Ecology & Ecosystems – A-Level Biology Revision Notes

Why Ecology Catches Students Out – An Examiner’s Perspective

Ecology looks like one of the more accessible topics in A-Level Biology. The language feels familiar – food chains, habitats, populations – and students often assume they can rely on general knowledge. This is precisely why ecology questions produce some of the worst answers I see as an examiner.

The problem is not complexity. It is precision. Ecology questions are built around causal chains where every step must cause the next. When a student writes about eutrophication, I am looking for a sequence of six or seven linked events, each following logically from the previous one. Skipping a step, reversing the order, or using vague language costs marks immediately. The same applies to succession, energy transfer calculations, and the nitrogen cycle.

Having marked ecology questions for both WJEC/Eduqas and Edexcel, I can tell you that the Eduqas 2024 examiner report flagged exactly this: students could not express ecological definitions precisely, struggled to relate NPP to photosynthesis, and many still found basic percentage calculations challenging. The OCR A 2023 report highlighted similar problems with applying knowledge to unfamiliar ecological contexts and choosing appropriate statistical tests.

On this page, I am going to walk you through every ecology topic your exam board requires – and more importantly, I am going to show you how each area is marked. This is not textbook summary. This is what I look for when I am sitting with a mark scheme and a pile of scripts.

Key Ecological Terminology – Definitions That Earn Marks

Every ecology question starts with terminology. Examiners test definitions directly and penalise imprecise language in extended responses. The Eduqas 2024 examiner report specifically flagged that students could not express ecological definitions from Component 1 accurately in Component 2 – synoptic recall of these terms is essential.

Energy Transfer Through Ecosystems – GPP, NPP and Efficiency

Energy transfer is one of the most calculation-heavy areas of ecology and one where examiners apply very strict marking conventions. You need to understand the equations, the correct units, and – critically – the correct language. Saying energy is “produced” or “lost” will cost you marks.

The Core Equations

Light energy enters an ecosystem through photosynthesis. The total amount of energy fixed by producers (converted from light energy to chemical energy in organic molecules) is called gross primary productivity (GPP). Producers use some of this energy for their own metabolic processes – primarily respiration. The energy that remains, stored in plant biomass, is called net primary productivity (NPP).

For consumers, the equivalent equation describes how much energy is incorporated into new biomass at each trophic level:

Why Only ~10% of Energy Transfers Between Trophic Levels

The efficiency of energy transfer between trophic levels is typically around 10%. This is because energy is transferred to the surroundings as heat during respiration at every trophic level, some material is not consumed (roots, bones), and some is lost in faeces and urine (material that was ingested but not assimilated).

Farming Practices and Energy Efficiency (AQA Specific)

AQA specifically asks how farming practices increase the efficiency of energy transfer. The key principle is reducing respiratory losses and preventing energy transfer to competing organisms:

• Restricting movement of livestock reduces energy transferred as heat through respiration (less muscle contraction)
• Keeping animals warm reduces energy transferred as heat to maintain body temperature (less thermoregulation)
• Removing competing organisms through pesticides and herbicides ensures more energy from producers reaches the target consumer
• Simplifying food webs – monocultures reduce the number of trophic levels, meaning less energy is transferred to the surroundings at each step

Ecological Succession – How Examiners Mark Each Step

Succession questions consistently appear as high-mark extended response questions (4–6 marks) on every exam board. Having marked hundreds of these, I can tell you that the difference between a Level 1 answer and a Level 3 answer is always the same: mechanism versus description. Students who describe what happens score poorly. Students who explain the mechanism – how each stage causes the next – score full marks.

Primary Succession – The Full Sequence Examiners Expect

Primary succession begins on bare, lifeless substrate with no soil – volcanic rock, new islands, exposed rock faces, sand dunes. The mark scheme expects you to follow this logical chain:

Secondary Succession

Secondary succession occurs where soil already exists – after fire, deforestation, abandonment of farmland, or flooding. Because soil is already present, secondary succession skips the pioneer stage on bare rock and begins at a later seral stage with grasses and herbaceous plants. It is significantly faster than primary succession.

Deflected Succession and Plagioclimax

When human activity (or other persistent factors) prevents a community from reaching its natural climatic climax, the resulting stable community is called a plagioclimax. This is maintained by deflected succession.

Examples include: moorland maintained by regular burning and grazing (would naturally become woodland), hay meadows maintained by annual mowing, and chalk grassland maintained by grazing sheep.

Eutrophication – The Exact Causal Chain for Full Marks

Eutrophication is one of the most frequently examined topics across all boards, and the mark scheme is brutally sequential. Each step must cause the next. Jumping from “fertilisers enter water” directly to “fish die” without the intermediate mechanism will typically earn 1 mark out of 5. Here is the complete sequence as it appears in mark schemes:

The Nitrogen Cycle – Which Bacteria Your Board Requires

The nitrogen cycle is examined differently across boards, and this catches students who switch between specifications or use general revision resources. The sharpest divide is whether your board requires you to name specific bacteria. Getting this wrong wastes valuable revision time or, worse, leaves gaps in your knowledge.

The Four Key Processes

Which Board Requires Which Bacteria?

The Phosphorus Cycle (AQA Only)

AQA is the only UK A-Level Biology board that explicitly requires the phosphorus cycle. Unlike nitrogen, the phosphorus cycle has no gaseous phase – phosphorus moves only through rock, soil, water and organisms.

Phosphate ions (PO₄³⁻) are released from rocks by weathering. Plants absorb phosphate ions from soil solution through their roots. Phosphorus passes through food chains in organic molecules (ATP, DNA, phospholipids). Decomposition returns phosphate to the soil. Mycorrhizae – mutualistic associations between fungi and plant roots – significantly increase phosphorus absorption by extending the effective root surface area.

Sampling Methods – When to Use Each Technique

The Eduqas 2024 examiner report specifically flagged that many candidates confused when to use transects versus randomly positioned quadrats. Choosing the wrong method, or failing to justify your choice, costs marks on practical methodology questions. Here is when to use each technique.

Quadrats (Random Sampling)

Used to estimate the abundance and distribution of sessile (non-moving) organisms across a habitat. Place a grid of numbered coordinates over the study area and use a random number generator to select positions for quadrat placement. Record frequency, density or percentage cover for each species. Calculate a mean from multiple quadrats.

Transects (Systematic Sampling)

Used to study changes in species distribution along an environmental gradient – for example, from the edge of a woodland to open grassland, or across a rocky shore from splash zone to low tide. A line transect records species touching the line at regular intervals. A belt transect uses continuous or interrupted quadrats along the line for more detailed data.

Mark-Release-Recapture (Motile Organisms)

Used to estimate the population size of mobile animals that cannot be counted directly. The method uses the Lincoln Index:

The Five Assumptions – And What Happens When They Fail

Examiners do not just want you to list assumptions. They want you to explain the consequences of each assumption being violated. This is where most students lose marks:

Conservation vs Preservation – A Distinction Examiners Test Directly

Many students use “conservation” and “preservation” interchangeably. They are not the same thing, and examiners test the distinction directly, particularly on OCR A and Edexcel B papers.

The key distinction is: conservation manages ecosystems (human involvement is planned and ongoing); preservation protects ecosystems (human involvement is minimised or excluded). OCR A mark schemes define conservation as maintaining “ecosystem/biodiversity/species/habitats” through “active/sustainable management” and preservation as leaving “ecosystems/habitats undisturbed.”

Exam Board Comparison – What YOUR Board Requires

This is the table I wish every student would check before starting ecology revision. The differences between boards are substantial – learning content your board does not examine wastes time, and missing content it does examine costs marks.

10 Common Mistakes Examiners See Every Year

These are not hypothetical errors. These are mistakes I see repeatedly when marking ecology scripts, confirmed by examiner reports from AQA, OCR, WJEC/Eduqas and Edexcel. If you can eliminate these ten errors, your ecology marks will improve significantly.