Unit 3: Major Terrestrial Ecosystems of the World

I. Orientation

A terrestrial ecosystem is a land-based system in which organisms interact with one another and with physical factors such as climate, soil, water, and relief. The global distribution of major ecosystems, or biomes, is governed chiefly by temperature and precipitation, with latitude, altitude, seasonality, and soil conditions modifying the pattern.

• Climate control: Mean temperature, annual rainfall, and their seasonal distribution determine growing conditions.
• Energy flow: Solar energy enters through photosynthesis and passes through producers, consumers, and decomposers.
• Water balance: Evapotranspiration, infiltration, runoff, and soil moisture influence vegetation density.
• Adaptation: Plants and animals develop structural, physiological, and behavioral traits suited to local conditions.
• Soil–vegetation relationship: Vegetation affects soil formation, nutrient cycling, and erosion, while soil properties influence plant growth.
• Spatial pattern: Biomes generally form broad latitudinal belts, but ocean currents, mountains, altitude, and human land use create local variations.

II. Tundra region — cold, treeless ecosystems

The tundra is a high-latitude or high-altitude biome characterized by very low temperatures, a short growing season, and the absence of continuous natural tree cover. Arctic tundra occurs mainly between the taiga and polar ice regions, while alpine tundra occurs above the climatic tree line.

A. Tundra region

The tundra region is controlled by severe cold, frozen ground, and limited biological productivity.

• Location: Arctic tundra forms across northern Alaska, Canada, Greenland’s margins, Scandinavia, and Siberia; alpine tundra occurs in high mountains such as the Himalayas and Andes.
• Climate: Winters are long and dark; summers are brief, with average warm-month temperatures generally below about 10°C in Arctic tundra.
• Permafrost: Permanently frozen subsoil restricts drainage and root penetration. Only the upper “active layer” thaws during summer.
• Vegetation: Mosses, lichens, sedges, grasses, and dwarf shrubs dominate; trees are absent because roots cannot develop effectively and the growing season is too short.
• Adaptations: Cushion-shaped plants reduce exposure to wind, while shallow roots and rapid flowering enable reproduction within a short summer.
• Animals: Reindeer or caribou, musk ox, Arctic foxes, lemmings, migratory birds, and polar bears use the region. Thick fur, fat storage, migration, and seasonal color changes aid survival.
• Nutrient cycling: Cold temperatures slow decomposition, so organic matter accumulates and soils are often nutrient-poor.
• Environmental change: Warming causes permafrost thaw, altered drainage, shrub expansion, greenhouse-gas release, and habitat changes for cold-adapted species.

III. Taiga region — the boreal coniferous forest

The taiga, or boreal forest, is the extensive conifer-dominated biome south of the Arctic tundra. It occupies a broad belt across North America and Eurasia and represents one of the world’s largest terrestrial carbon stores.

A. Taiga region

The taiga region combines long, cold winters with a short but warmer growing season that supports needle-leaved forests.

• Location: Major belts extend across Canada and Alaska, and across Scandinavia and Russia, approximately between 50° and 70° N.
• Climate: Annual precipitation is moderate, commonly about 300–850 mm, with much falling as snow. Winters may remain below freezing for several months.
• Vegetation: Spruce, fir, pine, and larch are characteristic. Needle-like leaves reduce water loss, and conical crowns shed snow.
• Soils: Podzolic soils are acidic and nutrient-poor because conifer litter decomposes slowly and heavy rainfall or snowmelt leaches nutrients.
• Animals: Moose, brown bears, wolves, lynx, beavers, and migratory birds are common. Many species have insulating fur, seasonal dormancy, or migration strategies.
• Fire ecology: Lightning-caused fires naturally renew stands, release nutrients, and create habitats for different forest-age communities.
• Human use: Timber extraction, pulp production, mining, hydropower, and transport corridors have altered large areas.
• Carbon role: Forest biomass and peatlands store substantial carbon; logging, drainage, and fires can convert these stores into atmospheric carbon dioxide.

IV. Deciduous forest — seasonal broadleaf woodland

Deciduous forests are woodlands in which many trees shed their leaves during an unfavorable season, usually winter or a seasonal dry period. Temperate deciduous forests are especially associated with moderate climates and clearly developed seasons.

A. Deciduous forest

The deciduous forest is shaped by seasonal temperature changes, fertile soils, and a relatively long growing period.

• Distribution: Temperate examples occur in eastern North America, western and central Europe, northeastern China, Korea, and Japan. Tropical dry forests may also be seasonally deciduous.
• Climate: Temperate forests commonly receive about 750–1,500 mm of precipitation annually, with warm summers and cold winters.
• Vegetation: Oak, beech, maple, birch, chestnut, and hickory form layered canopies. Leaf fall reduces winter water loss and prevents snow accumulation on branches.
• Soils: Leaf litter decomposes comparatively rapidly, producing humus and fertile soils such as Alfisols. Nutrient cycling is more active than in the taiga.
• Seasonal rhythm: Spring light reaches the forest floor before canopy closure, allowing herbs and flowering plants to grow. Autumn leaf fall returns nutrients to the soil.
• Animals: Deer, foxes, squirrels, bears, woodpeckers, and numerous insects occupy different layers and seasonal niches.
• Human transformation: Agriculture, urbanization, roads, and historical timber use have fragmented much original forest, although secondary regrowth is widespread.
• Ecological importance: These forests moderate local climate, protect watersheds, store carbon, and provide timber, fruits, recreation, and wildlife habitat.

V. Grasslands — ecosystems dominated by grasses

Grasslands are ecosystems in which grasses and herbaceous plants dominate, while trees are limited by insufficient rainfall, periodic fire, grazing, or seasonal drought. They include temperate grasslands and tropical savannas.

A. Grasslands

Grasslands are maintained by a balance among precipitation, evaporation, fire, herbivory, and the ability of grasses to regenerate from underground structures.

• Distribution: Temperate grasslands include the Prairies of North America, Pampas of South America, Steppes of Eurasia, and Veld of southern Africa. Savannas occur widely in tropical Africa, Brazil, and northern Australia.
• Climate: Temperate grasslands often receive roughly 250–900 mm of annual rainfall; savannas have warm temperatures but a pronounced wet and dry season.
• Vegetation: Grasses possess basal growth points and extensive roots, allowing them to recover after grazing or fire. Savanna grasses coexist with scattered drought-resistant trees.
• Soils: Temperate grassland soils, including Mollisols, are dark and rich in organic matter because dense roots die and decompose below ground.
• Fire and grazing: Periodic fires remove woody seedlings and recycle nutrients; grazing animals such as bison, wildebeest, and zebras maintain open structure.
• Adaptations: Deep roots withstand drought, narrow leaves reduce water loss, and many grasses become dormant during dry or cold periods.
• Food production: Grasslands support wheat, maize, cattle, and sheep, but cultivation can cause erosion and loss of deep soil carbon.
• Conservation: Ploughing, overgrazing, invasive species, and altered fire regimes threaten native grasslands; protected reserves and rotational grazing improve resilience.

VI. Tropical rainforest — warm, humid evergreen forest

Tropical rainforests are highly productive, multilayered forests found near the Equator where warmth and rainfall occur throughout most or all of the year. They contain exceptionally high biodiversity despite generally nutrient-poor surface soils.

A. Tropical rainforest

The tropical rainforest is sustained by year-round heat, abundant moisture, rapid nutrient cycling, and complex vertical structure.

• Distribution: Major areas occur in the Amazon Basin, Congo Basin, Southeast Asia, Central America, and parts of northern Australia.
• Climate: Temperatures commonly average about 24–27°C, while annual rainfall often exceeds 2,000 mm. There is little seasonal temperature variation.
• Structure: Emergent trees rise above a closed canopy; below it are the understory, shrub layer, and shaded forest floor. Each layer provides distinct habitats.
• Vegetation: Broad evergreen leaves maximize light capture. Lianas climb toward the canopy, while epiphytes such as orchids grow on branches without rooting in soil.
• Soils and nutrients: Heavy rain leaches minerals from soil, producing highly weathered soils. Most nutrients are held in living biomass and rapidly recycled through decomposition.
• Animals: Primates, jaguars, forest elephants, sloths, parrots, insects, amphibians, and reptiles occupy specialized niches.
• Adaptations: Drip-tip leaves shed water, buttress roots support tall trees, and camouflage, mimicry, or canopy locomotion reduce predation risks.
• Threats: Deforestation for cattle ranching, plantations, mining, roads, and logging causes habitat fragmentation, carbon release, biodiversity loss, and changed rainfall patterns.

VII. Deserts — ecosystems of water scarcity

Deserts are ecosystems where potential evaporation greatly exceeds precipitation, producing chronic moisture shortage. They may be hot or cold and are defined by aridity rather than temperature alone.

A. Deserts

Desert ecosystems demonstrate how organisms survive limited and unpredictable water availability.

• Distribution: Major hot deserts include the Sahara, Arabian, Kalahari, Australian, and Sonoran deserts. Cold deserts occur in Central Asia, the Patagonian region, and polar interiors.
• Climate: Many deserts receive less than 250 mm of annual precipitation, often in irregular events. Coastal deserts may remain cool because of cold ocean currents.
• Vegetation: Cacti, acacias, euphorbias, xerophytic shrubs, and ephemeral plants dominate. Reduced leaves, waxy surfaces, spines, and water-storing stems limit water loss.
• Animal adaptations: Camels conserve water through efficient kidneys and body-temperature regulation; rodents may be nocturnal and obtain water metabolically from seeds.
• Soils and surface processes: Sparse vegetation permits wind erosion, salt accumulation, and flash-flood erosion. Alluvial fans and playas commonly form after brief heavy rainfall.
• Productivity: Productivity is low overall but can increase rapidly after rain, when dormant annual plants germinate, flower, and produce seeds within weeks.
• Human use: Irrigation supports settlements and agriculture, but over-irrigation can cause salinization. Mining, roads, tourism, and groundwater extraction also disturb fragile surfaces.
• Environmental change: Desertification results from interacting climatic stress and land mismanagement, especially overgrazing, vegetation removal, and unsustainable water use.