Beyond Tigers: How the Sundarbans' Tidal Mangrove Web Keeps Asia's Largest Forest Alive

The Sundarbans sits across roughly 10,000 square kilometres of the Bengal delta, where the Ganga, Brahmaputra, and Meghna rivers meet the Bay of Bengal. Twice a day, tidal water floods the forest floor and retreats, depositing silt and carrying nutrients. This cycle, not the trees, not the tigers, is the actual engine of the system. The mangrove trees are the beneficiaries of that engine, and everything else is downstream of both.

Sundarbans mangroves belong to species that have solved a genuinely difficult engineering problem. Heritiera fomes, locally called sundari and the tree the forest is named after, grows aerial roots called pneumatophores that spike upward from the waterlogged mud to pull in oxygen. Rhizophora mucronata sends arching prop roots into the silt to anchor itself against tidal surge. These root systems do more than hold the trees up: they trap sediment, slow water velocity, and create the dense mat of organic material that the entire food chain below the canopy depends on.

The biological fact that makes this root architecture critical: mangrove sediment in the Sundarbans holds up to four times the carbon per hectare of a tropical rainforest. That carbon is not stored in the trees, it is stored in the anaerobic mud the roots create and stabilise. Lose the roots, lose the mud structure, and the carbon releases.

Mudskippers (Periophthalmodon schlosseri) are amphibious fish that spend most of their lives out of water. They breathe through their moist skin and the lining of their mouth, a process called cutaneous respiration. In the Sundarbans, they are among the most visible animals on any exposed mudflat, and among the most important. They feed on algae, detritus, and smaller invertebrates, and are themselves prey for kingfishers, herons, and the water monitor lizard (Varanus salvator). Remove them from the mudflat and you collapse the middle tier of that food chain.

Fiddler crabs (Uca species) do something even more structurally important: they aerate the mud. Their burrowing activity, millions of crabs across thousands of hectares, keeps the sediment oxygenated enough for the microbial communities that break down leaf litter. Sundarbans mangroves shed enormous quantities of leaves year-round. Without the fungal and bacterial decomposers working in oxygenated sediment, that litter would accumulate and acidify the soil. The crabs, in effect, are the forest's composting infrastructure.

Horseshoe crabs (Tachypleus gigas) use the Sundarbans shoreline as a breeding ground. Their blood, blue, copper-based, and extraordinarily sensitive to bacterial endotoxins, has been harvested by the biomedical industry for decades to test pharmaceutical sterility. Ecologically, their eggs are a critical food source for migratory shorebirds that stop along the Bengal coast.

The saltwater crocodile (Crocodylus porosus) is the largest living reptile on earth, and the Sundarbans population, now protected under Project Crocodile, represents one of the few recovering estuarine populations in South Asia. Adult males can exceed six metres and 1,000 kilograms. They are apex ambush predators in the water, but their ecological function extends beyond predation.

Crocodile basking sites on mudbanks are consistently used by nesting birds, because the crocodiles' presence deters terrestrial egg predators. Their wallowing creates pools that retain water during dry periods, providing habitat for fish, invertebrates, and amphibians. Their consumption of large fish, catfish, mullet, barramundi, prevents any single species from dominating the estuarine fish community and crashing the prey base below it. This is the classic keystone predator function, and in the Sundarbans it operates alongside, not in competition with, the tiger's terrestrial version of the same role.

The Irrawaddy dolphin (Orcaella brevirostris) is a freshwater and estuarine species found in the river channels of the Sundarbans. It has a rounded head, no beak, and a flexible neck, unusual for a dolphin, which allows it to turn its head sideways while navigating shallow, root-obstructed waterways. The Indian population is listed as vulnerable by the IUCN.

Irrawaddy dolphins engage in cooperative feeding with fishermen in parts of the Bengal delta, a behaviour documented in Myanmar and reportedly observed in the Sundarbans too, where dolphins herd fish toward nets in exchange for access to the catch. Whether this is true mutualism or opportunism is debated. What is not debated is that dolphin predation on fish schools in the river channels influences where those schools move, which in turn affects which areas of the forest receive nutrient input from fish waste and decomposing fish. The dolphins are, in a real sense, redistributing fertility across the waterway system.

Gangetic river dolphins (Platanista gangetica) also enter the Sundarbans through the northern river channels. They are functionally blind, their eyes have no lens, and navigate entirely by echolocation. They feed on bottom-dwelling fish and invertebrates, turning up sediment in the process and releasing nutrients that would otherwise remain locked in the riverbed.

The Bengal tiger (Panthera tigris tigris) in the Sundarbans has adapted to this tidal forest in ways no other tiger population has. Sundarbans tigers swim between islands, eat fish and crabs when ungulate prey is scarce, and tolerate salt water, behaviours not documented in any other tiger population. A 2019 study by the Wildlife Institute of India estimated the Sundarbans tiger population at around 96 individuals on the Indian side, with Bangladesh's Sundarbans holding a further estimated 114.

The tiger's ecological function is real: it controls spotted deer (Axis axis) and wild boar populations that would otherwise overgraze the forest understorey. But the tiger is the visible tip of a system that was already extraordinarily complex before the first tiger walked into it. The tidal cycle built the mud. The mud fed the mangroves. The mangroves created the root architecture. The root architecture sheltered the fish nurseries. The fish nurseries fed the dolphins, the crocodiles, the fishing cats, the otters, and the birds. The tiger arrived into a system already running.

The Sundarbans does not function because it has tigers. The tigers function because the Sundarbans has everything else. That distinction matters enormously for conservation: protect only the charismatic apex predator and you are protecting the flag while the country dissolves beneath it.