Mangrove Forest

The Mangrove Forest biome is a coastal wetland of salt-tolerant trees that grow in the intertidal zone, the band of shore that is covered by water at high tide and exposed at low tide. Mangroves are among the few woody plants that can live rooted in salty, waterlogged, oxygen-poor mud, and that tolerance is what sets the biome apart. They grow along the coasts and estuaries (the brackish zones where rivers meet the sea) of 123 countries across the tropics and subtropics, and although they cover less than 1 percent of the world's tropical forest area, the UN Environment Programme (UNEP) reports that they rank among the most carbon-rich ecosystems on Earth.

What builds the forest

A small number of tree species create the entire habitat, each occupying a different position along the shore. The red mangrove (Rhizophora mangle) grows closest to the sea and stands on prop roots, arching stilt-like roots that drop from the trunk and branches into the mud and stay partly submerged at high tide. Behind it, the black mangrove (Avicennia germinans) produces pneumatophores, pencil-thick roots that grow straight up out of the mud and take in air through small pores called lenticels while the tide is out. The white mangrove (Laguncularia racemosa) generally grows on higher ground that floods less often.

These trees survive in salt water by 2 main strategies. Salt excluders such as the red mangrove filter most of the salt at the root surface through ultrafiltration, a straining process that blocks salt before water enters the plant. Salt excretors such as the black mangrove take the salt up and push it back out through glands on the leaf surface, where it dries into visible crystals. Both approaches let the trees hold their internal water balance in mud whose salinity, or dissolved-salt content, would kill most land plants.

How the forest works

Mangrove forests are highly productive, meaning the plants convert a large amount of sunlight into new growth through primary production, the process by which plants build tissue from sunlight and carbon dioxide. Much of that growth returns to the ecosystem as fallen leaves. Rather than washing out to sea, the leaf litter is trapped among the roots, where bacteria and fungi break it down into detritus, the fine particles of decaying organic matter that form the base of the food web. Decomposition also releases tannins, natural plant compounds that stain the water brown and cut down how far light travels through it. Crabs, shrimp, worms, and filter-feeding invertebrates feed on the detritus, and fish and larger predators feed on them.

Shelter and predation shape where animals live in the forest. Juvenile fish, including snappers and grunts, crowd into the dense root tangle, where larger predators cannot follow, while predators such as barracuda patrol the more open water at the forest edge. This separation between protected structure and open hunting water is central to how the biome supports so much young fish life.

What lives there

The submerged roots give hard surfaces to a habitat otherwise made of soft mud, and they become encrusted with oysters, barnacles, sponges, tunicates (sea squirts), and anemones. Among that growth live juvenile snapper, grunt, and mojarra, along with small barracuda and, in the mud below, crabs and snails. The biome is an important nursery, a place where young animals grow before moving to adult habitat. Many fish that live on coral reefs or the open coast as adults spend their early lives among mangrove roots and then move offshore. According to UNEP, 1,533 species are associated with mangroves in some way. Water within the forest is usually warm, shallow, and turbid, or clouded with suspended sediment, and visibility is often reduced to a few meters by that sediment and the dissolved tannins.

Why the biome matters

Mangroves are a foundation species, an organism whose own physical structure creates the habitat that the rest of the community depends on. In the same way that corals build a reef, mangroves build the forest: the trees and their submerged roots turn open mud into a three-dimensional refuge, hold sediment in place, and supply the fallen leaves that feed the food web. Removing the mangroves does not simply subtract one species from the system, it removes the structure that holds everything else together, which is why the forest collapses when the trees are cleared.

That structure makes mangroves one of the ocean's most valuable nurseries. A 2025 global analysis in Communications Earth & Environment estimated that mangroves support an annual abundance of more than 700 billion juvenile fish and invertebrates, drawn from 37 commercially important species that rely heavily on the forest. A review drawing more than 11,000 statistical comparisons from 160 published studies found that mangroves and seagrass meadows provide the greatest nursery value of any coastal habitat, with the highest juvenile densities recorded in mangroves and seagrass alike. The 1,533 species that UNEP associates with mangroves range from these young fish to crabs, mollusks, birds, and mammals.

The fisheries built on those nurseries are large. The Florida Museum of Natural History reports that about 90 percent of the commercially important species in south Florida depend on the mangrove system for at least part of their life cycle, and Mapping Ocean Wealth estimates that 4.1 million small-scale fishers across 109 countries and territories work the mangroves and their adjacent waters, about 38 percent of all small-scale fishers in those countries. A study of the Gulf of California published in the Proceedings of the National Academy of Sciences found that mangrove-related fish and crab species made up 32 percent of the small-scale fishery landings there, and valued each hectare of mangrove fringe at about 37,500 US dollars a year in fishery catch alone.

Where the biome occurs

Mangroves cannot tolerate hard frost, so the biome is confined to warm coasts, roughly within 25 degrees of latitude north and south of the equator, although a decline in the frequency of hard freezes has let some populations spread toward the poles. Using satellite mapping, the Global Mangrove Watch estimated about 147,359 square kilometers (56,896 square miles) of mangrove forest worldwide in 2020, with 51 percent in the Asia-Pacific region, 29 percent in the Americas, and 20 percent in Africa. The forest needs fine, muddy sediment to root in, salt or brackish water, and a coastline sheltered from strong waves, conditions found at river mouths, lagoons, estuaries, and the calm leeward sides of islands.

Threats and conservation

Mangrove forests are among the most rapidly declining coastal ecosystems. UNEP reports a net global loss of about 5,245 square kilometers (2,025 square miles), or 3.4 percent of mangrove cover, between 1996 and 2020, driven mainly by clearing for aquaculture (especially shrimp farming), coastal development, pollution, and climate change. When the trees are removed, the mud they held in place erodes and the nursery habitat is lost with it. Rising sea level and warming compress the forest from the seaward side, and it can retreat inland only where undeveloped low ground remains, so seawalls and coastal building block that retreat. Mangroves also hold some of the largest carbon stores of any ecosystem, an average of roughly 1,000 tonnes of carbon per hectare including the soil, according to UNEP, so clearing them releases carbon that had been locked in the waterlogged ground for centuries.