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Ordinary Level Revision Notes
Leaving Cert Ordinary Level Biology
Chapter 4: General Principles of Ecology
Study Leaving Cert Ordinary Level Biology General Principles of Ecology with ExamsLogic revision notes, diagrams, summaries, and exam-focused practice.
Subtopics Covered
- Chapter Overview
- Learning Outcomes
- Ecological Hierarchies
- Environmental Factors
- Energy Flow in Ecosystems
- Trophic Levels and Pyramids of Numbers
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. Chapter Overview
Ecology is the study of how organisms interact with each other and with their environment. This chapter explains ecological terms, environmental factors, food chains, trophic levels, pyramids of numbers, nutrient recycling and human impacts on ecosystems.
ExamsLogic Big Idea: In ecology, always think in three directions: organisms, their environment, and how energy or nutrients move through the ecosystem.
2. Learning Outcomes
- I can define biosphere, ecosystem, habitat, niche and population.
- I can distinguish abiotic, biotic, climatic and edaphic factors.
- I can explain energy flow from the Sun through food chains.
- I can identify producers, consumers, decomposers and trophic levels.
- I can interpret pyramids of numbers.
- I can outline the carbon cycle and nitrogen cycle.
- I can explain human impact through pollution, conservation and waste management.
3. Ecological Hierarchies
Ecological terms describe different levels of organisation in nature.
| Term | Meaning | Example |
|---|---|---|
| Biosphere | All parts of Earth where life exists. | Land, water and atmosphere containing organisms. |
| Ecosystem | A community of organisms interacting with each other and with the physical environment. | Pond, woodland, grassland. |
| Habitat | The place where an organism lives. | A fox living in woodland. |
| Niche | The role of an organism in its ecosystem. | A bee pollinating flowers while feeding on nectar. |
| Population | All members of one species living in an area. | All rabbits in one field. |
Examiner Trap: Habitat and niche are not the same. Habitat is where an organism lives; niche is what it does in the ecosystem.
4. Environmental Factors
Environmental factors affect where organisms live, how many survive, and how ecosystems function.
| Factor type | Meaning | Examples |
|---|---|---|
| Abiotic factors | Non-living factors in the environment. | Light, temperature, water, pH, oxygen availability. |
| Biotic factors | Living factors or interactions with organisms. | Competition, predation, disease, food availability. |
| Climatic factors | Weather and climate-related factors. | Rainfall, wind, temperature, humidity. |
| Edaphic factors | Soil-related factors. | Soil pH, soil type, mineral content, water content. |
Examiner Tip: Edaphic means soil-related. If the question mentions soil pH or mineral content, it is an edaphic factor.
5. Energy Flow in Ecosystems
The Sun is the primary source of energy for most ecosystems. Producers capture light energy by photosynthesis. This energy then moves through food chains to consumers and decomposers.
| Term | Meaning | Example |
|---|---|---|
| Producer | Organism that makes its own food. | Grass, algae, green plants. |
| Herbivore | Consumer that eats plants. | Rabbit, cow, caterpillar. |
| Carnivore | Consumer that eats animals. | Fox, hawk, lion. |
| Omnivore | Consumer that eats plants and animals. | Human, pig, crow. |
| Decomposer | Organism that breaks down dead organisms and waste. | Bacteria and fungi. |
Food Chain Example: Grass → Rabbit → Fox. Grass is the producer, rabbit is the primary consumer and fox is the secondary consumer.
6. Trophic Levels and Pyramids of Numbers
A trophic level is a feeding level in a food chain. A pyramid of numbers shows the number of organisms at each trophic level.
| Trophic Level | Role | Example |
|---|---|---|
| 1 | Producer. | Grass. |
| 2 | Primary consumer. | Rabbit. |
| 3 | Secondary consumer. | Fox. |
| 4 | Tertiary consumer. | Hawk. |
Student Common Mistake: Arrows in food chains show the direction of energy flow, not simply “who eats who”.
7. Nutrient Recycling
Energy flows through ecosystems and is lost as heat, but nutrients are recycled. The carbon cycle and nitrogen cycle are two major nutrient cycles.
The Carbon Cycle
| Process | Effect on carbon dioxide |
|---|---|
| Photosynthesis | Removes carbon dioxide from the atmosphere. |
| Respiration | Releases carbon dioxide into the atmosphere. |
| Combustion | Releases carbon dioxide when fuels are burned. |
| Decomposition | Releases carbon compounds as dead organisms and waste break down. |
The Nitrogen Cycle
Nitrogen is needed to make proteins and nucleic acids. Most organisms cannot use nitrogen gas directly, so bacteria are very important in the nitrogen cycle.
| Process | Meaning |
|---|---|
| Nitrogen fixation | Nitrogen gas is converted into usable nitrogen compounds. |
| Nitrification | Ammonium compounds are converted into nitrates. |
| Assimilation | Plants absorb nitrates and use them to make proteins. |
| Decomposition | Dead organisms and waste release nitrogen compounds back into soil. |
| Denitrification | Nitrates are converted back into nitrogen gas. |
Examiner Tip: For nitrogen cycle questions, remember bacteria. Many nitrogen cycle processes are carried out by bacteria.
8. Human Impact on Ecosystems
Human activities can damage ecosystems, but careful conservation and waste management can reduce harm.
| Human Impact | Cause | Effect | Control |
|---|---|---|---|
| Water pollution | Sewage, fertilisers, chemicals. | Eutrophication, reduced oxygen, death of aquatic organisms. | Sewage treatment, reduce fertiliser runoff. |
| Air pollution | Vehicles, factories, burning fossil fuels. | Respiratory problems, acid rain, climate change. | Cleaner fuels, filters, public transport. |
| Habitat destruction | Deforestation, urbanisation, agriculture. | Loss of biodiversity. | Protected areas, reforestation, sustainable land use. |
| Waste accumulation | Overuse of plastics and poor disposal. | Pollution and harm to wildlife. | Reduce, reuse, recycle, safe disposal. |
Conservation: Conservation means protecting organisms, habitats and ecosystems to maintain biodiversity and ecosystem stability.
Waste Management: Waste management reduces environmental damage by reducing waste production, reusing materials, recycling and safely disposing of harmful substances.
9. Common Student Mistakes
- Confusing habitat with niche.
- Confusing abiotic and biotic factors.
- Forgetting edaphic means soil-related.
- Thinking arrows in a food chain show “eats” instead of energy flow.
- Forgetting decomposers recycle nutrients.
- Saying energy is recycled. Nutrients are recycled, but energy flows and is lost as heat.
- Mixing up photosynthesis and respiration in the carbon cycle.
- Forgetting plants usually absorb nitrogen as nitrates.
10. Examiner Traps
Trap 1: The Sun is the primary source of energy for most ecosystems.
Trap 2: Pyramids of numbers show number of organisms, not energy.
Trap 3: Energy is not recycled. Nutrients are recycled.
Trap 4: Nitrogen gas cannot be used directly by most plants.
Trap 5: Decomposers are usually bacteria and fungi.
11. Examiner Secrets
Secret 1: In definitions of ecosystem, include both organisms and physical environment.
Secret 2: In food chain questions, label producer, primary consumer and secondary consumer.
Secret 3: For pollution questions, structure your answer as cause → effect → control.
Secret 4: For nutrient cycles, learn the key verbs: photosynthesis, respiration, combustion, decomposition, fixation, nitrification, assimilation, denitrification.
12. Section A: Multiple Choice Practice
- What is the place where an organism lives called?
A. Niche B. Habitat C. Biosphere D. Population - Which of the following is an abiotic factor?
A. Predation B. Competition C. Temperature D. Disease - What is the primary source of energy for most ecosystems?
A. Soil B. Water C. The Sun D. Decomposers - Which organisms break down dead material?
A. Producers B. Decomposers C. Herbivores D. Carnivores - Which process removes carbon dioxide from the atmosphere?
A. Respiration B. Combustion C. Photosynthesis D. Decomposition
13. Section B: Structured Practice Questions
- Distinguish between habitat and niche. [4 marks]
- Give two abiotic factors and two biotic factors affecting an ecosystem. [4 marks]
- Explain why the Sun is the primary source of energy for most ecosystems. [3 marks]
- Construct a food chain with three trophic levels and label each trophic level. [6 marks]
- Explain the role of decomposers in nutrient recycling. [3 marks]
- Describe two processes in the carbon cycle. [4 marks]
- Describe the importance of bacteria in the nitrogen cycle. [4 marks]
- Give one cause, one effect and one control of water pollution. [3 marks]
14. Mark Scheme Answers
Section A:
1. B 2. C 3. C 4. B 5. C
Section B:
Q1. Habitat = place where organism lives [2]; niche = role of organism in ecosystem [2].
Q2. Two abiotic factors such as temperature/light/pH/water/oxygen [2]; two biotic factors such as predation/competition/disease/food availability [2].
Q3. Sun provides light energy [1]; producers use it in photosynthesis [1]; energy enters food chains [1].
Q4. Correct food chain with three organisms [3]; producer, primary consumer and secondary consumer labelled [3].
Q5. Decomposers break down dead organisms/waste [1]; release nutrients [1]; nutrients are reused by plants/organisms [1].
Q6. Any two explained: photosynthesis removes CO2, respiration releases CO2, combustion releases CO2, decomposition releases carbon compounds [4].
Q7. Bacteria carry out nitrogen fixation/nitrification/decomposition/denitrification [any two explained, 4].
Q8. Cause such as sewage/fertilisers [1]; effect such as eutrophication/oxygen loss/death of fish [1]; control such as sewage treatment/reducing runoff [1].
1. B 2. C 3. C 4. B 5. C
Section B:
Q1. Habitat = place where organism lives [2]; niche = role of organism in ecosystem [2].
Q2. Two abiotic factors such as temperature/light/pH/water/oxygen [2]; two biotic factors such as predation/competition/disease/food availability [2].
Q3. Sun provides light energy [1]; producers use it in photosynthesis [1]; energy enters food chains [1].
Q4. Correct food chain with three organisms [3]; producer, primary consumer and secondary consumer labelled [3].
Q5. Decomposers break down dead organisms/waste [1]; release nutrients [1]; nutrients are reused by plants/organisms [1].
Q6. Any two explained: photosynthesis removes CO2, respiration releases CO2, combustion releases CO2, decomposition releases carbon compounds [4].
Q7. Bacteria carry out nitrogen fixation/nitrification/decomposition/denitrification [any two explained, 4].
Q8. Cause such as sewage/fertilisers [1]; effect such as eutrophication/oxygen loss/death of fish [1]; control such as sewage treatment/reducing runoff [1].
15. Quick Revision Card
- Habitat: Where an organism lives.
- Niche: Role of an organism.
- Abiotic: Non-living factor.
- Biotic: Living factor.
- Edaphic: Soil-related factor.
- Producer: Makes its own food.
- Consumer: Eats other organisms.
- Decomposer: Breaks down dead matter and waste.
- Energy: Flows and is lost as heat.
- Nutrients: Recycled through ecosystems.
- Human impact: Cause → effect → control.
16. Interactive Simulator: Ecosystem Builder
Identify the Ecological Role
Clue: Grass makes its own food by photosynthesis.
Examiner Feedback
Ready
Choose the ecological role or term that matches the clue.
Score: 0/0
17. Mastery Checklist
- I can define biosphere, ecosystem, habitat, niche and population.
- I can distinguish abiotic, biotic, climatic and edaphic factors.
- I can explain energy flow through food chains.
- I can identify producers, consumers and decomposers.
- I can explain trophic levels and pyramids of numbers.
- I can outline the carbon cycle.
- I can outline the nitrogen cycle.
- I can explain human impacts on ecosystems using cause, effect and control.
- I can answer OL exam questions on ecology.