The Biosphere: Ecosystems and Biodiversity

What you'll learn

This revision guide covers the fundamental concepts of ecosystems, biodiversity and ecological processes tested in CIE IGCSE Environmental Management. You'll understand how organisms interact within ecosystems, how energy and nutrients flow through food chains and webs, and why biodiversity matters for ecosystem stability. The guide also addresses human impacts on ecosystems and the principles of conservation management.

Key terms and definitions

Ecosystem — A community of living organisms (biotic factors) interacting with each other and their non-living environment (abiotic factors) in a defined area.

Biodiversity — The variety of living organisms in an area, including species diversity, genetic diversity within species, and diversity of ecosystems.

Habitat — The specific place where an organism lives, characterised by particular physical and biological conditions.

Niche — The role an organism plays in an ecosystem, including its feeding relationships, behaviour and position in food chains/webs.

Producer — An organism (usually a green plant or algae) that makes its own food through photosynthesis, converting light energy into chemical energy.

Consumer — An organism that obtains energy by feeding on other organisms; primary consumers eat producers, secondary consumers eat primary consumers.

Decomposer — An organism (bacteria or fungi) that breaks down dead organic material and waste products, releasing nutrients back into the soil.

Food web — A diagram showing the interconnected feeding relationships between organisms in an ecosystem, consisting of multiple food chains.

Core concepts

Components of ecosystems

Ecosystems contain two main categories of components working together:

Biotic factors (living components):

• Producers: plants, algae, some bacteria
• Consumers: herbivores, carnivores, omnivores
• Decomposers: bacteria, fungi
• All organisms in competition for resources

Abiotic factors (non-living components):

• Light intensity and duration
• Temperature range
• Water availability
• Soil pH and nutrient content
• Air composition and humidity
• Topography and altitude

These factors determine which species can survive in an ecosystem. For example, mangrove ecosystems in the Caribbean have high salt tolerance species adapted to waterlogged, saline soils, whilst UK deciduous woodlands contain species adapted to seasonal temperature variation and moderate rainfall.

Organisms interact through feeding relationships, competition for resources, and symbiotic relationships. The balance between populations is maintained by availability of food, water, shelter and suitable breeding sites.

Energy flow in ecosystems

Energy enters ecosystems through photosynthesis:

Carbon dioxide + Water → Glucose + Oxygen (using light energy)

This energy flows through ecosystems in one direction:

1. Producers capture approximately 1-3% of light energy from the Sun
2. Primary consumers (herbivores) obtain energy by eating producers
3. Secondary consumers (carnivores) eat primary consumers
4. Tertiary consumers eat secondary consumers
5. Decomposers break down dead material at all levels

At each transfer between trophic levels, approximately 90% of energy is lost through:

• Respiration (heat energy)
• Movement and life processes
• Undigested material in faeces
• Incomplete consumption of organisms

Only about 10% of energy transfers to the next trophic level. This explains why:

• Food chains rarely exceed 4-5 trophic levels
• Producer biomass exceeds consumer biomass
• Pyramids of energy always form true pyramids

Energy flow diagrams show these transfers with arrows indicating direction of energy movement. Always remember energy is NOT recycled — it flows through and is eventually lost as heat.

Nutrient cycling

Unlike energy, nutrients are recycled through ecosystems. The two most important nutrient cycles for IGCSE are:

The Carbon Cycle:

• Carbon dioxide removed from atmosphere by photosynthesis
• Carbon stored in plant tissues (biomass)
• Carbon passes through food chains in organic compounds
• Carbon returned to atmosphere through respiration by all living organisms
• Carbon released through decomposition of dead material
• Combustion of fossil fuels and wood releases stored carbon
• Ocean absorption and release of CO₂

The Nitrogen Cycle:

• Nitrogen gas (N₂) makes up 78% of atmosphere but most organisms cannot use it directly
• Nitrogen fixation: bacteria in soil and root nodules convert N₂ into nitrates
• Plants absorb nitrates through roots to make proteins
• Nitrogen passes through food chains in proteins and DNA
• Decomposition: bacteria and fungi break down dead material, releasing ammonium compounds
• Nitrification: nitrifying bacteria convert ammonium to nitrites then nitrates
• Denitrification: denitrifying bacteria convert nitrates back to nitrogen gas (in waterlogged soils)

Human activities affecting these cycles include:

• Deforestation reducing carbon storage
• Fossil fuel combustion increasing atmospheric CO₂
• Fertiliser application adding nitrates to soil (leading to eutrophication if washed into water bodies)
• Intensive farming reducing soil organic matter

Ecological succession

Succession is the gradual change in species composition of an ecosystem over time.

Primary succession occurs on bare rock or newly formed land where no soil exists:

1. Pioneer species (lichens, mosses) colonise bare rock
2. Weathering and decomposition create thin soil
3. Grasses and small plants establish
4. Larger plants and shrubs colonise
5. Fast-growing trees appear
6. Climax community develops — a stable, mature ecosystem (e.g., oak woodland in UK)

Secondary succession occurs on previously vegetated land after disturbance (fire, farming abandonment, hurricane damage in Caribbean):

• Soil already present with seed bank
• Succession proceeds faster than primary succession
• Grasses and weeds appear first
• Shrubs and pioneer tree species follow
• Climax community eventually re-establishes

At each stage:

• Biodiversity increases
• Biomass increases
• Soil depth and nutrient content improve
• Microclimate becomes more stable
• Earlier species modify environment, making it suitable for later species

Biodiversity and ecosystem stability

Biodiversity exists at three levels:

1. Species diversity — the number and variety of different species
2. Genetic diversity — variation within species populations
3. Ecosystem diversity — variety of habitats and ecosystems in a region

High biodiversity provides:

• Greater ecosystem stability and resilience
• More complex food webs (if one species declines, others compensate)
• Genetic variation allowing adaptation to environmental change
• Ecosystem services (pollination, pest control, nutrient cycling)
• Potential sources of medicines, crops and other resources

Factors affecting biodiversity:

Positive factors:

• Habitat diversity creating varied niches
• Stable climate allowing species specialisation
• Large area supporting larger populations

Negative factors:

• Habitat destruction and fragmentation
• Pollution reducing species tolerance
• Climate change exceeding adaptation rates
• Invasive species outcompeting natives
• Overexploitation of species

Measuring biodiversity involves:

• Species richness (total number of species)
• Species abundance (number of individuals per species)
• Evenness (how equally abundant different species are)

Simpson's Diversity Index provides a numerical measure combining these factors, though calculation details are not typically examined at IGCSE level.

Conservation and sustainable management

Conservation aims to protect species and ecosystems whilst allowing sustainable human use.

In-situ conservation (on-site):

• National parks and nature reserves (e.g., Serengeti, Lake District)
• Marine protected areas restricting fishing
• Protection of breeding sites and migration routes
• Legal protection for endangered species
• Habitat restoration projects

Ex-situ conservation (off-site):

• Seed banks storing genetic material
• Botanical gardens maintaining plant collections
• Zoos breeding endangered species
• Gene banks preserving genetic diversity

Sustainable management strategies:

For forests:

• Selective logging rather than clear-cutting
• Replanting programmes maintaining forest cover
• Protection of primary forest areas
• Community forestry involving local people
• Certification schemes (FSC) ensuring sustainable practices

For fisheries:

• Quotas limiting catch sizes
• Minimum mesh sizes allowing young fish to escape
• Closed seasons during breeding periods
• Marine reserves as breeding refuges
• Reducing bycatch through selective fishing gear

For agriculture:

• Crop rotation maintaining soil fertility
• Reduced pesticide use protecting non-target species
• Hedgerow and woodland corridors maintaining connectivity
• Organic farming methods enhancing biodiversity
• Agroforestry combining trees with crops

Worked examples

Example 1: Energy transfer calculation

Question: A grassland ecosystem receives 2,000,000 kJ/m²/year of light energy. Only 2% is captured by grass through photosynthesis. If cattle (primary consumers) obtain 10% of the energy from grass, calculate the energy available to cattle per m² per year. [3 marks]

Answer:

• Energy captured by grass = 2% of 2,000,000 kJ = 0.02 × 2,000,000 = 40,000 kJ/m²/year [1 mark]
• Energy transferred to cattle = 10% of 40,000 kJ = 0.10 × 40,000 = 4,000 kJ/m²/year [1 mark]
• This shows why cattle require large grazing areas to obtain sufficient energy [1 mark]

Example 2: Interpreting a food web

Question: A pond food web shows: algae → water fleas → small fish → heron, and algae → pond snails → ducks. Explain what would happen to the small fish population if duck numbers increased significantly. [4 marks]

Answer:

• Increased duck numbers would consume more pond snails [1 mark]
• Fewer pond snails means less grazing pressure on algae, so algae population increases [1 mark]
• More algae provides increased food for water fleas, increasing their population [1 mark]
• More water fleas provide more food for small fish, so small fish population increases [1 mark]

Example 3: Nitrogen cycle application

Question: A farmer notices crop growth is poor despite adequate water and sunlight. Soil tests show low nitrate levels. Explain two ways nitrate levels could be increased naturally, without adding artificial fertiliser. [4 marks]

Answer:

• Plant legumes (beans, clover) which have nitrogen-fixing bacteria in root nodules that convert atmospheric nitrogen into nitrates [2 marks]
• Add compost or manure which decomposers break down, releasing ammonium that nitrifying bacteria convert to nitrates [2 marks]

Common mistakes and how to avoid them

• Confusing energy flow with nutrient cycling — Remember energy flows in one direction and is lost as heat; nutrients are recycled through decomposition. Never say "energy is recycled."

• Incorrectly drawing food chain arrows — Arrows show direction of energy flow FROM food source TO consumer. Write "grass → rabbit → fox" not the reverse.

• Mixing up producers and consumers — Producers make their own food through photosynthesis; they don't "consume sunlight." Use precise terminology.

• Forgetting the 10% rule applies to energy, not numbers — A decrease in producers doesn't mean exactly 10× fewer primary consumers; the 10% refers to energy transfer efficiency.

• Confusing succession terms — Primary succession starts on bare rock with no soil; secondary succession occurs where soil already exists after disturbance.

• Incomplete nutrient cycle descriptions — Always include the role of decomposers and bacteria; these organisms are essential for nutrient recycling but often forgotten.

Exam technique for "The Biosphere: Ecosystems and Biodiversity"

• Command word precision: "Explain" requires reasons/mechanisms (not just description); "Describe" needs characteristics/what happens; "Suggest" allows reasoned speculation using knowledge. Match your answer depth to the command word and marks available.

• Use case studies appropriately: Questions asking for examples need specific named locations (Amazon rainforest, Caribbean coral reefs, UK moorlands) with relevant detail. Generic answers lose marks.

• Draw clear, labeled diagrams: For food webs and cycles, use neat arrows with clear labels. Include all components asked for and ensure arrows point correctly. Check diagrams show what the question requires.

• Apply knowledge to unfamiliar contexts: You may face scenarios about ecosystems not studied in class. Apply principles learned (succession, energy flow, conservation methods) to the new context using information provided in the question.

Quick revision summary

Ecosystems consist of biotic and abiotic components interacting through feeding relationships and nutrient cycles. Energy flows unidirectionally through trophic levels with 90% lost at each transfer. Carbon and nitrogen cycle through ecosystems via photosynthesis, respiration, decomposition and bacterial processes. Succession describes ecosystem development from pioneer species to climax community. Biodiversity provides ecosystem stability and requires conservation through in-situ and ex-situ methods. Sustainable management balances human use with ecosystem protection through techniques like selective logging, fishing quotas and crop rotation. Understanding these interconnections is essential for environmental management.