What you'll learn
This revision guide covers ecosystems and biodiversity as tested in WJEC GCSE Geography. You'll learn how living organisms interact with each other and their physical environment, how energy and nutrients flow through natural systems, and why biodiversity matters globally. The content focuses on case study application, particularly tropical rainforests and one other major ecosystem.
Key terms and definitions
Ecosystem — A natural system where living organisms (plants, animals, microorganisms) interact with each other and with non-living components (water, soil, air, climate) in a specific area.
Biodiversity — The variety of all living organisms in a particular area, including the range of species, genetic variation within species, and variety of ecosystems.
Producer — An organism (usually a plant) that makes its own food through photosynthesis, forming the first trophic level in a food chain.
Consumer — An organism that obtains energy by eating other organisms; categorised as primary (herbivores), secondary (carnivores eating herbivores), or tertiary consumers.
Decomposer — An organism (bacteria, fungi) that breaks down dead organic material, releasing nutrients back into the soil.
Nutrient cycling — The continuous movement of nutrients (such as nitrogen, carbon, phosphorus) between living organisms and the physical environment.
Trophic level — The position an organism occupies in a food chain, numbered from producers (level 1) upwards through various consumer levels.
Interdependence — The relationship between different components of an ecosystem where each depends on others for survival; if one element changes, others are affected.
Core concepts
Components of an ecosystem
Ecosystems contain both biotic (living) and abiotic (non-living) components that interact continuously.
Biotic components include:
- Producers (plants, algae, phytoplankton)
- Consumers at various levels (herbivores, carnivores, omnivores)
- Decomposers (bacteria, fungi, some insects)
Abiotic components include:
- Climate factors (temperature, rainfall, sunlight, wind)
- Soil characteristics (pH, nutrient content, drainage, depth)
- Water availability and quality
- Relief and altitude
- Rock type influencing soil formation
These components are interdependent. For example, in tropical rainforests, heavy rainfall and high temperatures support rapid plant growth. Dense vegetation creates high humidity and intercepts sunlight, creating a shaded understory. When trees die, decomposers break down organic matter quickly in warm, moist conditions, returning nutrients to thin soils that support new growth.
Energy flows and food webs
Energy enters ecosystems through producers capturing sunlight via photosynthesis. This energy then flows through the ecosystem as organisms consume each other.
Food chains show simple linear feeding relationships: Leaf → Caterpillar → Bird → Snake
Food webs show the complex reality where organisms have multiple food sources and predators. Most ecosystems have interconnected food webs rather than simple chains.
Key principles of energy flow:
- Only approximately 10% of energy transfers between trophic levels
- The remaining 90% is lost through respiration, movement, heat, and waste products
- This limits food chains to typically 4-5 levels maximum
- Biomass (total mass of living material) decreases at each level
- Pyramids of biomass visually represent this decreasing energy
Nutrient cycling
Unlike energy (which flows through then leaves ecosystems), nutrients are recycled continuously within ecosystems.
The nutrient cycle involves:
- Nutrients in biomass — stored in living plants and animals
- Litterfall — dead organic material (leaves, branches, deceased organisms) falls to the ground
- Decomposition — decomposers break down organic matter
- Nutrient release — nutrients return to soil as humus
- Uptake — plant roots absorb nutrients from soil
- Return to biomass — nutrients incorporated into growing vegetation
In tropical rainforests:
- High temperatures and moisture accelerate decomposition
- Nutrients cycle rapidly (decomposition in 6 weeks vs 1 year in temperate forests)
- Most nutrients stored in biomass, not soil
- Soils remain nutrient-poor despite lush vegetation
- Removing vegetation breaks the cycle, causing rapid soil degradation
In temperate deciduous forests:
- Slower decomposition in cooler temperatures
- More nutrients stored in soil
- Distinct seasonal patterns (autumn leaf fall)
- Deeper leaf litter layer
Biodiversity and its importance
Biodiversity exists at three levels:
- Species diversity — variety of different species
- Genetic diversity — variation within individual species
- Ecosystem diversity — variety of different habitats and ecosystems
Tropical rainforests have the highest biodiversity globally because:
- Year-round warm temperatures and moisture
- Continuous growing season
- Complex vertical structure (stratification) creating multiple niches
- Stable conditions over millions of years allowing species evolution
- High productivity supporting food webs
Importance of biodiversity:
Economic value:
- Source of medicines (70% of cancer-fighting drugs originated from rainforest plants)
- Genetic resources for crop development
- Ecotourism revenue
- Sustainable timber and non-timber forest products
Environmental value:
- Climate regulation (carbon storage, oxygen production)
- Water cycle regulation
- Soil protection and nutrient cycling
- Pollination services
Intrinsic value:
- Ethical duty to protect species
- Cultural and spiritual significance
- Aesthetic and educational value
Threats to biodiversity
Direct threats:
Deforestation:
- Cattle ranching (largest driver in Amazon — 80% of cleared land)
- Commercial agriculture (soy, palm oil plantations)
- Logging for timber and paper
- Mining and resource extraction
- Infrastructure development (roads, dams)
Climate change:
- Shifting temperature and rainfall patterns
- Species unable to adapt quickly enough
- Coral bleaching in marine ecosystems
- Altered growing seasons disrupting food webs
- Increased frequency of extreme weather events
Indirect threats:
- Fragmentation isolating populations
- Pollution (agricultural runoff, industrial waste)
- Invasive species outcompeting native organisms
- Overfishing and overhunting
- Disease spread
Case study example — Amazon rainforest:
- Lost 17% of forest cover in past 50 years
- Deforestation rate of approximately 10,000 km² annually
- 400-500 indigenous tribes threatened
- Reduced rainfall affecting regional climate
- Carbon emissions from burning contributing to climate change
Conservation and sustainable management
Protection strategies:
Legal protection:
- National parks and nature reserves
- International agreements (CITES, CBD)
- Protected species legislation
- Enforcement against illegal logging and poaching
Sustainable management:
- Selective logging with replanting programmes
- Agroforestry combining trees with agriculture
- Ecotourism providing economic alternatives
- Payment for ecosystem services schemes
- Community forest management
Case study example — Costa Rica:
- Reversed deforestation from 26% to 52% forest cover (1983-2019)
- Payment for Environmental Services programme compensates landowners
- Ecotourism generates significant GDP
- Protected areas cover 25% of territory
- Biodiversity corridors reconnect fragmented habitats
Sustainable development balances:
- Economic needs of local populations
- Environmental protection
- Social and cultural considerations
- Long-term viability over short-term profit
Worked examples
Example 1: Extended response question (6 marks)
Question: Explain how nutrients are cycled in a tropical rainforest ecosystem. [6 marks]
Model answer: Nutrients in tropical rainforests are stored primarily in the biomass of living vegetation rather than in the soil [1]. When plants shed leaves or organisms die, organic matter falls to the forest floor as litter [1]. In the warm, humid conditions, decomposers such as bacteria and fungi rapidly break down this organic material [1], often within 6 weeks [1]. During decomposition, nutrients are released into the soil where they can be absorbed quickly by plant roots [1]. These nutrients are then incorporated back into growing vegetation, completing the cycle [1]. This rapid cycling means nutrients don't accumulate in the soil, explaining why rainforest soils are relatively nutrient-poor despite supporting lush vegetation [additional detail for development marks].
Mark scheme notes: Award 1 mark per valid point. Look for description of stages, understanding of speed/climate factors, and recognition of nutrient distribution pattern.
Example 2: Data interpretation (4 marks)
Question: Using the food chain below, calculate the energy available to the snake if the grass contains 50,000 kJ of energy. Show your working.
Grass → Grasshopper → Frog → Snake
Model answer: Energy transferred to grasshopper = 50,000 × 0.1 = 5,000 kJ [1] Energy transferred to frog = 5,000 × 0.1 = 500 kJ [1] Energy transferred to snake = 500 × 0.1 = 50 kJ [1] Therefore 50 kJ available to the snake [1]
Mark scheme notes: Award marks for correct application of 10% rule at each stage and correct final answer.
Example 3: Case study application (8 marks)
Question: Using a named example, assess the effectiveness of strategies used to manage tropical rainforest sustainably. [8 marks]
Model answer structure:
- Name specific location (e.g., Amazon rainforest, Brazil)
- Identify 2-3 different strategies with specific detail
- For each strategy: explain what it involves, give evidence of outcomes
- Evaluate effectiveness: what's worked well, limitations
- Balanced conclusion considering economic, environmental, and social factors
- Use data/facts to support points
Example points: Selective logging in Brazilian concessions allows only certain species/sizes harvested, with replanting requirements reducing degradation compared to clear-cutting, but enforcement remains challenging in remote areas. Debt-for-nature swaps have protected 30,000 km² but only work where countries have significant debt.
Common mistakes and how to avoid them
Confusing food chains with nutrient cycles — Remember energy flows through ecosystems (enters via sunlight, exits as heat), while nutrients cycle continuously within ecosystems. Use correct terminology for each process.
Stating biodiversity means "lots of plants and animals" — Be precise: biodiversity refers to variety of species, genetic variation, and ecosystem types. High biodiversity means high variety, not just large numbers.
Describing all deforestation identically — Distinguish between slash-and-burn subsistence farming, commercial cattle ranching, plantation agriculture, and logging. Each has different scales, impacts, and drivers.
Generic case study responses — WJEC rewards specific facts, figures, and place names. "A tropical rainforest" scores lower than "The Amazon rainforest in Brazil where cattle ranching accounts for 80% of deforestation."
Confusing interdependence with food chains — Interdependence is broader, including relationships like pollination, seed dispersal, habitat provision, not just feeding relationships.
Forgetting climate-biodiversity links — Always connect biodiversity levels to climate factors. Explain why tropical rainforests have higher biodiversity (year-round warmth, moisture) compared to other ecosystems.
Exam technique for "Ecosystems and Biodiversity"
Command word precision: "Describe" requires characteristics and features; "explain" needs reasons and processes; "assess" or "evaluate" demands weighing up different viewpoints with a judgement. For 6+ mark questions, explain means evaluation.
Case study specificity: Learn one tropical rainforest case study thoroughly with: location, biodiversity statistics, specific threats with data, named management strategies with evidence of outcomes. WJEC awards marks for accurate detail.
Diagram annotation: In questions about nutrient cycling or food webs, clear labelling with arrows and brief explanations scores highly. Show direction of movement clearly (arrows for energy flow, circular arrows for nutrient cycling).
Linking themes: Connect ecosystems to wider specification themes — climate change impacts, sustainable development conflicts, globalisation driving deforestation. Higher-level responses make these connections explicit.
Quick revision summary
Ecosystems contain interdependent biotic and abiotic components. Energy flows through trophic levels (with 90% loss at each stage) while nutrients cycle continuously via decomposition. Biodiversity, highest in tropical rainforests, has economic, environmental and intrinsic value. Major threats include deforestation for agriculture, logging, and climate change. Sustainable management strategies like selective logging, ecotourism, and legal protection aim to balance conservation with development needs. Success requires specific local context, enforcement, and community involvement.