Biology • Year 11 • Module 4 • Lesson 21
Human Impacts — Habitat Destruction, Fragmentation, Pollution and Introduced Species
Apply fragmentation, pollution and multi-stressor concepts to real data, a Great Barrier Reef case study, and a predict-and-justify scenario.
1. Interpret data — fragment size and bird species
A survey of woodland remnants in the WA Wheat Belt counted the number of native bird species in fragments of different size. 7 marks
| Fragment area (ha) | Native bird species recorded |
|---|---|
| 5 | 6 |
| 20 | 14 |
| 50 | 22 |
| 100 | 30 |
| 500 | 41 |
Representative data modelled on published Wheat Belt fragmentation studies.
1.1 Describe the relationship between fragment area and the number of native bird species. 2 marks
1.2 Calculate how many more native bird species the 500 ha fragment supports than the 5 ha fragment, and express the 5 ha count as a percentage of the 500 ha count. Show your working. 2 marks
1.3 Use the species-area relationship and the concept of minimum viable population to explain why the smallest fragments have lost the most species. 3 marks
2. Compare and contrast — eutrophication vs biomagnification
Complete the table using information from the lesson. Where a cell is already filled, use it as a clue for the adjacent row. 8 marks
| Criterion | Eutrophication | Biomagnification |
|---|---|---|
| Pollutant type | Excess nutrients (nitrogen, phosphorus) | |
| Key mechanism | Toxin accumulates in tissues and concentrates up each trophic level | |
| Organisms most affected | ||
| Direct cause of harm | Oxygen depletion (hypoxia) from bacterial decomposition of dead algae | |
| Example | DDT → eggshell thinning in eagles and peregrine falcons |
3. Case study — the Great Barrier Reef land–sea connection
Read the scenario and answer the questions that follow. 6 marks
Scenario. AIMS's 2017 Water Quality Report found that 35 rivers deliver about 10 million tonnes of sediment and 7,000 tonnes of dissolved nitrogen to the Great Barrier Reef (GBR) catchment each year, driven by land clearing and agriculture in Queensland. The nitrogen feeds phytoplankton blooms, which feed larval crown-of-thorns starfish (CoTS). CoTS outbreaks have killed about 40% of GBR coral since 1985, with each starfish eating up to 6 m² of coral per year. The damage originates on land, not on the reef.
3.1 Outline the cause-and-effect chain from land clearing to coral loss described in the scenario. 2 marks
3.2 Explain why this is described as a "land–sea connection", and why managing only the reef itself would not stop the damage. 2 marks
3.3 Suggest one management action on land that would reduce the nitrogen load, and explain its mechanism. 2 marks
4. Predict and justify — multi-stressor analysis
The Great Barrier Reef faces multiple stressors at once: ocean warming (causing bleaching), agricultural nutrient runoff, and crown-of-thorns starfish outbreaks. 5 marks
4.1 Identify which two stressors are directly linked by a cause-and-effect relationship, and state the chain. 2 marks
4.2 Predict what would happen to coral recovery if nutrient runoff were halved but sea temperatures kept rising. Justify your prediction using the idea of multi-stressor synergy. 2 marks
4.3 State why a Band 6 answer to a reef-decline question must identify at least two interacting stressors rather than one. 1 mark
Q1.1 — Relationship (2 marks)
As fragment area increases, the number of native bird species increases (a positive relationship) [1]. The increase is not proportional — species number rises steeply for small increases in area at first and then more gradually, consistent with the species-area relationship [1].
Q1.2 — Calculation (2 marks)
Working: 41 − 6 = 35 more species in the 500 ha fragment [1]. As a percentage: 6 ÷ 41 × 100 = 14.6% — the 5 ha fragment holds only about 15% of the species of the 500 ha fragment [1].
Q1.3 — Why small fragments lose the most (3 marks)
The species-area relationship states that smaller areas support fewer species, with loss accelerating below a critical threshold [1]. Small fragments can only hold small populations of each species, which often fall below the minimum viable population (MVP) [1]. Below MVP, populations are prone to extinction from inbreeding, genetic drift and random fluctuations, so the smallest fragments lose the most species — especially specialists that need large, continuous habitat [1].
Q2 — Compare-and-contrast table (8 marks — 1 per correct cell)
| Criterion | Eutrophication | Biomagnification |
|---|---|---|
| Pollutant type | Given (excess N and P nutrients) | Fat-soluble, stable toxins (DDT, mercury, PCBs) [1] |
| Key mechanism | Nutrients → algal bloom → algal death → bacterial decomposition → oxygen depletion [1] | Given (accumulates in tissues, concentrates up trophic levels) |
| Organisms most affected | Fish, crustaceans and aerobic organisms in the water; seagrass/coral (light loss) [1] | Apex predators (top of the food chain) [1] |
| Direct cause of harm | Given (oxygen depletion / hypoxia) | Toxic concentration in tissues → e.g. reproductive failure / eggshell thinning [1] |
| Example | Great Barrier Reef nutrient runoff; algal blooms / CoTS outbreaks [1] | Given (DDT → raptor eggshell thinning) |
8 cells to complete — 1 mark each.
Q3.1 — Cause-and-effect chain (2 marks)
Land clearing/agriculture → sediment and dissolved nitrogen wash into rivers → nitrogen enters the GBR catchment → phytoplankton blooms → more larval crown-of-thorns starfish survive → CoTS outbreak → starfish eat coral → coral loss (~40% since 1985) [1 for a logical chain; 1 for including the nitrogen→phytoplankton→CoTS→coral steps].
Q3.2 — Land–sea connection (2 marks)
It is a land–sea connection because a change made on land (clearing/farming) cascades through river water quality into the marine ecosystem hundreds of kilometres away [1]. Managing only the reef (e.g. removing starfish) would not stop the cause, because the nitrogen driving the outbreaks keeps arriving from the land; the source must be addressed on land [1].
Q3.3 — Land management action (2 marks)
Any one valid action with mechanism, e.g.: reduce fertiliser application / improve nutrient-use efficiency on farms — less nitrogen is available to run off into rivers [1]; or restore riparian (streamside) vegetation — plants take up nutrients and trap sediment before it reaches waterways, lowering the nitrogen load reaching the reef [1].
Q4.1 — Linked stressors (2 marks)
Nutrient runoff and crown-of-thorns starfish outbreaks are directly linked [1]: elevated nitrogen fuels phytoplankton blooms, which provide more food for larval CoTS, so more survive to adulthood and outbreaks occur, increasing coral predation [1].
Q4.2 — Prediction (2 marks)
Coral recovery would still be limited and the reef would likely keep declining [1]. Even with halved runoff, continued warming causes bleaching, and because stressors act synergistically, heat-stressed coral is less able to recover from CoTS damage and disease — so reducing one stressor alone does not allow recovery while another worsens [1].
Q4.3 — Why two interacting stressors (1 mark)
Because in the real world reef decline results from multiple stressors acting together, whose combined effect exceeds the sum of the parts (synergy); analysing only one stressor misrepresents the cause and the management needed [1].