Contents
1. Ecology Basics
| Term | Definition |
|---|---|
| Population | All individuals of one species in an area |
| Community | All populations of different species living in an area |
| Ecosystem | A community of organisms plus their physical environment |
| Habitat | The place where an organism lives |
| Producer | Organism that makes its own food by photosynthesis (plants, algae) |
| Consumer | Organism that obtains energy by eating other organisms |
| Decomposer | Organisms (bacteria, fungi) that break down dead organic matter |
Food chains and food webs
Arrows in a food chain show the direction of energy transfer: grass → rabbit → fox. The arrow means "is eaten by" or "energy flows to". A food web shows all feeding relationships in an ecosystem — multiple interconnected food chains.
The arrow shows the direction of ENERGY TRANSFER — from prey to predator, from food to feeder. "Grass → Rabbit" means energy transfers from grass to rabbit. Never draw the arrow pointing from predator to prey.
2. Energy Flow
Energy enters ecosystems through photosynthesis (producers absorb sunlight). Energy is lost at each trophic level — only about 10% is passed to the next level. The rest is lost as heat (respiration), in uneaten material, and in faeces.
Pyramid of numbers, biomass, and energy
| Pyramid type | What it shows | Always pyramid-shaped? |
|---|---|---|
| Numbers | Number of organisms at each trophic level | No — can be inverted (e.g. one oak tree → many insects) |
| Biomass | Total mass of organisms at each trophic level | Usually yes (can be inverted for phytoplankton) |
| Energy | Energy available at each trophic level | Always pyramid-shaped — energy always decreases |
3. Nutrient Cycles
Carbon cycle
- CO₂ removed from atmosphere by photosynthesis (producers).
- CO₂ returned by respiration (all living things), combustion (burning fuels), decomposition.
- Carbon stored in fossil fuels for millions of years — burning releases it rapidly (greenhouse effect).
Nitrogen cycle
- Plants absorb nitrate ions (NO₃⁻) from soil → make amino acids and proteins.
- Animals eat plants → obtain nitrogen in proteins.
- Decomposers break down dead organisms → ammonium ions (NH₄⁺).
- Nitrifying bacteria convert NH₄⁺ → NO₂⁻ → NO₃⁻ (nitrification).
- Nitrogen-fixing bacteria (in root nodules of legumes) convert N₂ gas → NH₄⁺ (nitrogen fixation).
- Denitrifying bacteria convert NO₃⁻ → N₂ gas (denitrification) — returns nitrogen to atmosphere.
4. Human Impact on the Environment
| Human activity | Environmental impact |
|---|---|
| Burning fossil fuels | CO₂ and SO₂ released → greenhouse effect / global warming; acid rain |
| Deforestation | Loss of biodiversity; less CO₂ absorbed; soil erosion; disrupted water cycle |
| Use of fertilisers | Eutrophication — nitrates and phosphates run off into water → algal bloom → algae die → bacteria decompose → use up O₂ → fish die |
| Use of pesticides | Bioaccumulation in food chains — toxic chemicals concentrate at higher trophic levels |
| Overfishing | Population collapse; disrupts food webs |
Fertiliser run-off → high nitrate/phosphate in water → algal bloom → algae block light → aquatic plants die → aerobic bacteria decompose dead plants and algae → bacteria use up dissolved oxygen → fish and other organisms suffocate and die.
5. Disease and Immunity
| Type of disease | Cause | Examples |
|---|---|---|
| Infectious | Pathogens (bacteria, viruses, fungi, parasites) | Malaria, tuberculosis, influenza, COVID-19 |
| Non-infectious | Genetic, lifestyle, environmental | Diabetes, cancer, coronary heart disease |
Immune response
- Non-specific defences: skin (physical barrier), mucus and cilia (trap pathogens), stomach acid (kills bacteria), phagocytes (engulf and digest pathogens).
- Specific immune response: lymphocytes recognise antigens on pathogens → produce specific antibodies → antibodies bind to antigens → pathogen destroyed. Memory cells remain for faster response on re-infection.
Vaccination
A vaccine introduces weakened/dead pathogens or their antigens → stimulates antibody production and memory cell formation → no disease symptoms. On future exposure: memory cells rapidly produce large amounts of antibodies → infection eliminated before symptoms develop.
When a sufficiently large proportion of a population is vaccinated, even unvaccinated individuals are protected because the pathogen cannot spread easily. This is especially important for protecting people who cannot be vaccinated (immunocompromised, newborns).
- Energy flow: only ~10% transferred per trophic level. Rest lost as heat/waste in respiration.
- Carbon cycle: removed by photosynthesis; returned by respiration, combustion, decomposition
- Nitrogen cycle: nitrogen-fixing (N2 to NH3), nitrifying (NH3 to NO3-), denitrifying (NO3- to N2), decomposers (protein to NH3)
- Vaccination: introduces antigens -> primary immune response -> memory cells formed -> rapid secondary response if re-exposed
- Antibiotics kill BACTERIA only (not viruses). Overuse leads to antibiotic-resistant strains (natural selection).
6. Common Exam Traps
Fertilisers are not directly toxic to fish. The kill mechanism is: fertiliser → algal bloom → bacteria decompose dead algae → bacteria use up dissolved oxygen → fish suffocate. The cause of death is oxygen depletion, not poisoning.
Each antibody binds to one specific antigen (lock-and-key). Antibodies produced against measles will not protect against influenza. This is why we need different vaccines for different diseases and why the flu vaccine changes yearly.
Pyramids of numbers and biomass can be inverted. The pyramid of energy is ALWAYS pyramid-shaped because energy is always lost at each trophic level — it can never increase going up the food chain.
Key Terms — Flashcard Review
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🎯 Practice Quiz — Test Yourself
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.