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Higher Level Revision Notes
Leaving Cert Higher Level Biology
Chapter 11: Ecology
Ecosystems, energy flow, cycles, sampling
Subtopics Covered
- Learning Outcomes
- Big Picture: Why Ecology Matters
- Core Ecology Terms
- Ecosystem Structure
- Abiotic and Biotic Factors
- Food Chains, Food Webs and Trophic Levels
What This Pack Includes
- Structured chapter notes formatted for ExamsLogic website reading
- Exam-focused diagrams, definitions, and worked examples
- Interactive practice sections carried over from the source notes
- Independent study guidance based on the official curriculum
- Print-friendly layout for future PDF export when needed
This publication is an independent educational resource developed by ExamsLogic and compiled by experienced educators. It is based on publicly available official curricula, including Cambridge, Pearson Edexcel, IB, and the Irish Leaving Certificate. This product is not endorsed by, affiliated with, or sponsored by any examination board or governing authority. All registered trademarks remain the property of their respective owners.
1. Learning Outcomes
- Define ecology, ecosystem, habitat, population, community, niche and biosphere.
- Explain how energy flows through food chains and food webs.
- Interpret pyramids of numbers and biomass.
- Describe competition, predation, parasitism, mutualism and symbiosis.
- Explain the carbon and nitrogen cycles at Higher Level standard.
- Describe ecological sampling using quadrats, transects and capture-mark-recapture.
- Evaluate human impacts on ecosystems and conservation strategies.
HL focus: Link every ecological term to examples, and in cycle questions include named processes such as photosynthesis, respiration, decomposition, nitrogen fixation, nitrification and denitrification.
2. Big Picture: Why Ecology Matters
Relationships
Ecology studies how organisms interact with each other and with non-living factors.
Energy Flow
Energy enters ecosystems mainly through photosynthesis and is transferred through feeding.
Balance
Cycles recycle matter, while human activities can disturb ecological balance.
3. Core Ecology Terms
| Term | Meaning | Example | Exam wording |
|---|---|---|---|
| Ecology | Study of interactions between organisms and their environment. | Studying a woodland ecosystem. | Include biotic and abiotic interactions. |
| Habitat | Place where an organism lives. | Pond, hedgerow, grassland. | Not the same as niche. |
| Population | Members of one species in an area. | All daisies in a field. | Same species only. |
| Community | All populations of different species in an area. | Plants, insects, birds and fungi in a wood. | Living organisms only. |
| Ecosystem | A community plus its physical environment. | Lake ecosystem. | Includes living and non-living components. |
| Niche | Role of an organism in its ecosystem. | Bee as pollinator and nectar feeder. | Role, not location. |
Examiner trap: Habitat means where an organism lives. Niche means what it does in the ecosystem.
4. Ecosystem Structure
5. Abiotic and Biotic Factors
Abiotic Factors
Non-living environmental conditions.
- Light intensity
- Temperature
- pH
- Water availability
- Oxygen concentration
- Soil mineral content
Biotic Factors
Living interactions between organisms.
- Competition
- Predation
- Parasitism
- Disease
- Food availability
- Pollination
Exam Tip: If the factor involves a living organism, it is biotic. If it is a physical or chemical condition, it is abiotic.
6. Food Chains, Food Webs and Trophic Levels
A food chain shows the transfer of energy from one organism to the next through feeding.
Producer→Primary consumer→Secondary consumer→Tertiary consumer→Decomposer
| Trophic level | Role | Example |
|---|---|---|
| Producer | Makes food by photosynthesis. | Grass, algae, trees. |
| Primary consumer | Eats producers. | Rabbit, caterpillar, zooplankton. |
| Secondary consumer | Eats primary consumers. | Frog, small fish. |
| Tertiary consumer | Eats secondary consumers. | Hawk, fox. |
| Decomposer | Breaks down dead organisms and waste. | Bacteria and fungi. |
Common mistake: Arrows in food chains show direction of energy flow, not the direction of eating.
7. Energy Flow and Pyramids
Energy is lost at each trophic level through respiration, heat loss, movement, waste and uneaten parts. This is why food chains are usually short.
HL focus: A pyramid of biomass is usually more reliable than a pyramid of numbers because it represents total mass of living material.
8. Interactions Between Organisms
| Interaction | Description | Example | Effect |
|---|---|---|---|
| Competition | Organisms require the same limited resource. | Plants competing for light. | Can reduce growth or survival. |
| Predation | One organism kills and eats another. | Fox eats rabbit. | Controls population size. |
| Parasitism | Parasite benefits, host is harmed. | Tapeworm in intestine. | Host loses nutrients or health. |
| Mutualism | Both organisms benefit. | Bees pollinate flowers and receive nectar. | Both gain advantage. |
| Symbiosis | Close long-term association between species. | Lichen: fungus and alga. | Often mutually beneficial. |
9. Carbon Cycle
Exam Tip: In carbon cycle answers, mention photosynthesis removes CO₂ from air, while respiration, decomposition and combustion release CO₂.
10. Nitrogen Cycle
| Process | What happens | Organisms involved |
|---|---|---|
| Nitrogen fixation | Atmospheric nitrogen gas is converted into useful nitrogen compounds. | Nitrogen-fixing bacteria in soil or root nodules. |
| Nitrification | Ammonium compounds are converted to nitrites and then nitrates. | Nitrifying bacteria. |
| Assimilation | Plants absorb nitrates and make proteins. | Plants. |
| Feeding | Animals obtain nitrogen by eating plants or other animals. | Consumers. |
| Decomposition | Dead organisms and waste are broken down to ammonium compounds. | Decomposers. |
| Denitrification | Nitrates are converted back to nitrogen gas. | Denitrifying bacteria, often in anaerobic soil. |
Examiner trap: Plants cannot use atmospheric nitrogen gas directly. They absorb nitrates from the soil.
11. Ecological Sampling Methods
| Method | Used for | How it works | Key exam point |
|---|---|---|---|
| Quadrat | Plants or slow-moving organisms. | Place quadrats randomly and count organisms or percentage cover. | Random placement reduces bias. |
| Line transect | Change across a habitat gradient. | Record organisms touching a line. | Useful across zonation, e.g. seashore. |
| Belt transect | Detailed change across a gradient. | Place quadrats at regular intervals along a line. | More detailed than line transect. |
| Capture-mark-recapture | Mobile animals. | Capture, mark, release, recapture and estimate population. | Marks must not harm animals or affect survival. |
HL formula: Estimated population = (number first captured × number second captured) ÷ number marked recaptured.
12. Interactive Simulator: Sampling Method Detective
Choose the Best Method
Scenario: Estimate the number of daisies in a school field.
Feedback
Ready
Choose the sampling method that matches the organism and investigation.
13. Interactive Simulator: Ecology Term Builder
Match the Definition
Definition: The role of an organism in its ecosystem.
Feedback
Ready
Precise definitions are easy marks in ecology.
14. Human Impact and Conservation
Negative Impacts
- Deforestation reduces habitats and biodiversity.
- Pollution damages air, water and soil.
- Overfishing reduces population sizes.
- Invasive species can outcompete native species.
- Greenhouse gas emissions contribute to climate change.
Conservation Strategies
- Protected areas and nature reserves.
- Sustainable harvesting and fishing quotas.
- Habitat restoration.
- Recycling and waste reduction.
- Captive breeding and seed banks.
Examiner Secret: The best conservation answers balance human needs with ecosystem protection. Use the word sustainable when appropriate.
15. High-Value Exam Guidance
Exam Tip 1: Always define ecology terms precisely. Habitat, niche, population and community are common definition questions.
Exam Tip 2: In food web questions, follow arrows carefully. Arrows show energy transfer.
Exam Tip 3: For sampling investigations, mention random sampling, large sample size and repeated measurements.
Examiner Trap 1: Do not say energy is recycled. Matter is recycled; energy flows through ecosystems and is eventually lost as heat.
Examiner Trap 2: Do not confuse biomass with number of organisms. Biomass means total mass of living material.
Examiner Trap 3: Plants absorb nitrates, not nitrogen gas directly.
Common Mistake 1: Writing “animals produce energy.” Correct: organisms release energy from food by respiration.
Common Mistake 2: Forgetting decomposers in food webs and nutrient cycles.
Examiner Secret 1: Ecology questions often combine definitions with data interpretation. Learn terms and practise reading graphs.
Examiner Secret 2: In cycle questions, process names score marks. Use the correct biological terms.
16. MCQs with Instant Answers
1. A habitat is:
Answer: B. A habitat is the place where an organism lives.
2. Which group makes its own food?
Answer: A. Producers make food by photosynthesis.
3. Which method is best for estimating daisies in a field?
Answer: B. Quadrats are suitable for plants or slow-moving organisms.
4. Which process converts nitrates to nitrogen gas?
Answer: B. Denitrifying bacteria convert nitrates to nitrogen gas.
17. Structured Questions
- Define habitat, population, community and ecosystem. [8]
- Explain why energy transfer between trophic levels is inefficient. [4]
- Describe the role of decomposers in the carbon cycle. [3]
- Describe the main stages of the nitrogen cycle. [8]
- A student wants to estimate the number of daisies in a field. Describe a suitable method and explain how bias can be reduced. [6]
- Explain how human activity can reduce biodiversity and suggest two conservation strategies. [6]
18. Mark Scheme
Q1. Habitat: place organism lives [2]; population: members of one species in area [2]; community: all populations of different species in area [2]; ecosystem: community plus physical environment [2].
Q2. Energy lost as heat [1]; respiration [1]; movement [1]; waste/uneaten parts/not all biomass transferred [1].
Q3. Decomposers break down dead organisms/waste [1]; release carbon dioxide by respiration [1]; recycle nutrients/carbon compounds [1].
Q4. Nitrogen fixation [1]; nitrification [1]; plant absorption of nitrates/assimilation [1]; protein formation [1]; feeding transfers nitrogen to animals [1]; death/waste [1]; decomposition to ammonium compounds [1]; denitrification returns nitrogen gas [1].
Q5. Use quadrats [1]; random placement [1]; count daisies/percentage cover [1]; repeat many times [1]; calculate mean [1]; random sampling reduces bias [1].
Q6. Human activity example: deforestation/pollution/overfishing/invasive species/climate change [any 2]; effect on biodiversity explained [2]; conservation strategies such as reserves, sustainable harvesting, habitat restoration, breeding programmes [any 2].
Q2. Energy lost as heat [1]; respiration [1]; movement [1]; waste/uneaten parts/not all biomass transferred [1].
Q3. Decomposers break down dead organisms/waste [1]; release carbon dioxide by respiration [1]; recycle nutrients/carbon compounds [1].
Q4. Nitrogen fixation [1]; nitrification [1]; plant absorption of nitrates/assimilation [1]; protein formation [1]; feeding transfers nitrogen to animals [1]; death/waste [1]; decomposition to ammonium compounds [1]; denitrification returns nitrogen gas [1].
Q5. Use quadrats [1]; random placement [1]; count daisies/percentage cover [1]; repeat many times [1]; calculate mean [1]; random sampling reduces bias [1].
Q6. Human activity example: deforestation/pollution/overfishing/invasive species/climate change [any 2]; effect on biodiversity explained [2]; conservation strategies such as reserves, sustainable harvesting, habitat restoration, breeding programmes [any 2].
19. Mastery Checklist
- I can define key ecology terms precisely.
- I can build and interpret food chains and food webs.
- I can explain energy loss between trophic levels.
- I can compare pyramids of numbers and biomass.
- I can describe the carbon cycle.
- I can describe the nitrogen cycle using correct process names.
- I can select suitable ecological sampling methods.
- I can evaluate human impacts and conservation strategies.