Wetland Biome
Wetlands are the most productive and most threatened ecosystems on Earth. Covering only approx. 6% of the land surface, they support roughly 40% of the world's species, filter the water we drink, protect coastlines from flooding, and store more carbon in their soils than all the world's forests combined. Swamps, marshes, bogs, fens, mangroves and floodplains — all different names for the same essential idea: land that stays wet enough, long enough, to develop its own unique community of waterlogged-adapted life.
- Area Coveredapprox. 6% of landSmall area, outsized ecological importance
- LocationEvery continentRiver floodplains, lake shores, coasts, upland bogs
- Annual RainfallHighly variableDefined by water saturation, not by rainfall amount
- Carbon Storage≈ 30% of soil CPeatlands store more carbon than all forests combined
- Species SupportDisproportionate40% of world's species breed, feed or shelter in wetlands
- Loss Since 1970> 35%Fastest-disappearing ecosystem on Earth
Wetland water input compared to other biomes
Wetlands are defined not by how much rain they receive but by whether water accumulates — from rain, rivers, groundwater or tides.
Wetlands receive water not only from direct rainfall but from surrounding catchments — rivers, groundwater springs and tides — making their water input much higher than rainfall figures alone suggest. A bog in rainy Scotland and a mangrove forest in monsoon-season Bangladesh both function as wetlands, despite very different climates.
What is the Wetland Biome?
A wetland is any habitat where the land is covered by water, or saturated with water, for long enough each year to affect the soil composition and the plant and animal communities that can survive there. This simple definition encompasses an extraordinary range of environments: from the shallow, sunny margins of tropical lakes to the dark, acidic sphagnum bogs of Scotland; from the tidal mangrove forests of Bangladesh to the vast floodplain forests of the Amazon that flood to 15 metres each wet season. What all wetlands share is water — and the fundamental reshaping of ecosystem function that water creates when it sits on or in the soil.
Wetlands are classified by their water source, chemistry and vegetation. Marshes are dominated by non-woody plants such as reeds, bulrushes and sedges, and receive water from rivers, lakes or rainfall. Swamps have a woody component — trees and shrubs growing in waterlogged soil. Bogs receive water only from rainfall, accumulating deep peat in acid, nutrient-poor conditions. Fens receive water from groundwater as well as rain, making them less acid and more fertile than bogs. Mangroves are coastal saltwater wetlands, dominated by salt-tolerant trees with specialised root structures. Floodplains are seasonally inundated river margins.
Where are Wetlands found?
Wetlands occur on every continent except Antarctica and in every climate zone from the Arctic to the tropics. The world's largest single wetland is the Pantanal — a vast freshwater wetland covering approx. 150,000–195,000 square kilometres across Brazil, Bolivia and Paraguay. The Amazon River basin contains enormous areas of seasonally flooded forest (várzea) and permanent swamp. The Mississippi River delta and the Florida Everglades are major North American wetland systems. In Europe, the Camargue delta of the Rhône in France, the Norfolk Broads in England, and the Danube delta in Romania are important examples. The Sundarbans mangrove forest spanning Bangladesh and India is the world's largest mangrove forest. In Africa, the Okavango Delta in Botswana — an inland delta that spreads over 15,000 km² in the middle of the Kalahari Desert — creates one of the most remarkable wildlife refuges on Earth. The Western Siberian Peatlands constitute the world's largest peat bog system, covering approx. 592,000 km².
The living filter: how wetlands clean water
One of the wetland's most important functions — and the one most directly beneficial to humans — is water purification. As water flows slowly through a wetland, it passes through a complex biological filter. Reed beds and other aquatic plants absorb nitrates and phosphates from agricultural runoff that would otherwise cause algal blooms in rivers and seas. Sediment carrying heavy metals and pollutants is trapped by plant stems and settles to the bottom. Bacteria in the oxygen-poor wetland mud process further pollutants. Studies have shown that natural wetlands can remove up to 92% of the nitrogen and 93% of the phosphorus from water passing through them. Some wastewater treatment systems now deliberately route water through constructed wetlands as a cheaper, lower-energy alternative to chemical treatment plants. The clean water flowing out of a healthy wetland may have entered as agricultural drainage or even partially-treated sewage.
Plants of the Wetland
Wetland plants have evolved remarkable strategies for surviving in waterlogged, often oxygen-depleted soils. Common reed (Phragmites australis), found on every continent, forms dense reedbeds by pumping oxygen down hollow stems to its roots — effectively creating an aerobic zone in the otherwise anaerobic mud. Water lilies photosynthesise from floating leaves while anchored in the mud by long stalks. Mangrove trees produce pneumatophores — finger-like roots that stick up out of the mud to absorb oxygen directly from the air. Sphagnum mosses — the foundation of peat bogs — acidify the water around them (preventing the growth of bacteria that would decompose the peat), can hold 20 times their own weight in water, and grow continuously upward while preserving lower layers in perpetuity. Carnivorous plants including sundews, butterworts and pitcher plants capture insects to supplement the nutrients that bog water lacks.
Animals of the Wetland
Wetlands support a disproportionately large fraction of the world's biodiversity — an estimated 40% of all species depend on wetlands for some part of their life cycle. Waterbirds are the most visible: herons, egrets, spoonbills, storks, ibises, flamingos, ducks, geese, swans, rails, moorhens, coots and kingfishers are all wetland specialists. Wetlands are essential stopover and wintering sites for migratory birds — a single shallow coastal lagoon can host hundreds of thousands of waders refuelling during a long migration. Freshwater fish use wetland margins and flooded forests as nurseries, where young fish can shelter from predators and find abundant food. Amphibians — frogs, toads, newts and salamanders — depend entirely on wetland water to breed, making them among the most wetland-dependent animal groups. Wetlands are home to several of the world's most endangered large animals: the Bengal tiger in the Sundarbans, the jaguar in the Pantanal, the pygmy hippo in West African swamps, the Florida panther in the Everglades, and the Chinese giant salamander — the world's largest amphibian — in mountain streams of China.
Wetland food web
Wetland food webs are among the most productive and complex in any ecosystem, driven by the combination of sunlight, shallow water and high nutrient availability. At the base are primary producers: aquatic plants (reeds, water lilies, pondweeds), algae (both microscopic phytoplankton and filamentous algae on surfaces), and sphagnum moss in bogs. Zooplankton — microscopic water fleas (Daphnia), copepods and rotifers — graze on phytoplankton and are in turn eaten by small fish, aquatic insects and filter-feeding invertebrates such as freshwater mussels. Aquatic invertebrates — dragonfly larvae, water boatmen, diving beetles, caddisfly larvae, mayfly nymphs, water snails and freshwater shrimp — form a critical middle layer, connecting the plant base to vertebrate predators. Small fish including sticklebacks, minnows and young carp feed on invertebrates and are eaten by kingfishers, grey herons, otters, pike and larger fish. Amphibians — frogs, toads and newts — feed on invertebrates and are eaten by herons, grass snakes, otters and large fish. Wading birds such as lapwings, redshanks and avocets probe the mud for invertebrates. Ducks and geese graze aquatic vegetation and filter invertebrates from water. Large predators including herons, ospreys, otters, crocodiles, alligators and — in tropical wetlands — tigers and jaguars top the food chain. Decomposers — anaerobic bacteria in waterlogged mud — process dead material, releasing nutrients that fuel the next cycle of plant growth, and producing methane as a by-product.
How animals adapt to Wetland life
Wetland animals have evolved remarkable adaptations to waterlogged environments, fluctuating water levels and the challenge of finding food in or under water. Herons stand motionless for long periods at the water's edge, using their spear-like bill to strike fish with lightning speed; their specialised neck vertebrae allow the bill to be fired forward like a harpoon. Otters have nostrils and ears that close when diving, dense, waterproof fur with a layer of trapped air for insulation, and webbed feet for powerful swimming; they can remain underwater for up to 4 minutes. The water vole — featured in Wind in the Willows — has a blunt nose and short ears that stay close to the head, reducing drag while swimming, and a long rudder-like tail. Frogs can breathe through their moist skin as well as their lungs, absorbing oxygen directly from the water and even from damp air — essential during hibernation in mud at the pond bottom. The African lungfish survives the annual drying of its wetland habitat by burrowing into the mud and secreting a mucus cocoon around itself, reducing its metabolic rate by 95% and breathing air through a modified swim bladder. It can survive for up to four years in this state, waiting for the rains to return. The purple heron's elongated toes spread its weight over floating vegetation, allowing it to walk on water lily pads. Many wetland birds — ducks in particular — have lamellae (comb-like structures) inside their bills that filter invertebrates and plant material from water, functioning like baleen plates in miniature.
Threats and conservation
More than 35% of the world's wetlands have been lost since 1970 — and the rate of loss is accelerating. Wetlands are disappearing three times faster than forests. The primary cause is drainage for agriculture: wetlands were historically seen as wasted land to be converted to productive farmland. Britain lost over 60% of its lowland peat bogs and significant proportions of its fens, salt marshes and coastal grazing marshes in the 20th century. In Indonesia and Malaysia, vast areas of tropical peatland have been drained and burned for palm oil plantations — releasing carbon stored for thousands of years and destroying habitat for orang-utans, proboscis monkeys and thousands of other species. Urban development encroaches on coastal wetlands. Water extraction from rivers reduces flows into wetland systems. Pollution — particularly agricultural runoff rich in nitrates — causes eutrophication, where excessive algae growth depletes oxygen and kills aquatic life. Climate change is altering water regimes, intensifying droughts in some regions and flooding in others.
Deeper dive: the Ramsar Convention and wetland carbon economics
The Ramsar Convention on Wetlands — signed in Ramsar, Iran, in 1971 — was one of the first modern international environmental treaties. Countries that sign it commit to the "wise use" of all their wetlands and to designating at least one Ramsar site of international importance. There are now over 2,400 Ramsar sites worldwide, covering more than 2.5 million square kilometres. Famous sites include the Everglades, the Camargue, Spain's Doñana marshes, Poyang Lake in China and the Pantanal.
Wetland conservation has gained new urgency from the economics of carbon storage. Peatlands store approximately 550 billion tonnes of carbon — but when drained, they release it rapidly. Indonesian peatland drainage releases an estimated 0.6 billion tonnes of CO₂ equivalent per year, making Indonesia one of the world's largest emitters. The economic case for conserving wetlands is increasingly compelling: healthy wetlands provide water filtration, flood control, fisheries, carbon storage and biodiversity at a fraction of the cost of the engineered alternatives. Studies have estimated the economic value of wetland services at US$14,000–$90,000 per hectare per year — far exceeding the value of converting them to farmland.
Wetlands are the kidneys of the landscape — filtering water, buffering floods, storing carbon and harbouring extraordinary wildlife. The open water habitats within and alongside them connect wetlands to the broader Grassland and Temperate Forest biomes in the great web of interconnected ecosystems that cover our planet.