A-Level · Biology · AQA · Mark scheme decoded

AQA A-Level Biology: Ecosystems, Populations, and Succession — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Biology specificationlast verified 3 July 2026

The short answer

In A-Level Biology, understanding ecosystems, populations, and succession is crucial for grasping how living organisms interact with their environment and each other. This section delves into the concepts of communities, ecosystems, niches, carrying capacity, population dynamics, and ecological succession.

The question

A biologist uses the mark-release-recapture method to estimate the population size of a certain bird species. In the first capture, 50 birds are marked and released. After one week, 60 birds are captured again, and 12 of them are found to be marked. Estimate the total population size.

[Paraphrased for study — not reproduced from any exam paper.]

4 marks

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

How marks are awarded

For calculation questions, marks are typically awarded for correct substitution of values into formulas, accurate arithmetic, and providing the final answer with appropriate units. For conceptual questions, marks are given for clear and concise explanations that demonstrate a deep understanding of the topic.

What the command words demand

Calculate: Perform a numerical calculation using given data and appropriate formulas.
Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Determine: Find or derive a specific value or quantity based on given information.
Analyze: Examine and interpret data or graphs to draw conclusions about the behavior of the system.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, perform calculations, and check your work.

Identify the given values.0 marks
Number of marked birds (M) = 50Total number of birds recaptured (C) = 60Number of marked birds in the recapture (R) = 12
Use the mark-release-recapture formula to estimate population size.1 mark
Population size (N) = (M × C) / R
Substitute the values into the formula.1 mark
N = (50 × 60) / 12
Perform the multiplication and division.2 marks
N = 3000 / 12N = 250

Final answer: 250 birds

Work through every step correctly and you earn all 4 marks.

Another worked example

A study is conducted to estimate the population of a non-motile plant species using quadrats. In a 10 m 2 area, 10 quadrats are placed randomly, and the number of plants in each quadrat is counted as follows: 5, 7, 6, 8, 4, 9, 6, 7, 5, 8. Calculate the average population density per m 2 .

3 marks

Sum the number of plants in all quadrats.1 mark
Total plants = 5 + 7 + 6 + 8 + 4 + 9 + 6 + 7 + 5 + 8Total plants = 65
Calculate the average population density per m 2 .2 marks
Average density = Total plants / (Number of quadrats × Area of each quadrat)Average density = 65 / (10 × 1)Average density = 6.5 plants/m 2

Final answer: 6.5 plants/m 2

Work through every step correctly and you earn all 3 marks.

Common mistakes

Confusing community with ecosystem.
Why it happens: Students sometimes mix up the definitions of community and ecosystem, leading to incorrect answers in questions involving interactions between species and their environment.
Fix: Remember that a community consists of populations of different species interacting with each other, while an ecosystem includes both biotic and abiotic components.
Misunderstanding the concept of carrying capacity.
Why it happens: Students may not fully grasp that carrying capacity is the maximum population size an environment can support indefinitely, leading to errors in population dynamics questions.
Fix: Carrying capacity is influenced by factors such as food availability, space, and abiotic conditions. When a population exceeds its carrying capacity, it may experience resource depletion and a decrease in size.
Incorrectly using the mark-release-recapture formula.
Why it happens: Students may use the wrong values or make arithmetic errors when applying the formula to estimate population sizes.
Fix: Always use the correct formula: Population size (N) = (Number of marked individuals × Total number recaptured) / Number of marked individuals in the recapture. Double-check your calculations for accuracy.
Failing to consider both abiotic and biotic factors in population dynamics.
Why it happens: Students may focus only on one type of factor, leading to incomplete or incorrect explanations of population changes.
Fix: Always consider both abiotic (e.g., temperature, precipitation) and biotic (e.g., competition, predation) factors when explaining population dynamics. Both types of factors can influence population sizes and interactions.
Misinterpreting data from quadrat or transect studies.
Why it happens: Students may not understand how to use data from quadrats or transects to estimate population densities, leading to incorrect calculations.
Fix: Practice using quadrat and transect data to calculate average population densities. Ensure you are summing the number of individuals in all quadrats and dividing by the total area covered.
Failing to explain the stages of ecological succession clearly.
Why it happens: Students may struggle to articulate the different stages of succession, leading to vague or incorrect descriptions.
Fix: Practice explaining the process of succession step-by-step. Start with pioneer species colonizing a new area, then describe how these species alter the environment to make it more suitable for other species, and finally, explain the development of a climax community.

Where the marks go

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question type	What you’re asked to do	Marks
Mark-Release-Recapture Calculation	Estimate a population size using the mark-release-recapture formula with the given capture data.	4
Population Density Calculation	Sum quadrat counts and calculate the average plant density per square metre.	3
Carrying Capacity Explanation	Describe and explain population change relative to carrying capacity after improved environmental conditions.	4
Calculate Percentage Increase	Work out the percentage rise in population size from an initial to a final value.	3
Total across these question types		14

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.