ReviewIndian Sundarbans mangrove forest considered endangered under Red List of Ecosystems, but there is cause for optimism
Graphical abstract
Introduction
Many of the world's ecosystems are experiencing severe and sustained decline in extent and condition, with intergovernmental bodies suggesting we are experiencing unprecedented environmental loss and deterioration (IPBES, 2019). Such rates of environmental loss are expected to have myriad impacts on biodiversity (Maxwell et al., 2016) and ecosystem service provision (Rounsevell et al., 2010), with subsequent impacts on human populations. Recently, tools have been developed to chart environmental health (Logan et al., in press) and assess the risk of ecosystem collapse. One tool that has gained traction internationally is the International Union for the Conservation of Nature (IUCN)'s Red List of Ecosystems (RLE) framework (Keith et al., 2013). Analogous to the IUCN Red List of Threatened Species, the RLE provides criteria to identify the risk of collapse of assessed ecosystems. While there are a number of challenges with adapting a species framework to broader ecosystems (e.g. Boitani et al., 2015), the RLE has been widely discussed theoretically (Keith et al., 2013; Murray et al., 2017) and used empirically to assess the condition of various ecosystems and locations (Marshall et al., 2018; Ferrer-Paris et al., 2019). Consequently, the RLE framework has been strongly promoted as assisting a shift from species-level to ecosystem-level conservation (Watson et al., 2020).
Conservation risk identification tools such as the RLE are particularly needed for large and dynamic ecosystems, such as the 4000 km2 Sundarbans mangrove forest in East India, that are at risk from a diverse range of threats. The Indian Sundarbans are part of a large deltaic complex that extends to mangrove forests in Bangladesh, and are a key biodiversity hotspot, home to iconic species including the Royal Bengal tiger (Panthera tigris), the fishing cat (Prionailurus viverrinus), the Ganges river dolphin (Platanista gangetica), the Irrawaddy river dolphin (Orcaella brevirostris), and the lesser known mangrove horseshoe crab (Carcinoscorpius rotundicauda) (Gopal and Chauhan, 2006; IUCN, 2013). Four million people rely on the Indian Sundarbans for ecosystem services (Ghosh et al., 2015); in particular, wild fisheries are the second biggest source of employment within the Sundarbans (Ghosh, 2012). Therefore, conservation of the Indian Sundarbans mangrove forest is critical for both wildlife and people.
The Sundarbans are a changing system under strong environmental and anthropogenic influence. Natural dynamics of accretion and erosion contribute to annual gains and losses in mangrove area (Hazra et al., 2002). Mangrove erosion between 1984 and 2018 in the Sundarbans totalled 136.77km2, with a progradation of 62.17km2 (Bhargava et al., 2020). The land has a long history of mangrove clearing. Populated since the 8th century, intensification of land use and mangrove clearing escalated from the late 19th century and continued throughout the 20th century (Ghosh et al., 2015). Since the 1970s, the declaration of the area as a World Heritage Site, along with other management and conservation tools, has helped to stabilise the mangrove area (Ghosh et al., 2015). However, as the local population continues to increase, and the impacts from agriculture, hydrological changes, illegal fishing including shrimp farming, and climate change intensify, there is a need to monitor and identify the status and drivers of change of this important mangrove system.
A holistic, standardised and quantitative risk assessment framework is essential for the Indian Sundarbans, because previous assessments have either generally considered single threats to the mangrove forest in isolation (e.g. salinity and erosion; Islam and Gnauck, 2009, Bera and Maiti, 2019), or have been qualitative in nature. For example, the Indian Sundarbans National Park was assessed using qualitative information to assign a conservation outlook status of ‘good with some concerns’ as part of the IUCN World Heritage Outlook (IUCN, 2017). However, quantitative tools are required to show transparency and set targets for future conservation interventions, and allow future changes in assessed variables to be rapidly incorporated into updated assessments when new data become available (Hill et al., 2016; McQuatters-Gollop et al., 2019). A holistic assessment is also required because the Indian Sundarbans face a range of threats that vary in their cause from the biophysical to the human, and originate both within and outside its borders. The RLE is a proven and transparent framework that has the ability to inform public policy (Alaniz et al., 2019; Bland et al., 2019), and can help the formulation of further conservation plans for the area.
This study represents the first quantitative and standardised environmental risk assessment for the Indian Sundarbans. The objectives of this study were to: (i) identify the defining ecological features of the ecosystem and review the key processes that threaten them, (ii) evaluate trends in key environmental and biotic variables relevant to the persistence of the ecosystem, (iii) assess the potential risk of ecosystem collapse, and (iv) evaluate the utility of the framework for assessing and managing coastal ecosystems.
Section snippets
Materials and methods
Classifying ecosystem boundaries is a key step and constraint of the RLE (Boitani et al., 2015). Assessments require detailed information on the target ecosystem, including its classification, spatial distribution, and descriptions of the abiotic and biotic environment, the ecosystem processes and the key threats acting upon the ecosystem (see Appendix A for the full assessment for the Indian Sundarbans).
Decline in distribution – criterion A
Mangrove extent changed from 2307 km2 (1968) to 1983 km2 (1989), 1926 km2 (2001), 1852 km2 (2014), and 1851 km2 (2016) (Fig. 3). The proportional and absolute rate of decline is 0.4% and 9.5 km2 year−1, respectively, representing a decrease in mangrove habitat of 20–21% over the last 50 years dependent on the method of calculation (proportional or absolute). Therefore, the ecosystem is assessed as Least Concern (<30% reduction) for sub-criterion A1. Linearly extrapolating 50 years into the
Conservation status of the Indian Sundarbans
The RLE framework allows the collation of current knowledge on the status of, and threats to, the mangrove ecosystem of the Sundarbans, India. We evaluate the utility of the framework for a dynamic coastal ecosystem. We show that long-term historic losses led to a status of Endangered with a plausible range between Vulnerable and Endangered for the ecosystem, and that continuing declines in juvenile fish populations are concerning. However, a significant slowing of past losses, limited areas of
Conclusions
Using a holistic, standardised and quantitative environment risk assessment framework, the Indian Sundarbans has been assessed as Endangered (with a plausible range between Vulnerable and Endangered), based primarily on substantial historical declines in extent. Despite this seemingly bleak outcome, there is cause for cautious optimism. Historically high rates of mangrove clearing have reduced, and management of the ecosystem, although under resourced, is laudable. However, the current
CRediT authorship contribution statement
RC, MS: Conceptualization; MS, AG, PB: Data curation; MS: Formal analysis, Visualization, Roles/Writing - original draft; MC: Data curation; All: Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Our assessment relies on the substantial efforts of many researchers who have helped to map mangrove forests and communities in the Sundarbans over several centuries, for which we are grateful. MS, MRC, RMP, MPT, CAB, MAH, EM, FA, and RMC were supported by The Global Wetlands Project, with support by a charitable organisation which neither seeks nor permits publicity for its efforts. MPT and RMC were supported by a Discovery Project from the Australian Research Council (DP180103124). AG
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