Survival of mangroves in response to ambient environment.pdf

Parana Journal of Science and Education (PJSE) – v.4, n.3, (34-41) April 18, 2018 https://sites.google.com/site/pjsciencea ISSN: 2447-6153 34 Spatial variation of mangrove diversity in Indian Sundarbans: A proxy to evaluate mangrove health to establish the concept of survival for the fittest ♯ ◊ † ¥ ♀ Arpita Saha1 , Suresh Kumar Agarwal2 , Prosenjit Pramanick1 , Sitangshu Roy1 , Sufia Zaman1 ☼ and Abhijit Mitra3 1 Department of Oceanography, Techno India University, Salt Lake Campus, Sector V, Kolkata 700091, West Bengal, India. 2 Pragyan International University, Ranchi, India; City Office: 2F Maurya Center, 48, Gariahat Road, Kolkata, 700019, West Bengal, India. 3 Department of Marine Science, University of Calcutta, 35 B. C Road, Kolkata, 700019, West Bengal, India. Abstract A study was conducted in ten selected stations of Indian Sundarbans to assess the health of mangroves using ecological indices as proxy. The stations are distributed in the western and central sectors of the deltaic complex. It is observed that the values of Shannon-Weiner Species Diversity Index are relatively high in the stations of western sector compared to those in the central sector. Mangroves, being brackish water loving species survived with a better biomass in the western Indian Sundarbans due to their exposure to low saline environment in the Hooghly estuarine mudflats. This is mainly because of the Farakka discharge in the Hooghly estuary, which cannot reach the Matla estuary in central Indian Sundarbans on account of the Bidyadhari siltation. Keywords: Indian Sundarbans, Mangroves, Shannon-Weiner Species Diversity Index. ◊ E-mail: [email protected] † E-mail: [email protected] ¥ E-mail: [email protected] ♀ E-mail: [email protected] ♯ E-mail: [email protected] ☼ E-mail: [email protected] Parana Journal of Science and Education (PJSE) – v.4, n.3, (34-41) April 18, 2018 https://sites.google.com/site/pjsciencea ISSN: 2447-6153 35 1. Introduction Mangroves are a group of salt tolerant woody plants with unique water use efficiency in brackish and saline environment. They provide a wide range of ecosystem services in the sector of natural disaster management, bioremediation, livelihood generation etc. [1-4]. The ecosystem services of mangroves can be divided into three broad headings namely:  Economic services (like generation of employment opportunities of tourism, timber production, honey and wax production, fisheries, etc.;  Socio-cultural services, which includes opportunities of exchanging ideas and promoting cultural values and beliefs as seen in case of Bonobibi festival in Sundarbans;  Environmental services like bioremediation, shelter for various flora and fauna, nutrient retention, carbon sequestration etc. Considering the wide range of benefits provided by mangroves, it is essential to know the health of the mangroves and the threats operating on this valuable reservoir of the planet. For this we have used three important ecological indices as proxies namely  Shannon-Weiner Species Diversity Index;  Index of Dominance;  Evenness Index This will help to prepare a composite health index map in the study area and identify regions facing high risk in terms of mangrove health. Because of the threats (of varied nature) operating on mangrove ecosystem many species has become endangered/ extinct [1]. The Indian Sundarbans sustains some 34 species of true mangroves [1, 5-6], which are not uniformly distributed throughout the entire Sundarban Biosphare Reserve (SBR) of 9630 km2. It has been reported that fresh water loving true mangroves floral species (like Sonneratia apetala, Heritiera fomes, Nypa fruticans) are dominant in the western and eastern sectors of Indian Sundarbans, whereas Rhizophora spp. are commonly found in the central sector. Salinity seems to be a key player in such distribution process [7-11]. Apart from salinity fluctuation mangrove ecosystems are threatened globally due to various anthropogenic activities and global climate change. Mangrove is the only marginal ecosystem which shares the resources with adjoining ecosystems [12]. Adverse effects on mangroves could lead to serious consequences for the adjoining fragile and important ecosystems such as coral reefs and sea grass beds. Damage to mangroves affects the sediment budget and promotes the coastal erosion. Moreover, the ecological and socio economic values offered by the mangroves are innumerable, immeasurable and incomparable. On this background, the present paper aims to evaluate the spatial variation of mangrove health in the framework of Indian Sundarbans linking with the stress (in terms of types and magnitude) operating in the selected stations. 2. Materials and methods 2.1. Site selection The present study was conducted in the Indian Sundarbans mangroves forest during March 2016. This study was conducted in 10 selected stations (Table 1). While selecting the stations we focused on anthropogenic threats and salinity as these factors regulate the diversity and distribution of mangroves. Quadrates of 10 m × 10 m were laid randomly up to 500m from Low Tide Level (LTL) and data from each one were recorded from 15 such quadrates. The floral species were identified considering the manual of Naskar [13] and confirmed from Botanical Survey of India (BSI), Kolkata (India). 2.2. Enumeration of Community Indices Relative Abundance (RA) was estimated as per the expression. RA = Abundance of a particular species/ sum of the abundance of all species ×100. Three common ecological indices were used in this study to determine the health of the mangroves. These are Shannon-Weiner Species Diversity Index, Index of Dominance and Evenness Index. Parana Journal of Science and Education (PJSE) – v.4, n.3, (34-41) April 18, 2018 https://sites.google.com/site/pjsciencea ISSN: 2447-6153 36 Table 1: Sampling stations with salient features. Longitude & Latitude Station Site Description Harinbari (Stn. 1) 88004'22.88'' E 21046'53.07'' N Situated in the western region of Indian Sundarbans almost in the middle of the Sagar Island; receives the water of the Hooghly River. Chemaguri (Stn. 2) 88008'49.01'' E 21039'42.88'' N Situated on the south-eastern side of Sagar Island and receives the water of the Mooriganga River. Sagar South (Stn. 3) 88004' 0.51'' E 21037'49.90'' N Situated on the south-western part of the Sagar Island at the confluence of the River Hooghly and the Bay of Bengal. Anthropogenically stressed zone due to presence of passenger jetties, fishing activities and pilgrimage. Lothian island (Stn. 4) 88019'8.47'' E 21039'08.04'' N Situated east of Bakkhali island; a Wildlife sanctuary; faces the River Saptamukhi. Prentice island (Stn. 5) 88017'3.62'' E 21042'43.31'' N Situated north of Lothian island; receives the water of the Saptamukhi River. Canning (Stn. 6) 88041'04.43'' E 22019'03.20'' N Sajnekhali (Stn. 7) 88048'15.78'' E 22006'34.19'' N Situated in the central part of the Indian Sundarbans and faces the mighty River Matla, a tide-fed river. Due to presence of fish landing stations, passenger jetties and busy market, the area is anthropogenically stressed. A Wildlife Sanctuary and a part of Sundarban Tiger Reserve; adjacent to River Bidhya and Gomor. Tourism pressure is extremely high in this station particularly during postmonsoon. Chotomollakhali 88054'26.71'' E (Stn. 8) 22010'40.00'' N Situated in the upper portion of Central Indian Sundarban adjacent to Jhila forest; receives the water of Rangabelia and Korankhali Rivers. Satjelia (Stn. 9) 88052'49.51'' E 22005'17.86'' N Situated adjacent to River Duttar in the upper region of Central Indian Sundarban facing western part of the Jhilla forest Pakhiralaya (Stn. 10) 88048'29.00'' E 22007'07.23'' N Situated adjacent to River Gomor; opposite to Sajnekhali Wild Life Sanctuary. Source: Mitra, 2013 [1]. Ten (10) quadrates were considered for the present study and the average value of each species was considered for RA and diversity indices estimation.  Shannon-Weiner Species Diversity Index was calculated as: 𝑆 𝐻 = − ∑ 𝑃𝑖. log 𝑃𝑖 𝑖=1 Where, H = Shannon-Weiner Species Diversity Index; 𝑛𝑖 𝑁 ni = Number of individuals of ith species and 𝑃𝑖 = N= total number of individuals of all the species in the quadrate). Parana Journal of Science and Education (PJSE) – v.4, n.3, (34-41) April 18, 2018 https://sites.google.com/site/pjsciencea ISSN: 2447-6153 37  Index of Dominance was calculated as per the expression: 𝑛𝑖 Index of Dominance = ∑ ( ) 𝑁 2 Where, ni = Number of individuals of ith species and Where, Evenness Index = 𝐻 log 𝑆 H = Shannon-Weiner Species Diversity Index and S = No. of species. N = total number of individuals of all the species in the quadrate.  Evenness Index was calculated as per the expression: 3. Results Figure 1. Spatial variation of ecological indices of mangrove floral community. Source: [This study]. It is observed that station 7 has the highest diversity index value. This is primarily because this station is a wildlife sanctuary and hence is under strict conservation measures. The evenness index is also high in this station because of the uniformity in the distribution of species. The index of dominance is lowest in this station, which reflects highest magnitude of stress on the mangroves of this station (Fig. 1). The order of Shannon-Weiner Species Diversity Index is  Stn. 7 (2.5962) > Stn 4 (2.4301) > Stn. 10 (2.3719) > Stn. 5 (2.3465) > Stn. 8 (2.3355)  > Stn. 3 (2.3032) > Stn. 1 (2.2463) > Stn. 9 (2.0961) > Stn. 2 (2.0873) > Stn. 6 (2.0584). The order of Index of Dominance is  Stn. 2 (0.1574) > Stn. 6 (0.1565) > Stn. 5 (0.1543) > Stn. 9 (0.1455) > Stn. 1 (0.1402) > Stn. 3 (0.1313) > Stn. 8 (0.1306) > Stn. 10 (0.1273) > Stn. 4 (0.1259) > Stn. 7 (0.0937) and the order of Evenness Index is of the order  Stn. 1 (0.9756) > Stn. 7 (0.9587) > Stn. 10 (0.9546) > Stn. 9 (0.9540) > Stn. 2 (0.9500) > Stn. 8 (0.9399) > Stn. 6 (0.9369) > Stn. 3 (0.9269) > Stn. 4 (0.9208) > Stn. 5 (0.8891), (Fig. 1). Parana Journal of Science and Education (PJSE) – v.4, n.3, (34-41) April 18, 2018 https://sites.google.com/site/pjsciencea ISSN: 2447-6153 38 4. Discussion Mangroves are halophytes with ̴70 trees species that grow in saline tidal wetlands on tropical and subtropical coastlines, Table (2). Table 2: Overview of Taxonomic diversity of World Mangroves. Order Arecaceae Caryophyllales Ericales Lamiales Family Palmae Myrsinaceae Species World distribution Salinity tolerance Nypa fruticans WP* Low Phoenix paludosa IM Mid annulata WP High rotundifolia IM High Plumbaginaceae Aegialitis Aegiceras corniculatum WP Mid floridum IM NA Pellicieraceae Pelliciera rhizophorae Americas Low Acanthaceae Acanthus ebracteatus WP NA ilicifolius WP Low alba WP Mid bicolor W. America High germinans AEP High integra Australasia Mid marina IWP High officinalis WP High rumphiana WP Mid schaueriana E. America NA agallocha IWP Low indica IM NA cylindrica WP Low exaristata Australasia High gymnorrhiza IWP Mid hainesii WP NA parviflora WP Mid sexangula WP Low Avicenniaceae Malpighiales Genus Euphorbiaceae Rhizophoraceae Avicennia Excoecaria Bruguiera Parana Journal of Science and Education (PJSE) – v.4, n.3, (34-41) April 18, 2018 https://sites.google.com/site/pjsciencea ISSN: 2447-6153 39 australis Australasia High decandra WP Low tagal IWP Mid Kandelia candel IM Mid Rhizophora apiculata WP Mid mangle AEP High mucronata IWP Low racemosa AEP Low samoensis W. America Mid stylosa WP High fomes IM Low globosa IM NA littoralis IWP Mid erectus AEP Mid Laguncularia racemosa AEP Mid Lumnitzera littorea WP High racemosa IWP High alba IWP Mid apetala IM Low caseolaris WP Low griffithii IM NA lanceolata WP Low ovata WP NA Ceriops Malvaceae Myrtales Sterculiaceae Combretaceae Lythraceae Sapindales Heritiera Conocarpus Sonneratia Myrtaceae Osbornia octodonta WP High Meliaceae Xylocarpus granatum IWP Low mekongensis WP Mid Source: Mitra and Zaman, 2016 [4] Mangrove forests in India are found along the coastline of nine maritime states and four union territories. The overall cover of mangrove forest is estimated to be 4740 km2, of which 58% is found along the east coast (Bay of Bengal), 29% is distributed along the west coast (Arabian Sea) and the remaining 13% in the Andaman and Nicobar Islands [14]. Diversity of Indian Mangroves was extensively studied by many workers, but considerable disparities still exist, for instance, Blasco [15] reported 58 mangrove species in the Indian territories, while Rao [16] listed 60 species from 41 genera and 29 families. Earlier studied [17] reported 35 true mangrove species, 28 mangrove associates and 7 back mangals. Many researchers [18-21] reported 59 species from 41 genera and 29 families. Jagtap et al. [22] reported 50 mangrove species. In Indian Sundarbans some 34 true species of mangroves have been reported by several workers [1-6]. In the present study conducted during 2016, Parana Journal of Science and Education (PJSE) – v.4, n.3, (34-41) April 18, 2018 https://sites.google.com/site/pjsciencea ISSN: 2447-6153 40 15 species of mangroves were documented from the selected quadrates, which exhibit highly significant spatial variation. Species richness was highest at Lothian Island (Stn. 4) and Prentice Island (Stn.5). This may be attributed to the location of these stations in and around the Wild Life Sanctuary region which is highly protected and free of anthropogenic pressure. The health of the mangroves as represented by Shannon Weiner Species-Diversity Index, Index of Dominance and Evenness Index clearly reflects the adverse impact of anthropogenic/natural stress on the ecosystem. High values of ShannonWeiner Species Diversity Index and Evenness Index represent a better environment in the study area, which in the present case is restricted at Sajnekhali (Stn. 7), Lothian Island (Stn. 4), Pakhiralaya (Stn. 10) and Prentice Island (Stn. 5). It is expected that Evenness Index will also vary in the similar sequence as with Shannon Weiner Species Diversity Index, but the picture is different in the present study. This is because of the non-uniformity amongst the population in some of the stations. The Index of dominance is a proxy for stressful environment, which is high in stations like Chemaguri (Stn. 2), Canning (Stn. 6) etc. and lowest at Sajnekhali (Stn. 7). The high values of Index of Dominance at Chemaguri (Stn. 2) and Canning (Stn. 6) clearly confirm the high level of stress operating on these stations, which in reality is occurring due to fish landing activities, tourism pressure, and existence of shrimp culture farms and operation of passenger vessels at frequent intervals. The lowest value of Index of Dominance at Sajnekhali (Stn. 7) is an indication of minimum stress operating on the mangrove floral community in this station. The reason is the location of the station in the Reserve Forest (RF) zone and is therefore under high degree of protection. 5. Conclusions The entire spectrum of results from the present study leads us to conclude that conservation of mangroves coupled with high degree of protection can be the only road map to preserve the diversity of this unique vegetation or restore the ecological health of mangrove ecosystem. The environment of a group of species is fittest/congenial if the values of Shannon Weiner and evenness indices are highest and index of dominance is lowest. References [1] Mitra, A. In: Sensitivity of Mangrove Ecosystem to Changing Climate. Publisher Springer New Delhi Heidelberg New York Dordrecht London, 2013 edition; ISBN-10: 8132215087; ISBN-13: 978-8132215080 (2013). [2] Mitra, A. and Zaman, S. Carbon sequestration by Coastal Floral Community, India. Published by The Energy and Resources Institute (TERI) TERI Press. ISBN 978-81-7993551-4 (2014). [3] Mitra, A. and Zaman, S. 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