Climate change and reduced freshwater supply have put Sundarbans on the ‘endangered’ ecosystem list

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The Sundarbans mangrove ecosystem in India is evaluated as “endangered” by a global team of researchers using the International Union for Conservation of Nature’s Red List of Ecosystems framework. The clearing of mangroves dating back to the 1800s and declining fish populations were the main historical threats. Ongoing threats such as climate change and reduced freshwater supply may further imperil this ecosystem, they said.

Researchers from India, Australia, Singapore and the United Kingdom evaluated the Indian Sundarbans ecosystem using the Red List of Ecosystems framework developed by the International Union for the Conservation of Nature to assess an ecosystem’s risk of collapse. The framework is analogous to the Red List of Threatened Species.

“Despite this seemingly bleak outcome, there is cause for cautious optimism” because of the slow down in historically high rates of mangrove clearing, and recently stabilising tiger populations. The researchers write in a study that with the stabilisation in mangrove extent over the last decade or so, tiger numbers are slowly increasing, and analysis of mangrove condition highlights that only a small proportion of the forest is classified as degraded.

“There are still ongoing threats to the ecosystem, which can still cause widespread degradation if not properly managed,” study co-author Michael Sievers told Mongabay-India. “Reduced freshwater and sediment supply to the ecosystem is an ongoing concern.”

Temperature, salinity, freshwater flow, nutrients and tidal amplitude are key drivers of mangrove ecosystem productivity and diversity. Natural factors such as increased wave action due to storminess and human-induced changes such as the reduction in sediments due to upstream dams are the major drivers of permanent loss of land (and mangroves) to water in the Sundarbans.

Study co-author Anwesh Ghosh, who works on marine microbial ecology, said the loss of forest cover is an immediate issue. “I fear that fast-changing river dynamics, seawater intrusion, and rainfall patterns could cause rapid changes to the ecosystem,” said Ghosh. “We might lose the high species diversity that we see now.”

The Sundarbans in the Bay of Bengal are the most extensive continuous mangrove forests globally, straddling India and Bangladesh. The United Nations Educational, Scientific and Cultural Organization World Heritage Site and biodiversity hotspot harbours diverse species including the royal Bengal tigers. Four million people rely on the ecosystem services of the Sundarbans, with wild fisheries the second biggest employment source within the region.

Key threats

The evaluation describes key threats to the mangrove ecosystem ranging from agriculture and aquaculture, biological resource use (such as honey collection, hunting, and poaching of tigers), climate change (such as enhanced salinisation due to sea-level rise, the severity of cyclones and drought), natural system modifications (dams, barrages, embankments), pollution (including accidental spills during oil transportation), as well as residential and commercial development (continued mangrove clearing).

However, Sievers, of the School of Environment and Science, Griffith University, Australia, underlines that there has been a “huge amount of great management and conservation efforts” in the Sundarbans. “These significant efforts need to be celebrated. We don’t want an assessment of ‘Endangered’ to suggest these efforts are not being successful,” Sievers said.

Andy Large, field-based physical geographer, who was not associated with the Red List of Ecosystems evaluation said the cause for optimism, is “refreshing”, referring to among other things “recently stabilising tiger populations” and that the “mangrove extent has stabilised”.

“This would indicate that the authors are not claiming that the entire ecosystem is stabilising and that there remain various causes for serious concern such as for example, declining fish populations,” Large, director, UKRI Global Challenges Research Fund’s Living Deltas Research Hub, told Mongabay-India. “It is an important differentiation to make so as not to miscast the message of the paper.”

Ritesh Kumar, director, Wetlands International South Asia, not associated with the study, notes the discourse on water management seldom takes into account water requirements for coastal ecosystems resiliency – rendering ecosystems such as Sunderbans vulnerable.

“While it is okay to be cautiously optimistic, Sunderbans remains subject to threats related to freshwater flow decline and pollution,” Ritesh Kumar told Mongabay-India. “With Sunderbans designated as Ramsar Site, there is a need to put in place an integrated management plan for maintenance of ecological character of the site – and addressing drivers of adverse change.”

Sundarbans is home to diverse species such as this brown-winged kingfisher. Photo credit: Dibyendu Ash/Wikimedia Commons.

Long-term, hands-on, and novel monitoring programs such as the Sundarbans Biological Observatory Time Series are crucial, the researchers note in their evaluation. Funding and capacity issues remain crucial barriers to achieving monitoring management goals.

Co-author Anwesha Ghosh adds that the ecosystem shows signs of degradation, but also that it is trying to bounce back.

“There is a lack of extensive information at both spatial and temporal scales,” Ghosh told Mongabay-India. “From the information that is available at hand, such as the Sundarbans Biological Observatory Time Series datasets, we see the system is trying to recover or the extent of the damage is less than we had previously estimated.”

“We hope that continued effort and stopping malpractices should help the Sundarbans revive faster from the degradation it is now facing,” Ghosh said.

Through the Sundarbans Biological Observatory Time Series program, set up by Indian Institutes of Science Education and Research-Kolkata, researchers Punyasloke Bhadury and Anwesha Ghosh are looking at changes in microbial community structure and stability in the Sundarbans estuaries to track changes in freshwater flow that could have implications for the sustainability of fisheries.

Risk of ecosystem collapse

Sievers explained that the Red List of Ecosystems assessments involve collating various data sources across multiple indicators relevant to the ecosystem of interest; in this case, the Indian Sundarbans. “Indicators are variables that can be used to evaluate the risk of ecosystem collapse,” Sievers said. “For instance, we assessed the mangrove area, levels of mangrove tree degradation, and mangrove tiger populations, among others.”

In the Red List of Ecosystems assessment, the authors define the “collapse threshold” as when a population reaches zero or the extent of the cover of habitat also reaches the same (zero) extent. For example, when tiger abundance is one of the indicators of disruption of biotic processes or interactions, the population decline to zero is a collapse threshold, as per the evaluation.

However, ecologist and environmental scientist Jack O’Connor argues that that collapse per se will occur before this – at a point where a system cannot recover to a previous, more healthy state.

O’Connor who was not involved in the RLE assessment elaborated that some writers in the scientific literature have referred to a phenomenon termed ‘flickering’ whereby variation (eg of nutrient concentrations in small water bodies) is seen around a level that is a harbinger of subsequent collapse over a threshold – beyond which a system moves to an alternate, degraded, stable state from which it is difficult to recover.

“Away from the Sundarbans this phenomenon has been recorded for freshwater ponds and given the dependency of rural populations in the Sundarbans on these sweet water resources, the implications of further degradation of the Sundarbans environment for the rural poor is also clear,” O’Connor, a scientist at United Nations University’s Institute for Environment and Human Security, told Mongabay-India.

“In summary, this study presents a useful effort to use the Red List of Ecosystems to assess ecosystem-scale risk,” added O’Connor. “The approach is still new, but we feel it is a promising tool to try and encapsulate such large-scale, complex processes.”

“While the authors provide some good information on the status of the Sundarbans, gaps remain to be filled given, for example, quantitative risk assessment in the paper is ‘NE’ ie not evaluated.”

Red List of Ecosystems assessment study co-author and IISER-Kolkata scientist Punyasloke Bhadury acknowledged that for some of the parameters information was not adequate and that all stakeholders working in Indian Sundarbans including the forest department, West Bengal government should be taken on board to address those parameters.

“Moreover, restoration strategies can be implemented on an experimental basis such as vegetation restoration in Indian Sundarbans which can lead to the recovery of declining components of the overall ecosystem,” Bhadury told Mongabay-India.

Mangrove, tigers and fish

Populated since the 8th century, intensification of land use and mangrove clearing escalated from the late 19th century and continued throughout the 20th century. Historically, the area of mangroves was estimated to be 6,588 sq km in 1776 and 6,068 sq km in 1873. These estimates suggest that the extent of mangroves has declined by 71.9% from 1776 to 2016, classifying the ecosystem as “endangered”, according to the study.

Study co-author Radhika Bhargava said the Sundarbans (both in India and Bangladesh) lost 24.55% of mangroves (136.77 sq km) due to erosion over the past three decades. The loss of land due to erosion is leading to a direct loss in the mangrove forests.

“Since 1987, the declaration of the area as a World Heritage Site, along with other management and conservation tools, has helped to stabilise the mangrove area,” said Bhargava. “The threats to mangrove forests have not completely gone away but they have reduced in the last 50 years.”

“So, we have to be cautious about how we manage the current threats,” said Bhargava.

Recent research based on satellite data indicates that the number of mangroves in the Sundarbans, spanning India and Bangladesh, has not reduced significantly in the last 30 years. However, there is evidence of a decline in the health of about 25% of the mangrove trees. And those areas will be more exposed to harm in the future, especially if extreme events such as cyclones become more common.

The assessment flags concern over the continuing declines in juvenile fish populations. Fishworkers’ unions have blamed depleting fish catch for affecting the livelihood of the traditional fishing communities and fueling migration to the west coast of India in recent years. Fish catch has reduced mainly in the rivers, creeks, and estuaries, while the marine catch in the Bay of Bengal has not been affected, a section of researchers have said.

Elaborating on the fisheries, Bhadury underscored the significance of mapping of the nursery grounds of coastal fisheries in Sundarbans while determining visible signals of climate change such as food availability for nursery fisheries.

“There are a number of areas in which information seems to be inadequate such as the long term effects of salinity changes on mangrove vegetation and resulting consequences for higher trophic levels,” Bhadury told Mongabay-India. “Moreover, coastal acidification and effects of the same on the biodiversity and ecosystem services of Indian Sundarbans are not fully understood and these need to be thoroughly studied.”

The latest “Status of Tigers, Co-predators and Prey in India” Report points out that the tiger population in the Sundarbans landscape “seems to be at stable density”. Together with Bangladesh, Sundarban holds about 200 tigers that have uniquely adapted to the mangrove forests. The report by the Indian government notes that it is important that this transboundary population is managed as a single population.

According to the latest data, the tiger count in the Sundarbans in India for the year 2019-2020 rose to 96, from 88 in 2018-2019. However, the report cautions that despite efforts by forest departments of both countries, joint patrolling and joint management activities have yet to commence. “In our rush to use waterways for economic transportation, we have to be careful that these do not become barriers to tiger (and other wildlife) dispersal.”



Four million people rely on the ecosystem services of the Sundarbans, with wild fisheries the second biggest employment source within the region. Photo credit: Kartik Chandramouli/Mongabay

Andy Large maintained that if there is a slight increase in the tiger population this is to be welcomed from a conservation and biodiversity point of view, but much more work needs to be done on correlating tiger populations to changing habitat status over a longer period before any conclusion as to population trends/threats.

“We would generally avoid using the term ‘stabilizing’ in relation to the Sundarbans though as it is, by definition, a truly dynamic system and one that instead should exist in a state of ‘dynamic equilibrium’.” Large said. “The issue today is the proximity of the Sundarbans ecosystem to a range of tipping points or thresholds which it is vital to avoid crossing.”

This article first appeared on Mongabay.

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