What this diagnostic is for
Use this page before you revise Unit 8 or right after finishing it. It has one short question per topic and a second for the three largest topics. Each answer names the study guide to read if you got it wrong.
These are original Marlbridge practice questions, not past exam questions. They are not calibrated against real exam results, so your total does not predict an exam score. Treat it as a map of what to revisit.
Calculator and data. A calculator is allowed on both sections of the Biology exam. Here a basic calculator is enough, and every number you need is in the question. All organisms and data are fictional.
How to work. Answer every question, with a short written reason, before you open any answer.
Question 1 (multiple choice · 8.1)
Farmland pigeons fly off when a person comes within about 10 m. Young pigeons of the same species in a city park do the same at first, but after a few weeks of people walking past harmlessly, they let people come within 2 m. Which statement best describes this change?
- (A) Habituation: a learned loss of a costly reaction to a harmless, repeated stimulus.
- (B) An innate response: city pigeons inherited a smaller escape distance, so they never needed experience.
- (C) Kinesis: the pigeons change their speed of movement in response to people.
- (D) Natural selection within each pigeon’s lifetime: each bird evolved a shorter escape distance.
Answer and explanation
Answer: (A). The response changes with experience, so it is learned: ignoring a harmless, repeated stimulus is habituation. Needless take-offs cost energy and feeding time, so the change can raise fitness.
- (B) is tempting, but the young park birds started with the long distance.
- (C) misuses kinesis, a change in speed or turning with stimulus strength. Here the bird simply stops fleeing.
- (D) mixes up individuals and populations. Individuals learn; populations evolve.
If you missed this: read “Innate and learned behaviour” in the responses to the environment study guide.
Question 2 (multiple choice · 8.2)
A sealed glass jar of pond water with algae, snails and bacteria stays alive for months on a sunny windowsill. Which statement is correct?
- (A) Energy and matter both cycle inside the jar, so nothing has to enter or leave.
- (B) Matter cycles inside the jar, but energy must keep entering as light and leaves as heat.
- (C) Energy cycles inside the jar, but matter must keep entering through the glass.
- (D) Decomposers return energy from dead snails to the algae, so the jar would survive in the dark.
Answer and explanation
Answer: (B). Carbon, nitrogen and phosphorus atoms move between the organisms and the water again and again. Energy flows one way: algae capture light, every organism releases heat in respiration, and heat cannot be used to build molecules.
- (A) is tempting because the jar is sealed, but glass lets light in and heat out.
- (C) swaps the two ideas. Matter cycles; energy flows.
- (D) is the classic error. Decomposers return nutrients, not energy. In the dark, nothing could photosynthesise.
If you missed this: read “Energy in, energy used, energy out” and “Matter cycles: biogeochemical cycles” in the energy flow study guide.
Question 3 (multiple choice · 8.2)
In a fictional forest food chain, the tertiary consumers make 15 kJ m⁻² yr⁻¹ of new biomass. Assuming about 10% transfer at each step, about how much net primary production must the producers make?
- (A) 150 kJ m⁻² yr⁻¹
- (B) 1500 kJ m⁻² yr⁻¹
- (C) 15 000 kJ m⁻² yr⁻¹
- (D) 150 000 kJ m⁻² yr⁻¹
Answer and explanation
Answer: (C). Producers → primary → secondary → tertiary consumers is three transfers, so 15 × 10 × 10 × 10 = 15 000.
- (A) uses one transfer only.
- (B) uses two transfers, as if tertiary consumers ate primary consumers.
- (D) counts four levels as four transfers. Count the arrows, not the levels.
If you missed this: read “Why so little energy reaches the top” and Worked example 1 in the energy flow study guide.
Question 4 (multiple choice · 8.3)
A fictional rodent reaches a new island with plenty of food and no predators. Its population of 300 doubles every 5 years. If growth stays exponential, what is the population after 15 years?
- (A) 900
- (B) 1200
- (C) 2400
- (D) 4800
Answer and explanation
Answer: (C). With a constant per capita rate, the doubling time is the same at any size. 15 years is three doublings: 300 → 600 → 1200 → 2400.
- (A) multiplies by the number of doublings (300 × 3) instead of doubling three times.
- (B) adds 300 every 5 years: linear growth, not exponential.
- (D) adds a fourth doubling.
If you missed this: read “Exponential growth” in the population ecology study guide.
Question 5 (short answer · 8.4)
Fictional water snails are kept in identical tanks at four densities, with the same daily food in each. The per capita death rate is 0.10 per week at every density.
| Snails per tank | 10 | 20 | 40 | 80 |
|---|---|---|---|---|
| Per capita birth rate / per week | 0.80 | 0.70 | 0.50 | 0.10 |
(a) Calculate the per capita growth rate at each density. (b) Estimate the carrying capacity of a tank, and give your reason. (c) Calculate dN/dt for a tank of 40 snails. (d) A cold night kills 25% of the snails in every tank. Is this factor density-dependent or density-independent? Explain.
Answer and explanation
(a) r = b − d: 0.70, 0.60, 0.40 and 0.00 per week.
(b) K ≈ 80 snails per tank, where births balance deaths (r = 0). The fixed food is shared by more snails, so each has less energy for eggs.
(c) dN/dt = rN = 0.40 × 40 = 16 snails per week.
(d) Density-independent. The fraction killed (25%) is the same at every density, even though more snails die in a crowded tank (20 of 80 against 5 of 20).
If you missed this: read “Carrying capacity”, “What limits growth: two kinds of factor” and “Logistic growth” in the effect of density study guide.
Question 6 (multiple choice · 8.5)
A fictional tree makes nectar and hollow thorns in which ants nest. The ants attack insects on its leaves. Trees with ants removed lose three times as much leaf area to insects, and ant colonies without these trees rarely survive. Which interaction is this?
- (A) Commensalism (+/0)
- (B) Mutualism (+/+)
- (C) Parasitism (+/−)
- (D) Predation (+/−)
Answer and explanation
Answer: (B). Both partners gain. The ants get food and shelter; the removal data show the tree keeps more leaf area for photosynthesis.
- (A) notices only the ants’ gain. The leaf-loss data show the tree is not “0”.
- (C) would need the ants to harm the tree overall; the nectar costs less than the leaves saved.
- (D) describes the ants and the insects they attack, not the ants and the tree.
If you missed this: read “Interactions between populations” in the community ecology study guide.
Question 7 (short answer · 8.5)
A student counts flowering plants in two fictional meadows.
| Meadow | Counts of each species | Total |
|---|---|---|
| 1 | 10, 10, 10, 10 | 40 |
| 2 | 25, 5, 4, 3, 2, 1 | 40 |
(a) Calculate Simpson’s diversity index for each meadow. (b) State which meadow has the higher species richness and which has the higher diversity index. Explain how both can be true.
Answer and explanation
(a) Meadow 1: Σ(n/N)² = 4 × 0.25² = 0.25. Index = 1 − 0.25 = 0.75.
Meadow 2: Σ(n/N)² = (25² + 5² + 4² + 3² + 2² + 1²) ÷ 40² = 680 ÷ 1600 = 0.425. Index = 1 − 0.425 = 0.575, so 0.58 to 2 decimal places.
(b) Meadow 2 has the higher richness (6 species against 4). Meadow 1 has the higher index, because the index also measures evenness: in Meadow 2 one species makes up 62.5% of the plants.
Watch for: forgetting to square. Without squaring, Σ(n/N) is always 1.
If you missed this: read “Describing a community: composition and diversity” and Worked example 1 in the community ecology study guide.
Question 8 (multiple choice · 8.6)
A flood scours two fictional streams. In stream X, the number of insect larvae falls by 20% and takes 6 weeks to return to normal. In stream Y, it falls by 50% and is back to normal after 2 weeks. Which statement is correct?
- (A) Stream X is more resistant; stream Y is more resilient.
- (B) Stream X is more resilient; stream Y is more resistant.
- (C) Stream X is both more resistant and more resilient, because it changed less.
- (D) The streams cannot be compared, because they recovered at different times.
Answer and explanation
Answer: (A). Resistance is how little a system changes when disturbed: X fell less. Resilience is how quickly and completely it recovers: Y bounced back faster.
- (B) swaps the two definitions.
- (C) assumes a small change also means fast recovery. The two measures can disagree.
- (D) is wrong: recovery time is how resilience is measured.
If you missed this: read “Diversity and resilience” in the biodiversity study guide.
Question 9 (multiple choice · 8.7)
Fertiliser washes into a lake. Which sequence of events is in the correct order?
- (A) Nitrate and phosphate rise → algae bloom → algae die → decomposers multiply and respire → dissolved oxygen falls → fish die
- (B) Nitrate and phosphate rise → algae bloom → algae use up the oxygen by photosynthesis → fish die → decomposers multiply
- (C) Nitrate and phosphate rise → fish die from the fertiliser → decomposers multiply → algae bloom on the nutrients released
- (D) Nitrate and phosphate rise → decomposers multiply → algae bloom → dissolved oxygen rises → fish die
Answer and explanation
Answer: (A). Extra nutrients remove the limit on algal growth. When the algae die, aerobic decomposers break them down, and their respiration uses up the oxygen.
- (B) is the most common error. Photosynthesis releases oxygen.
- (C) blames the fertiliser itself. The harm comes through the chain.
- (D) puts decomposers first and has oxygen rising.
If you missed this: read “Eutrophication” and Figure 2 in the disruptions study guide.
Question 10 (short answer · 8.7)
A fictional crayfish is carried from its home river to a lake on another continent, where it eats the same snails as a native crayfish. Native crayfish per trap fell from 120 before the invasion to 30 six years later.
| Measure | Home river | New lake |
|---|---|---|
| Introduced crayfish infected by its usual parasite | 40% | 2% |
| Introduced crayfish surviving to adulthood | 15% | 45% |
(a) Calculate the percentage change in native crayfish. (b) Use the data to explain why the introduced crayfish became invasive. (c) Predict one further effect on the lake community, and explain it.
Answer and explanation
(a) (30 − 120) ÷ 120 × 100 = −75% (a 75% decrease).
(b) Escape from enemies: its parasite was left behind (infection 40% → 2%), so three times as many survive (15% → 45%) and its population grows fast. It then outcompetes the native crayfish for the shared snails.
(c) Any reasoned prediction, for example: snails fall as more crayfish eat them, so the algae that snails graze increase (a trophic cascade).
If you missed this: read “Invasive species” in the disruptions study guide.
Your next step
Mark each question right or wrong. A short answer missed in one part still counts.
| Topic | Question(s) | If you missed it, read |
|---|---|---|
| 8.1 Responses to the environment | 1 | Study guide 8.1 |
| 8.2 Energy flow through ecosystems | 2, 3 | Study guide 8.2 |
| 8.3 Population ecology | 4 | Study guide 8.3 |
| 8.4 Effect of density on populations | 5 | Study guide 8.4 |
| 8.5 Community ecology | 6, 7 | Study guide 8.5 |
| 8.6 Biodiversity | 8 | Study guide 8.6 |
| 8.7 Disruptions in ecosystems | 9, 10 | Study guide 8.7 |
How to use your result
- Wrong answers first. Read the named section, then do that topic’s practice questions.
- Lucky guesses count as misses. If your written reason did not match the explanation, treat the question as wrong.
- No misses? Go to the mixed unit review, which joins the topics in longer questions.
- Missed Questions 4 and 5? Both depend on per capita rates. Start with Topic 8.3, then move to 8.4.