Bangladesh has not lost sight of that logic, putting a great premium on the ability of mangroves to stabilize shores and trap sediments. A low-lying country with a long, vulnerable coastline, Bangladesh is also land starved, with a crushing population density of 2,500 persons per square mile (2.6 square kilometers). By planting mangroves on delta sediments washed down from the Himalaya, it has gained over 300,000 acres (120,000 hectares) of new land on the Bay of Bengal. The plantings are relatively new, but there have been mangroves here for as long as the Ganges, Brahmaputra, and Meghna Rivers have been draining into the bay. The vast tidal woodland they form is known as the Sundarbans—literally "beautiful forest." Today, it's the largest surviving single tract of mangroves in the world.

In the forest's most luxuriant sections a dozen mangrove species, from feathery golpata palms to the towering sundri tree, form labyrinthine stands up to 60 feet (18 meters) tall. Beneath the sundri, the glutinous mud bristles with the tree's breathing roots. Twelve inches high (30 centimeters) and as thick as deer antlers, they grow so tightly together there's barely room to squeeze a foot between them. In drier areas, groves of semi-deciduous mangroves blaze red in the months before the monsoon. Spotted deer glide through the filtered shade, stopping abruptly when a troop of macaques shriek an alarm call. Woodpeckers hammer in the high branches, while on the forest floor dry leaves rustle with the scuttling of mud crabs. A butterfly called the Sundarban crow—charcoal with splashes of white—rests on a twig, opening and closing its wings like a prayer book.

Evening falls with the junk junk junk sound of nightjars, then all is quiet. Night belongs to the tiger. These forests provide one of the last remaining haunts for the Bengal tiger and its only saltwater habitat. According to local tradition, the tiger's name, bagh, must never be uttered. To speak it is to summon it. So people talk of mamu, uncle. Uncle tiger, lord of the Sundarbans.

Half a million Bangladeshis risk mamu's displeasure by coming into the Sundarbans each year to harvest its products. They come as fishermen, woodcutters, palm-frond cutters, cutters of thatching grass, harvesters of wild honey. The workers spend weeks at a time in the forest, living off its bounty as they earn a few taka for their labor. Seafood, fruits, medicines, tea, sugar, even the raw materials for beer and cigarettes are to be found in the Sundarbans larder.

Throughout the tropical world it's the same: Mangrove forests are the supermarkets, lumberyards, fuel depots, and pharmacies of the coastal poor. Yet these forests are being destroyed daily. One of the greatest threats to mangrove survival comes from shrimp farming. At first glance, shrimp might seem the perfect export for a poor country in a hot climate. Rich countries have an insatiable appetite for it (shrimp has overtaken tuna to become America's favorite seafood), and the developing world has the available land and right climate to farm it.

A prime location for shrimp ponds, though, happens to be the shore zone occupied by mangroves, an unhappy conflict of interests that has a predictable outcome: The irresistible force of commerce trumps the all-too-removable mangrove. To compound matters, shrimp farmers typically abandon their ponds after a few crop cycles (to avoid disease outbreaks and declining productivity) and move to new sites, destroying more mangroves as they go.

Mangrove-rich Brazil was slow to stake its claim in the bonanza. By the time shrimp fever hit Brazil's northeastern states, around the turn of the millennium, shrimp-farming pioneers such as Thailand, the Philippines, and Ecuador had been uprooting their mangroves for decades. Today, in the Brazilian port city of Fortaleza ponds the size of football fields crowd the landscape like rice fields. Paddle wheel aerators froth the water, and workers in kayaks fill feeding trays with fish meal. Even where mangroves have been spared, access to them is often blocked by the shrimp farms.

At the riverside settlement of Porto do Céu— "the gates of paradise"—an electrified fence shuts out villagers from their traditional harvesting grounds. But there is worse. The shrimp ponds have no lining, so salt water has percolated through the sandy soil and contaminated the aquifer beneath. The villagers have been forced to abandon wells that until recently drew sweet fresh water to the surface. The water is no longer sweet; it is salgada, saline, undrinkable.

At Curral Velho, a community to the west of Fortaleza, people have been finding a voice to oppose Big Shrimp. Demonstrations have been organized, land deals challenged, a public education center set up. Sister Mary Alice McCabe, an American nun who is helping the community in its struggle, says that one of the difficulties in raising awareness about carcinicultura—shrimp farming—is that most Brazilians aren't aware of the environmental damage it causes. "'Where does it happen, out at sea?' they ask. 'No, no, no,' we tell them, 'they're digging up your mangroves, they're destroying your coastline.' "