Ecosystems: Tropical Rainforests

What is an Ecosystem?

An ecosystem is a community of living organisms (plants, animals, microorganisms) interacting with each other and with their non-living (abiotic) environment (soil, water, climate, nutrients). Everything is interconnected — a change in one part affects the whole system.

The Nutrient Cycle

Nutrients cycle continuously through an ecosystem. In a tropical rainforest:

1. Trees and plants absorb nutrients from the soil through roots
2. Through photosynthesis, sunlight converts nutrients and CO₂ into organic matter (biomass — leaves, wood, roots)
3. When plants die, the litter (dead leaves, wood) falls to the forest floor
4. Decomposers (bacteria, fungi) break down the litter → nutrients are released back into the soil
5. Nutrients re-enter plant roots → cycle continues

Key feature of rainforest nutrient cycle: The nutrient cycle is very rapid and tight — hot, moist conditions make decomposition extremely fast. Almost all nutrients are stored in the biomass (the vegetation), not in the soil. This has a critical implication: if trees are removed, nutrients are removed with them, leaving nutrient-poor soils that quickly become infertile.

Tropical Rainforest — Distribution and Climate

Tropical rainforests are found in a belt around the Equator (approximately 10°N to 10°S): the Amazon Basin (South America), Congo Basin (Central Africa), SE Asia (Indonesia, Malaysia, Borneo), and parts of Central America and West Africa.

Climate: consistently hot (~26–28°C throughout the year) and wet (>2,000 mm rainfall per year, with no dry season). These conditions support extreme biodiversity.

Tropical Rainforest — Structure and Characteristics

Rainforests have a layered structure:

Adaptations of plants:

• Buttress roots: wide, flat root extensions help anchor very tall trees in shallow soils
• Drip tips: waxy, pointed leaf tips allow rain to run off quickly, preventing fungal growth
• Lianas: woody climbing vines reach the canopy without a thick trunk
• Epiphytes (air plants, e.g., orchids): grow on other plants to reach light; absorb water from rain, not soil

Biodiversity: The Amazon contains approximately 10% of all known species on Earth — estimated 40,000 plant species, 1,300 bird species, 3,000 fish species. This richness exists because of the stable, warm, wet climate that has persisted for millions of years.

Deforestation — Causes and Case Study (Amazon)

Deforestation is the permanent clearing of forest. In the Amazon, the rate peaked at ~40,000 km²/year (the area of Switzerland) in the 1990s but remains very high. Causes:

1. Commercial cattle ranching: the single biggest cause in Brazil — cleared land for beef export (Brazil = world's largest beef exporter)
2. Soya bean farming: for animal feed (mainly in Europe and China); often linked to ranching expansion
3. Logging: both legal (timber concessions) and illegal
4. Mining: Amazon has vast deposits of gold, iron ore, bauxite — clearing and pollution
5. HEP dams: the Belo Monte Dam (completed 2019) — flooded large areas of forest and displaced indigenous communities
6. Road building: Trans-Amazonian highway opened the interior → further settlement and deforestation

Effects of Deforestation

Environmental effects:

• Biodiversity loss: endemic species go extinct — many not yet discovered by science
• Climate change: trees store ~200 Gt of carbon; burning releases CO₂ — deforestation produces ~10% of global CO₂ emissions
• Water cycle disruption: trees recycle ~50% of Amazon's rainfall via transpiration → deforestation reduces rainfall, threatens drought
• Soil erosion: without tree cover, heavy rainfall erodes the thin topsoil → rivers become silted; land may become semi-arid
• Local climate change: the Amazon generates its own rainfall system — large-scale deforestation risks "dieback" — converting the Amazon from rainforest to savannah

Social effects:

• Displacement of indigenous communities (e.g., Yanomami, Kayapo peoples)
• Loss of traditional medicines (many pharmaceuticals derived from rainforest plants)

Sustainable Management of Tropical Rainforests

International Agreements

• REDD+ (Reducing Emissions from Deforestation and Forest Degradation): pays developing countries to keep forests standing; carbon credits sold on international markets

National Policies

• National Parks and Reserves: Brazil has over 10 million km² of legally protected Amazon rainforest
• IBAMA (Brazil's environmental agency): monitors and enforces against illegal logging and burning — satellite monitoring (PRODES system) tracks deforestation in real time
• Forest Code (Brazil): requires landowners in the Amazon to keep 80% of their land as forest — though enforcement has weakened at times

Selective Logging

• Only specified trees above a certain girth are felled; the rest of the forest remains
• Allows natural regeneration; maintains ecosystem structure
• More expensive and less profitable than clear-felling in the short term

Ecotourism

• Visitors pay to experience the rainforest → provides income for local communities
• Incentivises conservation (forest is worth more alive than cleared)
• Risk: too many visitors can damage the ecosystem

Agro-forestry

• Growing crops underneath a tree canopy rather than clearing the forest
• Maintains soil stability and nutrients; preserves some biodiversity; provides sustainable livelihoods

WJEC Exam Tips

• Know the layered structure of the rainforest with named layers
• Explain the tight nutrient cycle and its critical implication: removing trees = removing nutrients
• The Amazon case study should include specific named causes, effects and management strategies
• Evaluate question: "Is deforestation inevitable given the need for economic development?" — two sides + judgement