Biology • Year 11 • Module 4 • Lesson 14
Keystone Species, Introduced Species and Ecological Disruption
Apply keystone and invasive species concepts to real data, the dingo fence experiment, and a novel Australian scenario.
1. Analyse data from the dingo exclusion fence
Ecologists measured several indicators on both sides of the South Australian dingo exclusion fence. The table below summarises their findings. 8 marks
| Indicator | Inside fence (no dingoes) | Outside fence (dingoes present) |
|---|---|---|
| Kangaroo density (per km²) | 42 | 4 |
| Ground cover (%) | 18 | 37 |
| Fox activity index | 8.2 | 1.4 |
| Small native mammal abundance (index) | 12 | 46 |
| Soil erosion severity (1–5 scale) | 4.1 | 1.8 |
1.1 Describe the differences in kangaroo density and ground cover between the two sides of the fence. Include figures in your answer. 2 marks
1.2 Explain how the dingo fence data supports the claim that the dingo is a keystone species. In your answer, refer to the definition of keystone species and at least three indicators from the table. 3 marks
1.3 Using the data, explain the indirect pathway by which the absence of dingoes leads to lower small native mammal abundance. Your explanation should identify at least two steps in the cascade. 3 marks
2. Interpret a graph — rabbit population response to myxoma virus
The graph below is a stylised representation of European rabbit population in Australia from 1950 (myxoma virus introduction) to 1980. 6 marks
Stylised rabbit population index, Australia 1950–1980 (after Fenner & Fantini, 1999).
2.1 Describe the rabbit population trend between 1950 and 1955. 2 marks
2.2 Explain why the rabbit population recovered after the initial crash. Use lesson content in your answer. 2 marks
2.3 The graph shows the population stabilising at a lower level (around index 40) rather than returning to 100. Explain one reason why it does not return to pre-1950 levels, and one reason why it cannot be reduced to zero by myxoma alone. 2 marks
3. Apply to a new scenario — the Tasmanian devil and facial tumour disease
The Tasmanian devil is the world’s largest living carnivorous marsupial. It suppresses feral cat populations in Tasmania through competition and intimidation. Since the 1990s, devil facial tumour disease (DFTD) has reduced the Tasmanian devil population by approximately 80%. Researchers have observed increasing feral cat activity in areas where devils have declined. 7 marks
3.1 Based on lesson content, explain whether the Tasmanian devil is likely to be a keystone species. Use the definition of keystone species and evidence from the scenario. 3 marks
3.2 Predict two indirect effects of continued devil population decline on Tasmanian ground-nesting birds. 2 marks
3.3 DFTD is itself a unique transmissible cancer, not a fungal or viral pathogen. A researcher proposes using genetic selection to breed disease-resistant devils and release them into the wild. Compare this strategy to the biological control strategies discussed in the lesson: identify one similarity and one difference. 2 marks
Q1.1 — Describing fence data
Kangaroo density inside the fence (no dingoes) is 42 per km², approximately 10.5 times higher than outside the fence (4 per km²) where dingoes are present [1]. Ground cover inside the fence (18%) is approximately half that outside the fence (37%), indicating more vegetation degradation in the dingo-free zone [1].
Marking criteria: 1 mark for kangaroo density comparison including figures; 1 mark for ground cover comparison including figures. “10× higher” or “roughly 10 times” accepted.
Q1.2 — Dingo as keystone species
A keystone species has a disproportionately large impact on ecosystem structure relative to its biomass or abundance [1]. The dingo fence data shows that in the absence of dingoes, kangaroo density is ~10× higher, ground cover drops from 37% to 18%, and fox activity increases from 1.4 to 8.2 [1]. These are ecosystem-wide effects that span multiple trophic levels and ecological processes (vegetation, predator communities, soil), far greater than would be expected from the dingo’s own biomass [1].
Marking criteria: 1 mark for correct definition; 1 mark for citing at least two data points from the table; 1 mark for reasoning that the scale of impact is disproportionate to dingo biomass.
Q1.3 — Indirect pathway to lower native mammal abundance
Step 1: Without dingoes, fox activity increases (fox activity index rises from 1.4 to 8.2) because dingoes are no longer suppressing fox populations through mesopredator suppression [1]. Step 2: Higher fox (and feral cat) activity means higher predation pressure on small native mammals such as bilbies, dunnarts, and bandicoots [1]. Conclusion: Small native mammal abundance therefore falls (index drops from 46 to 12), even though the dingo does not directly prey on these small species [1].
Marking criteria: 1 mark for identifying dingo removal leads to increased fox/cat activity; 1 mark for linking increased fox/cat activity to higher predation on small mammals; 1 mark for correctly using the data to support the conclusion. Award 3 marks for a fully connected causal chain with data.
Q2.1 — Rabbit population trend 1950–1955
The rabbit population index crashed sharply immediately after the release of myxoma in 1950–1951, falling from 100 to approximately 5 (a ~95% reduction) [1]. Between 1952 and 1955 the population began a partial and oscillating recovery, rising to approximately 20–25 [1].
Marking criteria: 1 mark for describing the sharp initial crash with approximate figure; 1 mark for describing the subsequent partial recovery.
Q2.2 — Why rabbits recovered
Both the rabbit population and the myxoma virus co-evolved. Rabbits with genetic resistance to myxoma survived and reproduced in greater numbers, passing resistance alleles to offspring [1]. Simultaneously, the virus evolved lower virulence, since a virus that kills its host too quickly cannot spread to new hosts. These two evolutionary changes together reduced the kill rate of myxoma, allowing the rabbit population to recover [1].
Marking criteria: 1 mark for genetic resistance evolving in the rabbit population; 1 mark for virus virulence decreasing. Both required for full marks.
Q2.3 — Why not returned to 100, and why not zero
The population does not return to 100 because myxoma still kills a proportion of rabbits, and other controls (dingoes, foxes, hawks, calicivirus introduced later) continue to exert predation pressure [1 — any valid suppressive factor]. It cannot be reduced to zero by myxoma alone because resistant rabbits survive infection and reproduce; the virus itself evolves to become less lethal, reducing its effectiveness as a population control agent over time [1].
Marking criteria: 1 mark for reason it stays below 100 (ongoing suppression by myxoma or other factors); 1 mark for reason it cannot reach zero (evolution of resistance in rabbits / reduced virulence in virus). Accept other valid reasons.
Q3.1 — Is the Tasmanian devil a keystone species?
A keystone species has a disproportionately large impact on ecosystem structure relative to its biomass [1]. The scenario states that the decline of devil populations (80% reduction due to DFTD) has led to increased feral cat activity in affected areas [1]. This indirect effect on the cat population — and through cats on the ground fauna that cats prey upon — is an ecosystem-wide restructuring that goes well beyond the devil’s own direct feeding impact. The pattern is consistent with a keystone predator role via mesopredator suppression, analogous to the dingo’s role in mainland Australia [1].
Marking criteria: 1 mark for stating the keystone definition; 1 mark for citing evidence from the scenario (DFTD-driven decline → increased cat activity); 1 mark for reasoning that the scale of indirect impact qualifies as disproportionate. Accept confident “likely keystone” — certainty is not required.
Q3.2 — Indirect effects on ground-nesting birds
Two indirect effects: (1) Higher feral cat densities (released from devil competition/intimidation) would increase predation on nests, eggs, and chicks of ground-nesting bird species, causing population declines [1]. (2) If devil decline also leads to increased fox or feral rabbit activity, the resulting vegetation changes (overgrazing by rabbits) could reduce ground cover and nesting habitat quality for birds [1]. Accept any two ecologically valid indirect effects linked to the causal chain of devil decline → cat/mesopredator increase → bird impact.
Q3.3 — Similarity and difference with biocontrol
Similarity: Both strategies use a biological approach (a living organism — a disease-resistant devil — instead of a chemical agent) to address a biological threat to ecosystem function [1].
Difference: Biological control releases a non-native or novel organism to suppress a target species; the devil breeding program restores and strengthens an existing native species population rather than introducing a new one. Additionally, biocontrol programs are irreversible in principle; releasing resistant devils does not introduce a new organism and could theoretically be stopped if unintended effects emerged [1]. Accept other valid contrasts (scale, target, reversibility).