These eleven questions cover the skills in human influence on ecosystems: reading a change, following a causal chain, comparing options, judging a sample and separating evidence from recommendation. All data are invented for practice.
Work on paper with the answers closed. Questions run from easier to harder, and each answer shows every step. Log repeated slips in the mistake log and retest queue.
Questions
1. An invented survey finds a mean of 50 birds per transect in 2015 and 35 in 2025. Calculate the percentage change.
Show answer
Difference = 35 − 50 = −15. Percentage change = −15 ÷ 50 × 100 = −30%, a 30% decrease.
2. Invented coral cover at a reef fell from 60% to 45%. Give the fall in percentage points and the percentage change.
Show answer
Fall in percentage points = 60 − 45 = 15 points. Percentage change = −15 ÷ 60 × 100 = −25%. The two are different: 15 points is the absolute drop, and 25% is the drop relative to the starting cover.
3. Put these in order to show eutrophication: dissolved oxygen falls; nitrate from fertiliser enters a pond; algae grow rapidly; submerged plants die; bacteria increase; light is blocked.
Show answer
Nitrate from fertiliser enters a pond, algae grow rapidly, light is blocked, submerged plants die, bacteria increase, dissolved oxygen falls.
4. Dissolved oxygen in an invented pond fell from 7.5 mg/dm³ to 1.5 mg/dm³ after an algal bloom. Calculate the percentage decrease.
Show answer
Decrease = 7.5 − 1.5 = 6.0. Percentage = 6.0 ÷ 7.5 × 100 = 80%.
5. Explain why the number of bacteria increases and the oxygen level falls after plants die in an enriched pond.
Show answer
Dead plants and algae are a food source for decomposing bacteria, so the bacteria multiply. They respire aerobically and use up dissolved oxygen faster than it is replaced, so the oxygen concentration falls.
6. Invented scenario. A nutrient-rich ditch drains from a farm into a lake. Option A is a strip of dense vegetation beside the ditch to take up nutrients before they reach the lake. Option B is to install pumps that add air to the lake. The criteria are: (1) reduces the nutrients entering the lake, (2) keeps working without a continuous energy supply. Compare the options and reach a conclusion.
Show answer
Option A: criterion 1, yes, because plants take up nitrate and phosphate before the water enters the lake. Criterion 2, yes, because the plants keep growing without added energy.
Option B: criterion 1, no, because nutrients still enter the lake and only the symptom is treated. Criterion 2, no, because the pumps need power to run.
Conclusion: Option A meets both criteria. It does not clean nutrients already in the lake, which is a limit.
7. A student places five random 1 m² quadrats and counts 4, 7, 5, 8 and 6 plants. The field is 150 m². Estimate the total number of plants.
Show answer
Total = 4 + 7 + 5 + 8 + 6 = 30. Mean = 30 ÷ 5 = 6 per m². Estimate = 6 × 150 = 900 plants.
8. Give two limits of the estimate in question 7 and say how you could improve it.
Show answer
(1) Only 5 m² out of 150 m² was sampled, about 3.3%, so the result depends on chance. (2) Counts may vary with time or season, or some plants may be missed. Improve it by using more quadrats and placing them at random across the whole field. (5 ÷ 150 × 100 = 3.33%.)
9. An invented survey of two forest plots with three quadrats each finds 20 tree species in an unlogged plot and 12 in a logged plot. By what percentage is the species count lower in the logged plot? What can you conclude?
Show answer
Difference = 20 − 12 = 8. Percentage lower = 8 ÷ 20 × 100 = 40%. The data show fewer species in the logged plot. With only one plot of each type and three quadrats each, they do not prove that logging caused the difference, because the plots may have differed before logging.
10. Classify each statement as evidence, interpretation or recommendation. (a) Mean oxygen was 2.5 mg/dm³ downstream. (b) Runoff from the farm caused the low oxygen. (c) The farm should stop using nitrate fertiliser.
Show answer
(a) Evidence, a measurement. (b) Interpretation, an explanation that goes beyond the measurement. (c) Recommendation, a statement of what should be done, which depends on values and costs as well as the data.
11. A student writes: “The data prove the mangrove must be protected.” Rewrite the sentence using the data from assessing evidence for a habitat change, where saplings fell from 14 to 9 per m².
Show answer
“The mean number of saplings fell from 14 to 9 per m², a decrease of about 36% (−5 ÷ 14 × 100 = −35.7%). This is consistent with habitat damage, but it does not show the cause, and whether to protect the area is a decision that also depends on values and costs.”
If you got these wrong
Use the error type to choose what to revisit.
- Percentage change or percentage points (questions 1, 2, 4, 9): go back to assessing evidence for a habitat change.
- Missing links in a chain (questions 3, 4, 5): revisit explaining eutrophication through a causal chain.
- Options judged on preference, not criteria (question 6): try comparing conservation options using specified ecological criteria again.
- Estimates and sampling limits (questions 7, 8, 9): work through interpreting biodiversity sampling limitations. The probability tree board and inheritance model board show how a model differs from a small observed sample.
- Evidence mixed with opinion (questions 10, 11): reread separating environmental science evidence from political recommendations.
If the same slip keeps returning after a second attempt, working through your own answers with a teacher is often quicker than re-reading. Our teachers offer online one-to-one Biology tuition.