What you'll learn
This topic examines how human populations affect the planet and how biological knowledge helps address critical challenges. You'll explore population dynamics, food security, sustainable resource management, and the biological approaches to solving global problems. Understanding these concepts is essential for both your exam and appreciating real-world applications of biology.
Key terms and definitions
Biodiversity — the variety of different species living in a habitat or ecosystem
Food security — having access to sufficient, safe, and nutritious food to maintain a healthy life
Sustainability — meeting present needs without compromising the ability of future generations to meet their own needs
Biological control — using natural predators, parasites or diseases to control pest populations instead of chemical pesticides
Eutrophication — excessive nutrient enrichment of water bodies leading to algal blooms and oxygen depletion
Carbon footprint — the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event
Monoculture — growing a single crop species over a large area year after year
Bioaccumulation — the build-up of toxic substances in organisms through the food chain, increasing in concentration at higher trophic levels
Core concepts
Human population growth and its impacts
The global human population has grown exponentially from approximately 1 billion in 1800 to over 8 billion today. This growth results from:
- Improved medical care reducing death rates
- Better sanitation and hygiene preventing disease spread
- Increased food production supporting larger populations
- Reduced infant mortality rates
Consequences of population growth:
Population increase directly impacts resource consumption and waste production. Key effects include:
- Greater demand for food, water, and energy
- Increased land use for agriculture, housing, and industry
- More waste generation requiring disposal
- Rising greenhouse gas emissions contributing to climate change
- Habitat destruction reducing biodiversity
- Depletion of non-renewable resources
Different countries experience varied population growth rates. Many developed nations show slow or negative growth, while some developing countries maintain rapid expansion. This creates global inequalities in resource distribution and environmental impact per capita.
Food security and agricultural challenges
Food security requires adequate food production, distribution, and access for all populations. Several factors threaten this:
Factors affecting food security:
- Population growth increasing demand
- Climate change disrupting growing seasons and rainfall patterns
- Water scarcity limiting irrigation
- Soil degradation reducing crop yields
- Pest and disease outbreaks destroying harvests
- Political conflicts disrupting food distribution
- Economic inequality limiting food access
Increasing food production:
Modern agriculture employs various methods to increase yields:
Fertilisers: Chemical fertilisers provide essential minerals (nitrogen, phosphorus, potassium) that plants need for growth. While they increase crop yields significantly, overuse causes environmental problems including eutrophication of waterways and soil degradation.
Pesticides: These chemicals kill organisms that damage crops. Insecticides target insects, herbicides kill weeds, and fungicides prevent fungal diseases. However, pesticides can harm non-target species, accumulate in food chains through bioaccumulation, and pests may develop resistance requiring stronger chemicals.
Biological control: This sustainable alternative uses living organisms to control pests. Examples include:
- Ladybirds eating aphids on crops
- Parasitic wasps laying eggs in pest caterpillars
- Bacteria (Bacillus thuringiensis) producing toxins harmful to specific insects
Advantages include no chemical pollution, specificity to target pests, and long-term effectiveness. Disadvantages include slower action than pesticides and potential ecological disruption if control organisms become invasive.
Hydroponics: Growing plants in mineral nutrient solutions without soil allows:
- Precise control of nutrient delivery
- Water conservation through recirculation
- Year-round production in controlled environments
- Higher yields per area than traditional farming
- Crop production in areas with poor soil quality
Sustainable food production
Monoculture farming grows single crops over large areas, maximising efficiency but reducing biodiversity and increasing vulnerability to pests and diseases. Sustainable alternatives include:
Crop rotation: Different crops planted in sequence across seasons prevents soil nutrient depletion and breaks pest life cycles. For example, rotating cereals with legumes (which fix nitrogen) maintains soil fertility naturally.
Intercropping: Growing multiple crop species together increases biodiversity, reduces pest spread, and maximises land use efficiency.
Organic farming: Avoiding synthetic pesticides and fertilisers, using natural alternatives like compost and biological control. This protects biodiversity but typically produces lower yields requiring more land.
Selective breeding: Traditional method of breeding organisms with desirable characteristics over many generations. Used to develop:
- Crop varieties with higher yields, disease resistance, or better nutritional content
- Livestock producing more meat, milk, or eggs
- Organisms adapted to specific environmental conditions
Process involves selecting parents with desired traits, breeding them, and selecting offspring showing the strongest expression of those traits for subsequent breeding.
Genetic engineering and biotechnology
Genetic modification (GM) involves transferring genes from one organism to another, even between different species, creating transgenic organisms. This differs from selective breeding by directly manipulating DNA rather than selecting from existing genetic variation.
Process of genetic modification:
- Identify and isolate the desired gene using restriction enzymes
- Insert gene into a vector (often bacterial plasmid or virus)
- Transfer vector into target organism's cells
- Select successfully modified cells
- Grow modified cells into whole organisms
Examples of GM applications:
- Golden rice: Contains genes for beta-carotene production (converted to vitamin A in humans), addressing vitamin A deficiency in populations dependent on rice
- Bt crops: Contain bacterial genes producing proteins toxic to specific insects, reducing pesticide need
- Herbicide-resistant crops: Allow farmers to use herbicides killing weeds without harming crops
Advantages of GM crops:
- Increased yields feeding growing populations
- Enhanced nutritional content addressing deficiencies
- Reduced pesticide use benefiting environment
- Crops tolerant to drought or poor soils
Concerns about GM crops:
- Unknown long-term health effects on humans
- Potential reduction in biodiversity
- Gene transfer to wild relatives creating "superweeds"
- Corporate control of seed supplies affecting farmers
- Ethical concerns about manipulating nature
Environmental sustainability and resource management
Sustainability requires balancing human needs with environmental protection for future generations.
Reducing environmental impact:
Water management: Freshwater comprises only 2.5% of Earth's water, with much frozen in ice caps. Sustainable water use involves:
- Efficient irrigation systems (drip irrigation) reducing waste
- Rainwater harvesting and storage
- Water recycling and treatment for reuse
- Protecting watersheds and aquifers from pollution
Waste management: Modern societies generate enormous waste volumes. Strategies include:
- Recycling: Reprocessing materials (paper, glass, metals, plastics) into new products, conserving resources and reducing landfill
- Composting: Biological decomposition of organic waste producing nutrient-rich soil amendment
- Anaerobic digestion: Bacteria breaking down organic waste without oxygen, producing biogas (methane) for energy
- Reducing consumption and designing products for longevity
Greenhouse gas reduction: Carbon dioxide, methane, and other greenhouse gases trap heat, causing climate change. Reduction strategies include:
- Renewable energy (solar, wind, hydroelectric) replacing fossil fuels
- Energy efficiency improvements in buildings and transport
- Reforestation and habitat restoration increasing carbon storage
- Reducing meat consumption (livestock produce significant methane)
- Carbon capture technologies
Biodiversity conservation:
Maintaining biodiversity provides ecosystem services including pollination, pest control, water purification, and climate regulation. Conservation methods include:
- Protected areas: National parks and reserves preventing habitat destruction
- Captive breeding programmes: Breeding endangered species in zoos for potential reintroduction
- Seed banks: Storing plant genetic diversity for future use (e.g., Svalbard Global Seed Vault)
- Wildlife corridors: Connecting habitats allowing species migration and gene flow
- International agreements: Treaties like CITES regulating trade in endangered species
Deforestation impacts: Clearing forests, particularly tropical rainforests, causes:
- Biodiversity loss through habitat destruction
- Increased atmospheric CO₂ (trees store carbon)
- Soil erosion and degradation
- Disrupted water cycles
- Loss of indigenous communities' livelihoods
Sustainable forestry practices include selective logging, replanting programmes, and protecting old-growth forests.
Measuring and reducing carbon footprints
A carbon footprint accounts for all greenhouse gas emissions associated with an activity, product, or individual. Measurement considers:
- Direct emissions (e.g., burning fuel in vehicles)
- Indirect emissions (e.g., electricity generation)
- Embodied emissions (manufacturing, transport, disposal of products)
Reducing personal carbon footprint:
- Using public transport, cycling, or walking instead of driving
- Choosing locally-produced, seasonal food
- Reducing meat and dairy consumption
- Improving home insulation and energy efficiency
- Purchasing fewer manufactured goods
- Supporting renewable energy sources
Life cycle assessments evaluate environmental impacts of products from raw material extraction through manufacture, use, and disposal, identifying improvement opportunities.
Worked examples
Example 1: Explain why biological control is often preferred over chemical pesticides in sustainable farming. (4 marks)
Model answer: Biological control uses natural predators or parasites to control pest populations (1 mark). It does not involve chemicals that can pollute soil and water or harm non-target species (1 mark). Biological control agents are specific to target pests, reducing environmental damage (1 mark). Pests are less likely to develop resistance compared to chemical pesticides, providing longer-term effectiveness (1 mark).
Mark scheme notes: Award marks for any four valid points comparing biological control advantages to pesticide disadvantages. Accept equivalent statements about sustainability, environmental impact, or effectiveness.
Example 2: A farmer wants to increase crop yields. Describe how genetic modification could be used and evaluate whether this is better than selective breeding. (6 marks)
Model answer: Genetic modification involves inserting genes from another organism into crop plants (1 mark), such as genes for pest resistance or drought tolerance (1 mark). This is faster than selective breeding because desired traits appear in one generation rather than many (1 mark). GM can introduce traits from any organism, even different kingdoms, whereas selective breeding only uses existing variation within the species (1 mark).
However, selective breeding is more natural and publicly accepted (1 mark), while GM raises concerns about unknown long-term effects on human health and ecosystems (1 mark).
Mark scheme notes: Award up to 4 marks for description of GM and comparison with selective breeding. Award up to 2 marks for evaluation (advantages/disadvantages). Accept other valid evaluative points such as cost, accessibility, or regulatory approval.
Example 3: Eutrophication is a consequence of excess fertiliser use. Describe the process of eutrophication and explain its effects on aquatic ecosystems. (5 marks)
Model answer: Excess fertiliser washes into waterways through surface runoff (1 mark). Increased nutrients cause rapid algal growth forming blooms on the water surface (1 mark). Algae block light from reaching underwater plants, which die (1 mark). Bacteria decompose dead plants and algae, using oxygen during aerobic respiration (1 mark). Oxygen depletion kills fish and other aerobic organisms (1 mark).
Mark scheme notes: Award marks for correct sequence and explanation of cause-effect relationships. Accept "nutrient enrichment" for fertiliser runoff. Require mention of both light blocking and oxygen depletion for full marks.
Common mistakes and how to avoid them
Confusing genetic modification with selective breeding: GM directly transfers genes between organisms including different species; selective breeding only works with existing genetic variation within a species over many generations. Always specify which technique you're discussing.
Stating that fertilisers directly kill aquatic life: Fertilisers cause eutrophication leading to oxygen depletion which kills organisms. Describe the complete process showing understanding of the biological mechanisms involved.
Claiming GM foods are proven dangerous or proven safe: Examiners want balanced evaluation. State that long-term effects are uncertain and there are both potential risks and benefits. Avoid absolute statements either way.
Forgetting that sustainability involves future generations: Don't just describe resource conservation—explicitly state that sustainability means meeting current needs without compromising future generations' ability to meet their needs.
Mixing up bioaccumulation and eutrophication: Bioaccumulation is concentration of toxins up food chains; eutrophication is nutrient enrichment causing algal blooms. Use the correct term for each context.
Writing vague statements about "helping the environment": Be specific about mechanisms—state exactly what process causes what effect (e.g., "reduces carbon dioxide emissions" not just "better for the environment").
Exam technique for "B6: Global Challenges"
Command word awareness: "Describe" requires stating what happens in a logical sequence; "Explain" requires reasons using scientific knowledge; "Evaluate" demands both advantages and disadvantages with a reasoned conclusion. Address the specific command.
Use data when provided: Questions often include graphs or tables about population growth, yields, or emissions. Always reference specific values from data to support answers, stating trends with numbers.
Link concepts explicitly: Connect population growth → increased demand → food security challenges → agricultural methods → environmental impacts. Examiners reward students showing how concepts interrelate rather than treating them as isolated facts.
Balance in evaluation questions: When asked to evaluate (GM crops, biological control, sustainability measures), provide equal consideration of advantages and disadvantages. Typically award 1-2 marks for positives, 1-2 for negatives, and 1 for a conclusion.
Quick revision summary
Global human population growth increases resource demand and environmental impact. Food security requires sustainable agricultural methods including biological control, hydroponics, and crop rotation. Genetic modification transfers genes between organisms creating crops with enhanced traits, though concerns exist. Sustainability demands reducing carbon footprints, managing waste through recycling and composting, and conserving biodiversity through protected areas and breeding programmes. Eutrophication from excess fertiliser depletes oxygen in waterways. Understanding these interconnected challenges is essential for developing solutions balancing human needs with environmental protection.