 Kandelia Rhizophorace 5.829 candel ae 0 0 0 19 Nypa fruticans Arecaceae 1.429 0 0 0 20 Phoenix Arecaceae 3.756 11.01 7.153 8.449 21 22 23 24 25 padulosa Rhizophora mucronata Sonneratia apetala Sonneratia caseolaris Xylocarpus granatum Xylocarpus mekongensis Rhizophorace ae 4.534 0 0.286 0.154 Lytharaceae 1.166 0.674 2.575 2.611 Lytharaceae 2.073 1.124 2.718 2.611 Meliaceae 0.907 1.348 1.287 0.922 Meliaceae 0.907 1.124 1.573 1.075 The different biodiversity indices and their values from 2008-2011 is given in table 2. Table 2: Biodiversity Indices of the Island from 2008 to 2011. Sl. No. 1 2 3 4 Biodiversity Indices Year Simpson's Index of Dominance Simpson's Index of Diversity Shannon's diversity index Normalized Shannon's Index 2008 2009 2010 2011 0.054 0.076 0.059 0.065 0.946 0.924 0.941 0.935 2.994 2.675 2.867 2.592 0.93 0.879 0.928 0.865 The investigation shows a drastic decrease of Rhizophoraceae family members, considered to be true mangroves, from 2008 to 2009. The reason can be the influx of salt water during the advent of cyclone AILA on May, 2009. The increase of invasive species like Derris trifoliate [11] can also be in response to the stress imposed on the ecosystem due to the natural disaster. Derris sp have high salinity tolerance [12], so increase in this undergrowth species reflects an increase in salinity due to the influx of salt water during the storm surge. The changes in biodiversity pattern at Jharkhali island from 2008-2011is depicted in Fig 3. Figure 3: Changes in mangrove biodiversity in Jharkhali Island from 2008-2011. Later in 2010 and 2011, there is a reduction in invasive species and comparative increase in true mangrove (Rhizophoraceae) members, but the ecosystem fails to achieve the previous condition. Mangroves are indeed very fragile ecosystem [13], and this fragility is also evident not only from the changes in floral composition but also for exclusion of critically endangered mangrove species namely Kandelia candel (Rhizophoraceae), Heretiera fomes (Malvaceae) and Nypa fruticans (Arecaceae), from the ecosystem after AILA. There is a steady increase of salt tolerant species of Avicinnia sp [14], in the ecosystem (6.7% in 2008, 14% in 2009, 15% in 2010 and 18% in 2011), indicating probably an increase in surface water and soil salinity of the region. Salinity raise is also responsible for exclusion or
4 Degradation and Deterioration of Mangrove Biodiversity at Indian Sundarbans: A Case Study from Jharkhali Island, WB, India decrease in diversity, abundance of many true mangrove species and rhizophoraceae members like H. fomes and Ceriops decandradue to increased susceptibility of the plants to top dying diseases [15]. There is a increased dominance and decreasing diversity trend from 2008 to 2011, elucidated by Simpson s Index of Dominance (SD) and Simpsons Index of Diversity (SDi), Shannon-Weiver Index (SWI) and Normalized Shannon s Index (SN) in Fig 4, Fig 5, Fig 6 and Fig 7 respectively. Figure 2: Simpson's Index of Dominance Figure 5: Simpson's Index of Diversity Figure 3: Shannon-Weiver Index Figure 4: Normalized Shannon's Index As is evident from Fig4-7, there is a progressive increase in Simpson s Dominance indices from 2008-2011, with the highest value is recorded in 2009. Simpson s Index of Dominance is a heterogeneity index that is inversely proportional to the heterogeneity as encountered in the ecosystem and Simpson s Index of Diversity is directly proportional to heterogeneity formulated by Gini, to avoid the inverse relationship in the former [16]. So progressive increase in dominance index and decrease of diversity index in 2009 conclusively indicate a decrease in heterogeneity in the mangrove ecosystem of Jharkhali Island. The reason can again be the influx of salt water during AILA, 2009 which facilitate the proliferation of only salinity tolerant floral assemblages. But the ecosystem progressively recovered from the stress in the later years showing the resilience of mangrove ecosystem. As dominance increases the Simpson s Indices for Diversity decreases showing that dominance of few species is decreasing the species diversity of the ecosystem. Normalised Shannon s Index gives weight age to the rarer species of the ecosystem and is an Information- Statistic Index with moderate sensitivity to sample size, depicting the overall health of the ecosystem [17]. Shannon-Weiver Index equates heterogeneity with the uncertainty regarding the selection of an individual of a species at random from a population [17, 18].The reduction of SWI from 2008 to 2009 is due to the stress imparted by the natural disaster, but it recovers at 2010, the anomaly is again a decrease of biodiversity, heterogeneity and increase in dominance pattern in 2011. Anthropogenic pressure due to increased ecotourism, development of aquaculture farms and natural causes of erosion of banks and raise in salinity may be responsible for this. The condition can easily be understood from two pictures of same area, taken in 2008 and 2011 in Fig 8 and Fig 9.
ABHIROOP CHOWDHURY AND SUBODH KUMAR MAITI 5 Figure 8: Mangroves at Jharkhali Jetty in 2008. Figure 5: Mangrove deforestation near Jharkhali jetty on 2011. The deforestation of the patch depicted in Fig 8, was done by Forest department for setting up a Propagation Center for endangered mangroves, Herobhanga beat, Matla range, South 24 Parganas, West Bengal. The question remains that weather this sort of conservation strategies developed at expense of existing natural patch of mangroves, is beneficial or detrimental. The deforested part of the soil is left with saline blank which makes the land even unsuitable for propagation of even the salt tolerant halophytes. Deforestation of mangrove is rampant from 1986 to 2009 in Jharkhali, which is continuing till date. This is evident from the statistics and remote sensing data, which shows that the forest cover in Jharkhali at 2009 is 5.02 Km2, whereas the figures in 1986 reflects an area of 17.09 km2 and there is an increase in settlement area from 29.1 km2 to 63.8 km2 with effective deforestation of 12.4 km2 in 2009[19]. 4. Conclusion Mangroves are unique ecosystem which acts as the first barrier against any coastal natural disasters, can resist pollution load, and are ecologically resilient. But we should understand that this ecosystem is also very fragile. Increasing anthropogenic pressure coupled with natural causes like raise in salinity and changes in geological parameters are putting mangroves under heavy pressure. The study is focused on Jharkhali Island, which has also undergone transformation to accommodate ever raising population pressure in expense of the mangroves. This scenario is a replication of similar situations in other parts of sundarbans. 5. Recommendations Management plans are in serious need that put emphasis on in-situ conservation of mangrove ecosystem outside the Tiger Reserve area, in the buffer zone of Sundarban Biosphere Reserve. The mangrove biodiversity is dwindling, and true mangrove varieties are getting excluded from the ecosystem, which need to be conserved 6. Acknowledgements I am indebted to Prof. Harendranath Bhattacharrya, of Department of Applied Geology and Environmental System Management, for giving me the opportunity to work at Jharkhali, during 2008. I am thankful to Prof. Raman Sukumar, chairman of department of Ecological Sciences, Indian Institute of Science, Bangalore, to permit and motivate me to work in Sundarban mangroves, as a Project Assistant during 2009-2010. This work was not possible without the guidance of Prof. Pranabes Sanyal, ex-field director, Sundarban Biosphere reserve, while working under him in World Bank funded non-technical project in Sundarban delta during 2010-2011. References [1] F. Blasco, P. Saenger and E. Janodet, Mangroves as indicators of coastal change. Catena, Volume. 27, PP. 167-178, 1996. http://www.sciencedirect.com/science/article/pii/03 41816296000136 [2] C.D. Donato, J. B. Kauffman, D. Murdiarso, S. Kurnianto, M. Stidham and M. Kanninen, Mangroves among the most carbon-rich forests in the tropics, Nature Geoscience, Volume 4, PP 293-297, 2011. DOI: 10.1038/NGEO1123. [3] T. G. Jagtap, V. S. Chavan and A. G. Untawale, Mangrove Ecosystems of India: A Need for Protection, AMBIO. Volume. 22. Issue. 4, PP. 252-254, 1993. [4] UNEP, Keeping Track of Our Changing Environment: From Rio to Rio+20 (1992-2012).
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