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3.7.4
Populations in Ecosystems
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
36
Total marks
85
Marks / Q
2.4
Accessibility
71.0%
Mastery
36.3%
Student strength
56.4%
01
At a glance
3.7.4 · Populations in Ecosystems
8YRSYNTHESIS
3.7.4 (Populations in Ecosystems) appeared in 8 of the 8 years between 2017 and 2024, contributing 36 questions and 85 marks across Papers 1, 2 and 3. APPLICATION dominates the mark distribution at 52.9% of total marks. The accessibility–mastery gap sits at 34.7 percentage points (71.0% vs 36.3%) — most students reach partial credit, but full marks remain harder to secure. The largest single question observed is worth 5 marks, signalling that AQA expects complete hierarchical accounts in this sub-section. Mastery varied year-to-year, lowest in 2019 (27.0%) and highest in 2023 (54.5%).
Access–mastery gap
+35 pp
Lowest mastery
2019 · 27.0%
Highest mastery
2023 · 54.5%
02
Question type split
By marks · compound to dominant
85MARKS
85
marks
Application52.9%45 marks
Knowledge30.6%26 marks
Calculation16.5%14 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
21TERMS
Tier 1 · Always credit
climax community
Tier 2 · Sometimes credit
Lincoln indexlife spanpioneer speciesbiodiversityrandom coordinatesquadratlarge samplemeaninterspecific competitionnot visible to predators
Reject · Never credit
to fit on scalenumbers are largemore accuratemore comparableblocks all of the lightmark-release-recapture (not for plants); 'scale up' alone (must specify how); 'several' quadrats (insufficient — must say large sample or ≥10)'more food' alone (must be 'more food sources')'results are significant/not significant' alone (disqualifies first mark)'harder to capture fish' (without specifying marked fish)maximum capacity; habitat; population; ecosystem
04
Question patterns
Recurring formats & tariff structure
0PARAGRAPHS
05
Year-over-year presence
P1 + P3 · 2017–2024
8YEARS
| Year | Questions | Total marks | Mean accessibility | Mean mastery |
|---|---|---|---|---|
| 2017 | 6 | 11 | 62.5% | 30.7% |
| 2018 | 6 | 12 | 66.3% | 39.7% |
| 2019 | 2 | 9 | 90.5% | 27.0% |
| 2020 | 3 | 6 | — COVID | — COVID |
| 2021 | 7 | 20 | — COVID | — COVID |
| 2022 | 3 | 5 | 62.3% | 45.0% |
| 2023 | 2 | 4 | 91.5% | 54.5% |
| 2024 | 7 | 18 | 74.6% | 32.0% |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
26TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| climax community | 3 | 2018, 2019, 2024 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| Lincoln index | 2 | 2017, 2024 | |
| life span | 2 | 2017 | |
| pioneer species | 2 | 2018, 2019 | |
| biodiversity | 2 | 2019, 2023 | |
| random coordinates | 2 | 2019, 2024 | |
| quadrat | 2 | 2019, 2024 | |
| large sample | 2 | 2019, 2024 | |
| mean | 2 | 2019, 2024 | |
| interspecific competition | 2 | 2020, 2024 | |
| not visible to predators | 2 | 2022, 2024 |
Commonly rejected language
| Term | Times rejected | Years | Why rejected |
|---|---|---|---|
| to fit on scale | 1 | 2018 | |
| numbers are large | 1 | 2018 | |
| more accurate | 1 | 2018 | |
| more comparable | 1 | 2018 | |
| blocks all of the light | 1 | 2018 | |
| mark-release-recapture (not for plants); 'scale up' alone (must specify how); 'several' quadrats (insufficient — must say large sample or ≥10) | 1 | 2019 | |
| 'more food' alone (must be 'more food sources') | 1 | 2021 | |
| 'results are significant/not significant' alone (disqualifies first mark) | 1 | 2021 | |
| 'harder to capture fish' (without specifying marked fish) | 1 | 2021 | |
| maximum capacity; habitat; population; ecosystem | 1 | 2023 | |
| throwing quadrats | 1 | 2024 | |
| Punnett squares | 1 | 2024 | |
| 'scale up' alone | 1 | 2024 | |
| intraspecific | 1 | 2024 | |
| direct competition | 1 | 2024 |
Marks in this sub-section are typically awarded for precise terminology and correct application of biological principles. Sequential mark schemes — where each mark requires building on the previous one — are common in multi-mark questions; stating the first step without progression rarely earns more than one mark. Calculation marks are typically split between method (correct setup and value extraction) and answer (accurate numerical result), allowing partial credit when arithmetic errors occur.
07
Where students lose marks
Examiner-anchored error patterns
4CASE STUDIES
Conceptual errors
- Sterile male mosquitoes believed to transmit disease — in 2017, a significant number of students described sterile males as a risk because they could still carry and transmit the virus to humans; sterile males cannot transmit disease because they do not bite; only female mosquitoes feed on blood; the correct mechanism by which the sterile male technique reduces the population is through competitive mating — sterile males out-compete wild males for females, reducing viable egg production (2017 P2 Q02.1)
- Mark-release-recapture proposed for sessile plant populations — in 2019, some students suggested using MRC to estimate marsh plant population size; MRC assumes animals move freely between the two sampling events so that marked individuals mix throughout the population; plants cannot disperse after marking and so do not mix, invalidating the equal-probability-of-recapture assumption; the correct method is systematic or random quadrat sampling with scaling up (2019 P2 Q09.1)
- Succession described without stating that species change their own environment — in 2019, "more hospitable" or "more habitable" were not credited; the mark required explaining that each seral community modifies abiotic conditions — increases soil depth and organic matter, reduces light intensity — such that it becomes outcompeted by the next community; the mechanism is species-driven environmental change, not a passive improvement in conditions (2019 P2 Q01.1)
- "Carrying capacity" replaced by "maximum capacity" — in 2023, "maximum capacity" was rejected; "carrying" is the specific ecological term for the maximum population size an environment can sustain given its available resources; substituting "maximum" loses the ecological precision the mark requires (2023 P2 Q07.1)
Vocabulary errors
- Log scale justification given as "to fit on scale," "numbers are large," or "more accurate" — in 2018, all three of these phrasings were explicitly rejected; the credited reason for using a logarithmic scale is that it allows comparison of populations with very different absolute sizes or that it linearises exponential growth to make rate comparisons visible; vague appeals to scale or accuracy do not earn the mark (2018 P2 Q03.1)
- "Habitat" or "population" written instead of "niche" or "community" — in 2023, these substitutions were rejected; the question required the specific ecological vocabulary distinguishing an organism's functional role and resource use (niche) from the assemblage of species in an area (community); using the wrong tier of the ecological hierarchy loses both marks (2023 P2 Q07.1)
- "Scale up" stated without specifying the calculation method — in both 2019 and 2024, "scale up" alone was rejected; the mark required specifying that the mean count per quadrat is multiplied by the number of quadrats that fit into the total area; giving the intention without the method scored zero for that mark point (2019 P2 Q09.1, 2024 P2 Q05.1)
- Interspecific competition called intraspecific — in 2024, this substitution was rejected; interspecific competition occurs between different species competing for the same resource; intraspecific competition occurs within a species; these produce different ecological predictions and cannot be interchanged (2024 P2 Q05.2)
Application errors
- MRC precaution given as a reason or assumption rather than as an action — in 2022, students described why a precaution matters or stated that a condition must be met, rather than describing what the investigator should do; the mark required a practical action ("allow time between sampling events for marked individuals to disperse") rather than a statement that the population must be closed or that marking must not affect survival (2022 P2 Q05.2)
- Sand dune succession x-axis read as distance from shore rather than time since exposure — in 2018, students read the progression from right to left on the succession diagram, identifying trees as pioneer species and grasses as the climax community; the axis represented increasing time or distance from the sea, with pioneers at the exposed seaward end and climax woodland at the inland end; reversing the sequence produced answers that were the exact inverse of the correct account (2018 P2 Q04.2)
- Interspecific competition not linked to photosynthesis or a specific organic molecule — in 2024, stating that species competed for "light" was insufficient; the mark required linking reduced light to reduced photosynthesis and to a specific consequence such as less glucose, less ATP, or reduced growth; stopping at "light" left the mechanism incomplete (2024 P2 Q05.2)
- Random quadrat placement described as physically throwing quadrats — in 2024, "throwing quadrats" was rejected; random placement requires using a random number generator or random coordinate table to select positions, not arbitrary physical throwing, which may introduce investigator bias toward accessible or visually prominent areas (2024 P2 Q05.1)
High-impact failures · examiner narrative
2017 P2 Q02.12 marks
Why releasing sterile males reduces the mosquito population. Only approximately 10% of students scored both marks. The dominant error was believing that sterile males could still transmit disease or that "the population eventually dies out" without explaining the mechanism. The correct account requires two linked steps: sterile males compete with wild males for females, and matings with sterile males produce no viable offspring, so population size falls over successive generations. Students who answered in terms of disease transmission were not answering the question at all.
2018 P2 Q04.24 marks
Sand dune succession from a figure. Only 7% scored maximum marks. The most frequent error was reading the x-axis of the succession diagram in reverse — identifying trees as pioneer species and grasses as the climax community. Students who read the axis correctly still lost marks by omitting the role of abiotic change: the credited account required stating that each seral community alters soil depth, organic content, or light availability, enabling the next successional community to establish. Describing what species are present at each stage without explaining the mechanism of replacement scored at most two of the four marks.
2019 P2 Q09.15 marks
Sampling method for a marsh plant population. Only 7% scored maximum marks. Two errors dominated: proposing mark-release-recapture (rejected, as plants cannot disperse) and writing "scale up" without specifying that the mean count per quadrat must be multiplied by the number of quadrats fitting the total area. Students who named quadrats and random sampling but stopped before describing the calculation step scored three marks at most. The question required a complete operational description from random placement through to the final population estimate.
2024 P2 Q05.23 marks
Interspecific competition between two plant species. Only 10% scored maximum marks. Three errors clustered: calling the interaction intraspecific rather than interspecific; stating competition for light without linking reduced light to reduced photosynthesis; and omitting a named organic molecule when the question asked for a specific consequence of reduced photosynthesis. The mark scheme awarded one mark for naming interspecific competition, one for linking light reduction to photosynthesis, and one for naming a specific product (glucose, ATP) whose reduction caused slower growth or lower fitness.
08
Difficulty signals
Performance metric synthesis
35PP GAP
Mean accessibility
71.0%
Mean mastery
36.3%
Mean student strength
56.4%
The accessibility–mastery gap of 34.7 percentage points characterises this sub-section's difficulty profile. Most students reach partial credit; full marks remain harder to achieve. Within 3.7 (Genetics, populations, evolution and ecosystems), 3.7.4 ranks 3 of 4 sub-sections by mean mastery (1 = hardest). Mastery trajectory is rising across the cohort window: 30.7% in 2017 → 32.0% in 2024 (+1.3 percentage points). Mean mastery was lowest in 2019 (27.0%) and highest in 2023 (54.5%).