Ecology — GCSE Biology Revision
Everything you need to revise ecology for GCSE Biology: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Ecosystems and Interdependence
An ecosystem is the interaction of a community of living organisms (biotic factors) with the non-living parts of their environment (abiotic factors). Examples of biotic factors include plants, animals, and microorganisms. Abiotic factors include light intensity, temperature, moisture levels, and soil pH. All organisms in an ecosystem are interdependent; a change in one population can affect others. For instance, a decrease in predator numbers might lead to an increase in prey, which could then overgraze plants, impacting the entire food web.
Sampling Techniques and Distribution
Ecologists use sampling techniques to study the distribution and abundance of organisms. Quadrats are square frames used for estimating population size of immobile organisms like plants. They are placed randomly to avoid bias. Transects are lines used to study how species distribution changes across an area, for example, from a shaded to a sunny spot. By systematically placing quadrats along a transect line, changes in species abundance can be observed in relation to abiotic factors.
Food Chains, Webs, and Pyramids
Food chains show the feeding relationships between organisms, starting with a producer (e.g., plant) which makes its own food. Primary consumers (herbivores) eat producers, secondary consumers eat primary consumers, and so on. Food webs show multiple interconnected food chains within an ecosystem, demonstrating complex feeding links. Pyramids of biomass represent the total mass of living material at each trophic level, typically showing a decrease in biomass at higher levels due to energy loss.
Decomposition and Nutrient Cycles
Decomposers, mainly bacteria and fungi, break down dead organic matter and waste products. This process, known as decomposition, returns vital nutrients (like carbon and nitrogen) to the soil, which can then be absorbed by producers. The carbon cycle describes the movement of carbon through the atmosphere, oceans, and living organisms via processes like photosynthesis, respiration, and combustion. The water cycle involves evaporation, condensation, precipitation, and transpiration.
Key facts
- Biotic factors are living components of an ecosystem.|Abiotic factors are non-living components of an ecosystem.|Quadrats and transects are used for ecological sampling.|Producers make their own food, usually through photosynthesis.|Decomposers recycle nutrients by breaking down dead matter.|Energy is lost at each trophic level in a food chain.
Key terms
- Ecosystem::The interaction of a community of living organisms (biotic) with the non-living parts of their environment (abiotic).|Producer::An organism, usually a plant, that produces its own food, typically by photosynthesis.|Consumer::An organism that obtains energy by feeding on other organisms.|Decomposer::An organism, such as bacteria or fungi, that breaks down dead organic matter and waste products.|Biodiversity::The variety of all the different species of organisms on Earth, or within a particular ecosystem.|Trophic level::The position an organism occupies in a food chain.
Common mistakes
- Confusing biotic factors (living) with abiotic factors (non-living).|Drawing food chains incorrectly, e.g., arrow pointing from predator to prey.|Thinking all energy is transferred between trophic levels (actual transfer is about 10%).|Assuming quadrats should always be placed visibly near organisms.
Exam tips
- When asked about food chains, ensure arrows point in the direction of energy flow (from organism being eaten to organism eating it).|For sampling, explain why random sampling is important to avoid bias and ensure validity.|Remember to link environmental changes (abiotic) to their impact on living organisms (biotic) and vice-versa.|Practice calculating estimated population size from quadrat data (mean number per quadrat x total area / quadrat area).
Quick quiz
1. Which of these is an abiotic factor?
- A. Predator
- B. Pathogen
- C. Temperature
- D. Decomposer
Show answer
C. Temperature — Abiotic factors are the non-living parts of an environment. Temperature is a non-living component, while predators, pathogens, and decomposers are living (biotic).
2. In a food chain, what do the arrows represent?
- A. The direction of growth
- B. The flow of energy
- C. The movement of organisms
- D. The increase in population
Show answer
B. The flow of energy — Arrows in a food chain show the transfer of energy from the organism being eaten to the organism that eats it.
3. Which term describes organisms that break down dead organic matter?
- A. Producers
- B. Primary consumers
- C. Decomposers
- D. Predators
Show answer
C. Decomposers — Decomposers, such as bacteria and fungi, are responsible for breaking down dead organisms and waste products, returning nutrients to the ecosystem.
4. Why is random sampling important when using quadrats?
- A. To ensure all species are counted
- B. To make the experiment faster
- C. To avoid bias and make results representative
- D. To only count rare species
Show answer
C. To avoid bias and make results representative — Random sampling ensures that each area has an equal chance of being sampled, which helps to avoid biased results and makes the data more representative of the entire habitat.
5. Which of these is a typical characteristic of a pyramid of biomass?
- A. It gets wider at higher trophic levels.
- B. It shows a decrease in biomass at higher trophic levels.
- C. It always starts with consumers at the bottom.
- D. It represents the number of individuals at each level.
Show answer
B. It shows a decrease in biomass at higher trophic levels. — Pyramids of biomass generally show a decrease in total mass of living material at successive trophic levels because only about 10% of energy is transferred, meaning less biomass can be supported.
Exam-style questions
Describe how to use a quadrat to estimate the population size of daisies in a school field. (4 marks) [4 marks]
Show mark scheme
- Place quadrats randomly across the field (e.g., using random numbers for coordinates).
- Count the number of daisies within each quadrat.
- Calculate the mean number of daisies per quadrat.
- Multiply the mean by the total area of the field and divide by the area of one quadrat.
Explain the role of decomposers in nutrient cycling within an ecosystem. (3 marks) [3 marks]
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- Decomposers (e.g., bacteria/fungi) break down dead organic matter and waste products.
- They release nutrients (e.g., mineral ions, carbon) back into the soil or atmosphere.
- These nutrients are then available for uptake by producers (e.g., plants) for growth.
A food chain is grass → rabbit → fox. Explain why the biomass of foxes is much less than the biomass of grass. (4 marks) [4 marks]
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- Energy is transferred from one trophic level to the next.
- Only about 10% of the energy from one level is transferred to the next.
- Energy is lost at each level through respiration, movement, and heat.
- Also, not all parts of an organism are eaten or digested, so energy is lost in waste products. Less energy means less biomass can be supported.
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