A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Ecosystems, Populations, and Succession — mark scheme explained
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.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the given values.
- S2
Number of marked birds (M) = 50
- S3
Total number of birds recaptured (C) = 60
- S4
Number of marked birds in the recapture (R) = 12
- S5
Step 2: Use the mark-release-recapture formula to estimate population size.
- S6
Population size (N) = (M × C) / R
- S7
Step 3: Substitute the values into the formula.
- S8
N = (50 × 60) / 12
- S9
Step 4: Perform the multiplication and division.
- S10
N = 3000 / 12
- S11
N = 250
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the given values.
- S2
Number of marked birds (M) = 50
- S3
Total number of birds recaptured (C) = 60
- S4
Number of marked birds in the recapture (R) = 12
- S5
Step 2: Use the mark-release-recapture formula to estimate population size.
- S6
Population size (N) = (M × C) / R
- S7
Step 3: Substitute the values into the formula.
- S8
N = (50 × 60) / 12
- S9
Step 4: Perform the multiplication and division.
- S10
N = 3000 / 12
- S11
N = 250
Final answer: 250 birds
Common mistakes
- Confusing community with ecosystem. — 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. — 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. — 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. — 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. — 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. — 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
- Full worked solution (all marking points)4 marks