Mapping Long-Term Changes in Mangrove Species Composition and Distribution in the Sundarbans

Abstract

Abstract:The Sundarbans mangrove forest is an important resource for the people of the Ganges Delta. It plays an important role in the local as well as global ecosystem by absorbing carbon dioxide and other pollutants from air and water, offering protection to millions of people in the Ganges Delta against cyclone and water surges, stabilizing the shore line, trapping sediment and nutrients, purifying water, and providing services for human beings, such as fuel wood, medicine, food, and construction materials. However, this mangrove ecosystem is under threat, mainly due to climate change and anthropogenic factors. Anthropogenic and climate change-induced degradation, such as over-exploitation of timber and pollution, sea level rise, coastal erosion, increasing salinity, effects of increasing number of cyclones and higher levels of storm surges function as recurrent threats to mangroves in the Sundarbans. In this situation, regular and detailed information on mangrove species composition, their spatial distribution and the changes taking place over time is very important for a thorough understanding of mangrove biodiversity, and this information can also lead to the adoption of management practices designed for the maximum sustainable yield of the Sundarbans forest resources. We employed a maximum likelihood classifier technique to classify images recorded by the Landsat satellite series and used post classification comparison techniques to detect changes at the species level. The image classification resulted in overall accuracies of 72%, 83%, 79% and 89% for the images of 1977, 1989, 2000 and 2015, respectively. We identified five major mangrove species and detected changes over the 38-year (1977-2015) study period. During this period, both Heritiera fomes and Excoecaria agallocha decreased by 9.9%, while Ceriops decandra, Sonneratia apelatala, and Xylocarpus mekongensis increased by 12.9%, 380.4% and 57.3%, respectively.

Climate Variability and Mangrove Cover Dynamics at Species Level in the Sundarbans, Bangladesh

“…The Sundarbans is at the Ganges delta where the three mighty river systems: Ganges, Brahmaputra and Meghna converge [15,21,22], and is characterized by a complex pattern of tributaries [23]. The Sundarbans extends between 21°32′ to 22°40′N latitude and 88°05′ to 89°51′E longitude ( Figure 1) and covers an area of around 10,000 km 2 .…”

“…In this regard, remote sensing techniques could play an important and effective role in identifying and mapping mangrove species composition changes with an acceptable accuracy and it also has advantages over traditional methods [15,[27][28][29][30][31][32]. To identify mangrove forests, a number of satellite sensors have been used, including Landsat TM/ETM/OLI [15,33,34], SPOT [35,36], CBERS [37], SIR [36], ASTER [38], and IKONOS and Quick Bird [39]. According to Wang and co-authors [40], conventional satellite imagery such as Landsat and SPOT have been used for mapping mangrove species with mixed results.…”

“…The free availability of archived images enables the development of useful techniques in its use, with this in mind; Landsat imagery were used in this study for mangrove species classification. Even though it has limitations, such as coarse resolution, many researchers have used this imagery successfully in mangrove mapping at species level (for example [15,33,42] …”

“…A process called signature extension, suggested by Foody [43,44], was undertaken to use these samples, and this allowed us to use samples for unchanged areas for such purposes. The detailed methodology for this is given in Ghosh et al [15] Training samples of between 51 and 141 for each species were randomly and carefully selected, stratified by land cover class based on true representation and distribution. These sample points were used to create sample polygons of 3 × 3 or 5 × 5 pixels depending on the class homogeneity.…”

“…The pattern of change of the Sundarbans mangrove forest in Bangladesh is commonly associated with the change in species level composition of mangroves and seaward expansion in some parts of the forest [15]. Due to high population densities in its immediate vicinity, landward expansion of the Sundarbans are limited.…”