Impact of Climate Change on the Offshore Nijhum Island of Bangladesh

The study aims to assess the diversity of flora and fauna in the offshore Nijum Island of Bangladesh and examine the probable impact of climate change. Both primary and secondary data were used to identify the plants and animals and to predict future changes in the ecosystem. It was found that Keora (Sonneratia apetala) is the main tree species followed by Kakra (Bruguiera gymnorhiza), Gewa (Excoecaria agallocha), Hargoza (Acanthus ilicifolius), Khalisha (Aegiceras corniculatum) and Bain (Avicennia Officinalis). There are several patches of Hogla (Typha elephantina) throughout the area. It was found that tropical cyclone Aila caused mass devastation, altered vegetation composition and facilitated biological invasion. The climatic factors include sedimentations, inundation stress and increased salinity at landward zones. The study recommended several adaptive measures to overcome the challenges.

section_content: Nijhum is an offshore island in the Bay of Bengal, located in the extreme south of Hatia island separated by the Hatia channel. It is a scenic treasure trove with having 20 km long sandy and grassy beach. Accredited on the estuarine Meghna River and the Bay of Bengal, Nijhum is a virgin island constituted of intertidal mudflats and sandflats. The island is dissected by small creeks or canals and its central part is under cultivation and human habitation. It is a cluster of several small accreditations mainly Char Osman, Char Kamla, Char Muri and Ballar Char. It came under the human settlement in 1969 and the Forest Department began aforestation in 1972 with mangrove species. Now it has a large deep green forest with native and early successional tree species. This island was declared a National Park in 2001 and now is one of the attractive tourist spots for its rich faunal and floral diversity. This island could be the next prime tourist spot after St. 

Martin's Island. The biodiversity of this offshore island is still unexplored. The study aimed to examine the existing flora and fauna on this island. 

The environmental parameters with the direct influences on this island in terms of global climate change are sea-level rise, natural calamities like cyclones, and rise in temperature and salinity. Bangladesh faces unpredictable climate change impacts for a long that is subject to multiple stressors originating from a rapidly changing environment (Rahman 2020, Rahman 2021a, Rahman 2022c, Rahman 2023 ) and social dynamics (Mitchell et al. 2015, Rahman et al. 2009, Rahman 2021b, Rahman 2022a, b). The study also predicted the upcoming impact of climate change on the ecosystem.
section_name: Introduction

section_content: Both primary and secondary data were used for this study. The secondary data were collected from Forest Department. Local people were personally interviewed to identify the flora and fauna. A focus group discussion was done incorporating the respondents from diverse stakeholders. A total of ten key informants were interviewed to understand the future projections and mitigation strategy. Content analysis was done to analyze the data.
section_name: Methodology

section_content: The most important type of tree planted on the island is Keora (Sonneratia apetala) also known as Kerfa locally, which has fast-growing roots holding the sandy soil. Kakra (Bruguiera gymnorhiza), Gewa (Excoecaria agallocha), Hargoza (Acanthus ilicifolius), Khalisha (Aegiceras corniculatum) and Bain (Avicennia Officinalis) are the co-dominant species of this forest. There are several patches of Hogla (Typha elephantina) throughout the area. 

The main attraction in this successional mangrove forest is the herd of about 5000 spotted deer. The natural beauty has been enhanced by monkeys, wild boar, wild buffaloes, fishing cats, snakes, turtles, tortoises, Bengal monitor, black lizard, yellow monitor, oriental small-clawed otter, clawless otter and a huge number of migrated winter birds. Tidal mudflats are very important habitats for water birds. 

Oysters of various natures and snails can be easily seen on this island. The water bodies are the ideal habitat for Hilsha fish, Zebra fish and Hamilton fish (Baila fish in the local language). 

There is a large breeding colony of a black-crowned night heron, pond heron, grey heron, purple heron, cattle egret, little egret, lesser whistling-duck, bar-headed goose, cotton pygmy-goose, common shelduck, ruddy shelduck, tufted duck, water cock, and a wide variety of shorebirds, ring-billed gull, herring gull, noisy gull, sea terns, hawks, swallows, falcons, small cranes, local nightingales and king storks.
section_name: Flora and fauna

section_content: The respondents opined that the species composition, natural regeneration, species richness, and vertical and horizontal structure of this successional mangrove forest will undergo major changes under the predicted climate impacts. The sea level rise in the Bay of Bengal is faster than in other parts of the world. Sea level rise will cause a major threat to this successional mangrove ecosystem through sediment erosion, inundation stress and increased salinity are landward. As the sea level rises, the existing concentration of salinity and the distribution of freshwater in mangrove areas will be changed. The mangrove ecosystem will respond by changing productivity, canopy closure, tree coverage and species diversity, or by migrating. Sea level rise will bring drastic changes in the livelihoods and socioeconomic conditions of the inhabitants of these areas. 

Their valuable arable land will likely be lost. Even a limited rise in sea level will seriously affect the people through loss of land; accelerated erosion along the coasts and in river mouths; increased salinity, changes in the physical characteristics of tidal rivers and increased vulnerability to flooding. Communities living on this island will be climatic refugees increasing pressure on the mainland. 

The frequency of occurring cyclones increased by 26% over the past 120 years in the Bay of Bengal, which may be increased further with the intensification of El Nino in the upcoming days. Aila caused mass devastation on this island with twothree metres high surges sweeping over the whole area. A tidal surge of 15-20 feet inundated Nijhum Island during Sidr. These cyclones uproot, topple stems, break off trunks and defoliate the canopy. Sediments carried by storm surges are deposited on the forest floor as the surge recedes, causing plant mortality by interfering with root and soil gas exchange, leading to the eventual death of the plants. 

Storm surges weaken the potentiality of natural regeneration by reducing the viability of seeds, seedling germination and seedling recruitment. 

Invasive plant species like lantana can rapidly colonize disturbed areas and causes slower-growing of native plant species. The cyclone damages or alters the structural diversity and spatial pattern of forests. The density of mortality (>5 cm diameter at breast height) ranges from 14-100% (depending on the intensity) and averages 47. 7%. The reductions in the total basal area range from 9-100%. 

Mortality increases by 9% during post-cyclone 7-18 months. Inter-specific differences in susceptibility to wind damage appear to be a primary factor contributing to spatial patterns in mortality (Sherman 2001). 

With the increase of salinity, the tree mortality rate will be accelerated as the production of new leaves, leaf longevity and the leaf area, net photosynthesis rate, stomata conductance and transpiration rate of leaves --all will be reduced. 

The deer, often in groups will come to the nearby locality by swimming rivers and canals to quench their thirst with sweet water. Many of the deer will die of drowning or being caught by crocodiles and the people, or even bitten by dogs. 

The breeding habitat of fishes like Hilsha and other crustaceans will be destroyed with the intrusion of salinity. They lay their eggs and stay up to the juvenile stage in the freshwater. The leaves stem, and roots of mangrove vegetation provide a vital shelter for predators and nourishment for young fish, shrimps, and crabs. 

Without this environment, only a handful would survive. Mangrove trees, a crucial component, provide shelter and nutrients to their ecosystems. They provide habitats to young fish, crabs molluscs. Hundreds of migratory bird species nest in mangrove forests. 

Animals inhabit mangrove forests. The mangrove trees provide not only support to countless food webs; they are also indirectly responsible for the survival of the most primary planktonic and epiphytic algal food chains, which in turn provide carbon for the mangrove tree. Salinity is one of the most important factors in mangrove forest growth and distribution. High salinity diminishes the number of species and their size and only a few species can exist and grow (Qiu & Qian1997). 

The mangrove ecosystem of Nijhum Island is a sustainable resource that provides a huge number of people with food, tannins, fuel wood, timber, medicines and other ethnobotanical values. Mangrove offers protection of property and life from storms and coastal erosion. Sea level rise induced by global warming could alter substantially the status of mangrove forests, with serious consequences for coastal protection and resource management of this island.
section_name: Responses of biodiversity to climatic stressors

section_content: Adaptive management can be effective to overcome the problems raised by climate change. Adaptation is often a traumatic process triggered by disaster rather than a gradual process of adjustment. However, the pragmatism of the people of Electronic copy available at: https://ssrn. com/abstract=4408979

Bangladesh in adapting to difficulties in the past with limited resources should prove of great value in the identification of appropriate adaptive responses. The nexus of pragmatism, education and community participation can provide an excellent base for efforts. The following measures could be taken to overcome the challenges: *Understanding the intrinsic links between climate change and impact; * Strong commitment to sustainable development at all levels of society; pushing forward research on climate change and preventive measures;
section_name: Adaptive measures