Biomes

Wetland

Transitional water/land areas
1,966 Animals
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Overview

Understanding This Category

Wetlands are ecosystems in which water inundates or saturates the soil at or near the surface for long enough to create hydric (oxygen-poor) soils and to support plant and microbial communities adapted to saturated conditions. They occur along the continuum between terrestrial and fully aquatic environments, driven by distinctive hydrology, soils, and wetland-adapted vegetation.

Wetlands form where water stays, seasonally or year-round, changing soil, plants, and animals. Low oxygen in wet soil slows decay and makes hydric soils, favoring plants like sedges, rushes, reeds, mangroves, and water-tolerant trees, creating marshes, swamps, bogs, and fens. Wetlands slow floods, hold and release water, filter sediments and excess nutrients, and improve downstream water. They are wildlife hubs and store lots of carbon in peat, so they are fragile to drainage, fire, and warming.

Key Characteristics

Persistent or seasonal saturation/inundation that controls ecosystem processes (distinct wetland hydrology)
Hydric, often anaerobic soils with characteristic redox features (e.g., mottling, gleying)
Dominance of hydrophytic (water-tolerant) vegetation adapted to low-oxygen soils
High capacity for water storage and flow regulation, often reducing floods and buffering droughts
Strong biogeochemical filtering of sediments, nutrients, and some contaminants; high productivity in many wetland types
In peat-forming wetlands, slow decomposition leads to peat accumulation and substantial long-term carbon storage
Climate

Climate Conditions

Wetlands are places where soils stay wet from frequent rain, high water tables, river overflow, coastal flooding, or underground water. They occur from tropical to subarctic regions and often have high humidity, fog, and small temperature swings near the ground. In cold areas, yearly freeze-thaw controls water and plant life; in the tropics, warm, wet seasons keep growth going.

Temperature

Highly variable by latitude: ~5-15°C (tropical coastal/riverine wetlands) to ~25-45°C (temperate/continental wetlands); subarctic peatlands often ~30-50°C annual swing between winter lows and summer highs.

Average High
Typical mean warm-season highs: ~25-32°C (tropical); ~20-28°C (temperate); ~12-20°C (subarctic/taiga peatlands).
Average Low
Typical mean cool-season lows: ~18-24°C (tropical); ~-5 to 5°C (temperate); ~-20 to -5°C (subarctic/taiga peatlands).
Extremes
Heat: ~35-45°C possible during drought/heat waves (especially inland marshes). Cold: ~-20 to -50°C possible in continental and subarctic peatlands. Water presence can buffer daily extremes locally but does not prevent regional extremes.

Precipitation

Broad range depending on wetland type and setting: ~500-1,500 mm/yr common for many temperate marshes/swamps; ~1,500-3,500+ mm/yr for humid tropical wetlands; peat-forming bogs often ~700-2,000 mm/yr with precipitation exceeding evapotranspiration for much of the year. Some wetlands persist with lower rainfall (~250-600 mm/yr) if sustained by groundwater, snowmelt, rivers, or tides.

Pattern
Often year-round precipitation in humid regions; strongly seasonal in monsoonal/savanna climates (pronounced wet season flooding, dry season drawdown). Many inland wetlands rely on spring snowmelt and seasonal river flooding; coastal wetlands can be driven by tidal cycles plus storm seasons.
Humidity
Generally high near the surface (often ~70-95% RH common), with frequent fog/mist in cool seasons or mornings; high evapotranspiration in warm seasons further elevates local humidity.
Seasonality

Seasonal change in wetlands is mainly hydroperiod (how deep, long, and when water floods). In temperate/subarctic wetlands, winter ice slows decay and spring snowmelt floods trigger spawning and bird migration; summer droughts lower water, expose mudflats, and raise peatland fire risk. In tropical wetlands, wet seasons widen flooding and fish movement; dry seasons shrink pools and favor drier-edge plants.

Growing Season

Growing season varies by latitude and flooding. Tropical wetlands 10–12 months; temperate 5–8 months (150–240 days); subarctic peatlands 2–4 months (60–120 days). In cold regions it goes from post‑thaw to late summer/fall; in monsoonal tropics peak growth is early‑mid wet season when water is plentiful but not deeply flooding vegetation.

Seasons

Seasonal Changes

Spring (Thaw & Flood Pulse)

Late winter to late spring; timing varies by latitude and elevation (earlier in lowlands, later in boreal/temperate high latitudes).

Rising temperatures; ice-out where applicable; high water from snowmelt and seasonal rains; widespread inundation of floodplains and marsh platforms; increased turbidity and nutrient inputs; rapidly warming shallow water in sunlit areas.

Major hydrologic reset: floodwater reconnects channels, backwaters, and floodplains; transports sediments/organic matter and redistributes nutrients; triggers primary production as light and temperature increase; replenishes groundwater; expands spawning and nursery habitat; can also flush accumulated pollutants downstream.

Amphibian breeding migrations to vernal pools and shallow marsh edges; mass calling and egg-laying Fish move into inundated floodplain and vegetated shallows to spawn; juvenile recruitment increases Waterfowl and shorebird spring migration stopovers; heavy foraging on invertebrates and seeds Emergence of aquatic insects (e.g., mayflies, midges) begins, supporting birds and bats Mammals (e.g., muskrats, beavers) repair lodges/dams; increased foraging as vegetation greens up

Summer (Peak Growth & High Biological Activity)

Late spring through late summer; longest days and warmest temperatures.

Warm water and air; high plant growth (emergent reeds/sedges, floating-leaved plants, swamp canopy); water levels may remain high in rain-fed wetlands or gradually decline in seasonal wetlands; potential stratification in deeper ponds; increased decomposition; occasional droughts and heat waves can reduce surface water.

Maximum primary productivity; strong nutrient uptake and filtration by vegetation and microbial communities; high carbon fixation (especially in peat-forming systems where anoxia slows decay); oxygen can fluctuate strongly (daytime supersaturation from photosynthesis, nighttime hypoxia); risk of algal blooms increases with nutrient loading and warmth.

Nesting and chick-rearing for many waterbirds; brood-rearing in dense emergent cover High foraging activity on abundant insects and aquatic invertebrates; bat and swallow activity peaks Fish and amphibian larvae growth accelerates; juveniles use vegetated refuges to avoid predation Reptiles (where present) bask and reproduce; turtles nest on nearby uplands Beavers maintain dams and canals; muskrats harvest emergent vegetation; increased territorial behavior in many species

Autumn (Cooling, Drawdown & Migration)

Late summer to early winter; shortening days and cooling temperatures.

Lower sun angle; cooling water; increased storms in many regions; water levels often drop in seasonal wetlands as evapotranspiration declines and inflows change; leaf fall in forested swamps adds detritus; first frosts begin; clearer water may occur as algal production declines.

Shift from production to detrital food webs: large inputs of leaf litter and senescing macrophytes fuel microbial decomposition; nutrient release can increase late-season; reduced plant uptake; fall turnover in deeper waters can redistribute oxygen and nutrients; exposure of mudflats creates feeding habitat for migrants.

Southbound migration of waterfowl and shorebirds; intense feeding on seeds, tubers, and invertebrates in exposed shallows Fish may concentrate in remaining pools/channels as water recedes; increased predation pressure Amphibians and reptiles seek overwintering sites (burrows, mud, deep pools); reduced activity after frosts Mammals cache food and increase foraging; beavers store branches; muskrats build/fortify houses Many insects enter diapause; emergence declines; dragonflies and other migrants may pass through

Winter (Dormancy, Ice & Low Oxygen Risk)

Early winter through late winter; strongest in temperate/boreal wetlands; minimal in tropical wetlands.

Cold temperatures; short days; ice cover and snow in colder climates; reduced inflow; low light; vegetation dormant; water may persist under ice in deeper areas while shallow seasonal wetlands may freeze solid or dry if autumn drawdown was strong.

Greatly reduced primary production; slowed decomposition; anoxic conditions can develop under ice (winterkill risk) due to continued respiration and limited gas exchange; peatlands can store carbon as decay slows; physical disturbance from ice can shear vegetation and reshape channels.

Many amphibians overwinter in mud or underwater with reduced metabolism; some frogs tolerate freezing in uplands Fish overwinter in deeper pools; reduced feeding; may suffer mortality if oxygen drops Waterfowl either migrate out or concentrate in spring-fed/open-water refugia Mammals use lodges/burrows; beavers remain active under ice accessing cached food; muskrats use push-ups/air pockets Invertebrates persist as eggs/larvae in sediments; diapause is common

Day Length: Day length varies strongly with latitude: in temperate/boreal wetlands, long summer days drive peak photosynthesis, rapid plant growth, and high food availability; shortening days in autumn cue migration, diapause, and plant senescence; minimal winter daylight limits primary production and can exacerbate low-oxygen conditions under ice. In tropical wetlands, photoperiod changes are modest and seasonality is often driven more by rainfall than day length, though day length still helps time breeding and migration for some species.

Where Found

Global Distribution

Wetlands are found on every continent, even in polar areas, where water stays near the soil long enough to make wet soils and water-loving plants. They are common in flat floodplains, deltas, coasts (estuaries, mangroves, tidal marshes), glacial hollows, and northern peatlands. Big areas are in tropical river basins, the boreal/Arctic peat belt, and large temperate floodplains and lake edges.

≈1.7% of Earth's surface (≈6% of global land area) of Earth's Surface
≈8.9 million km² (varies by definition, mapping method, and inclusion of coastal wetlands/peatlands) Total Area

Notable Locations

Pantanal (Brazil/Bolivia/Paraguay) Everglades (Florida, USA) Okavango Delta (Botswana) Sudd (South Sudan) Congo Basin peatlands (Central Basin, DR Congo/Republic of the Congo) Western Siberian Lowland peatlands (Russia) Hudson Bay Lowlands (Canada) Sundarbans mangroves (India/Bangladesh) Mekong Delta wetlands (Vietnam/Cambodia) Danube Delta (Romania/Ukraine) Mesopotamian Marshes (Iraq/Iran) Prairie Pothole Region (USA/Canada) Kakadu wetlands (Northern Territory, Australia)
Conservation

Conservation Status

Globally threatened and in widespread decline; many wetland types (especially peatlands, coastal marshes, and floodplain wetlands) have been heavily reduced, fragmented, and degraded, with strong biodiversity and carbon-storage consequences.

Rapidly Declining Trend
Ongoing net loss and degradation; while rates vary by region and wetland type, global assessments indicate continued decline driven mainly by drainage/conversion, water diversion, and coastal development (often on the order of ~0.5-1% per year in rapidly developing landscapes). Loss Rate

Primary Threats

Habitat Loss Agricultural Expansion Natural System Modification Pollution Infrastructure Urbanization Climate Change Invasive Species Resource Depletion Logging Mining Overfishing Human Disturbance Hunting Wildlife Trade Disease Human-Wildlife Conflict

Protection Efforts

  • Ramsar Convention listing and management of Wetlands of International Importance, including national wetland policies and site monitoring
  • Expansion and improved management of protected areas encompassing wetlands, with enforcement of land-use and hunting/fishing regulations
  • Environmental flow programs and basin-scale water allocation reforms to restore seasonal inundation and sediment delivery
  • Pollution controls: wastewater treatment upgrades, agricultural nutrient management, pesticide restrictions, and riparian buffer restoration
  • Coastal wetland protection measures: setback zones, limiting shoreline hardening, and enabling "managed realignment" where appropriate
  • Peatland rewetting and paludiculture (wet agriculture) to cut CO2 emissions, reduce fire risk, and recover wetland function
  • Invasive species prevention and control (biosecurity, early detection/rapid response, targeted removals)
  • Indigenous and community-led wetland stewardship, co-management, and livelihood diversification tied to wetland conservation
  • Nature-based solutions financing (flood mitigation, water purification, carbon/peatland climate finance) and restoration incentive programs
Fun Facts

Did You Know?

A wetland doesn't have to be "wet" all the time: Some wetlands are seasonal-dry for part of the year-yet still have hydric soils and water-adapted plants.

Some wetlands are fueled by rain, not rivers: Bogs can be "ombrotrophic," meaning they get most of their water and nutrients from rainfall, making them acidic and nutrient-poor.

Wetlands can clean water like a living filter: Microbes, plants, and sediments can trap particles, transform nitrogen compounds, and bind some pollutants-one reason wetlands can improve downstream water quality.

Wetlands can be both carbon sinks and methane sources: They often lock away carbon in peat, but low-oxygen soils also produce methane-so their climate role depends on water level, temperature, and disturbance.

Peat is basically time stored in mud: In waterlogged, low-oxygen conditions, dead plant material decomposes extremely slowly, building layers that can preserve pollen, seeds, and even human artifacts.

Trees can "breathe" in flooded soil: Mangroves and swamp trees often use specialized roots (like pneumatophores) that pull oxygen from the air when the ground is waterlogged.

Peat growth is slow-motion geology: A common rule of thumb is ~1 millimeter of peat accumulation per year-so 1 meter of peat can represent around 1,000 years of ecosystem history.

Pantanal scale check: At roughly ~150,000+ km² (varying by flood extent and definitions), the Pantanal is comparable in area to England and can look like an inland sea during peak floods.

Sundarbans scale check: Around ~10,000 km², the Sundarbans is about the size of Jamaica-except it's a labyrinth of tidal channels and salt-tolerant forests.

Okavango Delta scale check: The Okavango's seasonal wetland footprint can reach ~15,000 km²-similar to the size of Connecticut-despite being in a semi-arid region.

Small footprint, big influence: Although wetlands occupy only a single-digit percent of Earth's land area, they can strongly regulate floods and water quality across entire river basins.

Mangrove nurseries as "apartment complexes": A mangrove shoreline can function like stacked habitat-roots, trunks, and canopy each providing different living space for fish, crabs, insects, birds, and reptiles.

Largest tropical wetland: The Pantanal (Brazil/Bolivia/Paraguay) is widely cited as the world's largest tropical wetland, swelling dramatically in the rainy season.

Largest contiguous mangrove forest: The Sundarbans (India/Bangladesh) is the biggest continuous mangrove ecosystem on Earth.

Biggest peatland system: The Western Siberian Lowland contains the largest peatland complex on the planet, storing vast amounts of carbon in deep peat.

Largest tropical peatland complex (known): The Congo Basin's Central Basin peatlands form the largest mapped tropical peatland complex, a major carbon store discovered and measured only recently (2010s).

Among the most productive ecosystems: Many wetlands rival (and can exceed) rainforests and coral reefs in primary productivity-especially marshes and mangroves-because shallow water and nutrients power rapid growth.

Carbon "heavyweights": Peatlands cover only a small fraction of Earth's land (~3%), yet store an outsized share of global soil carbon-often summarized as roughly twice the carbon contained in the world's forests' biomass.

Wetland Animals

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