Biology • Year 11 • Module 4 • Lesson 14
Keystone Species, Introduced Species and Ecological Disruption
Lock in the core vocabulary, the keystone vs invasive species distinction, and the key Australian examples from this lesson.
1. Term–definition match
Match each term in the left column to its correct definition. Write the letter of the correct definition next to each term. 8 marks
| Term | Answer | Definition options (A–H) |
|---|---|---|
| 1. Keystone species |
A. A species moved to a new ecosystem where it lacks co-evolved predators, competitors, and pathogens. B. The deliberate release of a natural enemy to suppress an invasive species population. C. A species whose removal causes disproportionate ecosystem-wide restructuring relative to its biomass. D. The hypothesis explaining why introduced species often thrive: they leave behind co-evolved natural enemies. E. The reduction of smaller predator populations due to the presence of a larger apex predator. F. A bare rocky seafloor created when sea urchins overgraze kelp in the absence of sea otters. G. A series of population-level effects triggered by the addition or removal of a top predator. H. A pest-control strategy that uses a living organism rather than a chemical agent to reduce a target population. |
|
| 2. Introduced species | ||
| 3. Enemy release hypothesis | ||
| 4. Mesopredator suppression | ||
| 5. Trophic cascade | ||
| 6. Biological control | ||
| 7. Urchin barren | ||
| 8. Biocontrol agent |
2. Classify the species
For each species below, place a tick in the correct column(s). A species may qualify for more than one column. 10 marks (1 per row, deduct 0.5 for each incorrect tick)
| Species | Keystone species | Introduced species (to Australia) | Subject of biological control |
|---|---|---|---|
| Sea otter | |||
| Dingo | |||
| European rabbit | |||
| Cane toad | |||
| Prickly pear (Opuntia stricta) | |||
| African elephant | |||
| Bee (pollinator) | |||
| Cactoblastis cactorum moth | |||
| Myxoma virus | |||
| Fox |
3. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version on the line provided. 10 marks (1 for T/F, 1 for correction where needed)
3.1 A keystone species is defined by being the most abundant species in its ecosystem. T / F
3.2 The dingo exclusion fence in South Australia shows that kangaroo density is higher outside the fence (where dingoes are present) than inside. T / F
3.3 According to the enemy release hypothesis, introduced species thrive partly because they leave behind their co-evolved predators. T / F
3.4 The cane toad was successfully used to control beetle pests in Queensland sugar cane fields. T / F
3.5 The Cactoblastis cactorum moth is regarded as one of the most successful biological control programs in history. T / F
4. Function recall
Answer each question in 1–2 sentences using precise lesson vocabulary. 8 marks (2 each)
4.1 What is the function of sea otters in a North Pacific kelp forest ecosystem?
4.2 What is the ecological function of dingoes in suppressing fox and cat populations?
4.3 What role did the myxoma virus play after its introduction to Australia, and why did its effectiveness decline?
4.4 Why is biological control considered irreversible once an agent has been released?
5. Complete the comparison table
Fill in the missing cells using information from the lesson. 8 marks
| Species | Year introduced to Australia | Primary ecological impact | Control measure used |
|---|---|---|---|
| European rabbit | Myxoma virus; calicivirus (RHDV) | ||
| Cane toad | 1935 | ||
| Prickly pear (Opuntia stricta) | 1800s | Covered 24 million ha; smothered pasture and native ground cover |
Q1 — Term–definition match
1 C • 2 A • 3 D • 4 E • 5 G • 6 B • 7 F • 8 H
Note: Biocontrol agent (8) is a subset of biological control (6); the distinction is that “biocontrol agent” refers to the organism released, while “biological control” refers to the strategy as a whole. Both H and B are closely related — if a student argues either mapping with justification, accept it.
Q2 — Classify the species
| Species | Keystone | Introduced (Aus) | Biocontrol subject |
|---|---|---|---|
| Sea otter | ✓ | ||
| Dingo | ✓ | ✓ (arrived ~4,000 yrs ago with humans) | |
| European rabbit | ✓ | ✓ | |
| Cane toad | ✓ | ||
| Prickly pear | ✓ | ✓ | |
| African elephant | ✓ | ||
| Bee | ✓ | ||
| Cactoblastis moth | ✓ (released as biocontrol) | ||
| Myxoma virus | ✓ (released as biocontrol) | ||
| Fox | ✓ |
Q3 — True / false with correction
3.1 False. A keystone species is defined by having a disproportionately large impact on ecosystem structure relative to its biomass or abundance — not by being the most abundant.
3.2 False. Kangaroo density is higher inside the fence (where dingoes are absent), approximately 10× higher than outside where dingoes are present.
3.3 True.
3.4 False. The cane toad failed to control beetle pests in sugar cane. It did not reach the cane flowers where the beetles lived, and instead thrived in the wider environment.
3.5 True.
Q4.1 — Function of sea otters
Sea otters prey on sea urchins, keeping urchin populations in check. Without otters, urchin populations explode and overgraze kelp, creating urchin barrens. With otters present, kelp forests persist and support hundreds of fish, invertebrate, and seabird species.
Q4.2 — Ecological function of dingoes suppressing foxes and cats
Dingoes kill and suppress the populations of foxes and feral cats (mesopredators). This is called mesopredator suppression. Because foxes and cats are devastating predators of small native mammals, birds, and reptiles, dingo suppression of these mesopredators indirectly protects native fauna.
Q4.3 — Myxoma virus: role and declining effectiveness
The myxoma virus was released in 1950 as a biological control agent against European rabbits. It initially killed about 99.8% of infected rabbits. Over time, both the rabbit population and the virus evolved: rabbits developed genetic resistance, and the virus evolved lower virulence (since a dead host cannot spread the virus). As a result, myxoma still suppresses rabbit numbers but no longer controls them effectively alone.
Q4.4 — Why biological control is irreversible
Once a biological control agent is released into the environment, it becomes a living, self-reproducing component of that ecosystem. Unlike a chemical pesticide that degrades, a living organism cannot be recalled. It may spread beyond the intended area, evolve, or shift to attack non-target species over time. This irreversibility is a key risk of biocontrol programs.
Q5 — Comparison table
| Species | Year introduced | Primary impact | Control measure |
|---|---|---|---|
| European rabbit | 1859 | Overgrazing, soil destabilisation by warrens, competition with native herbivores (bilbies, bandicoots), prevention of tree seedling regeneration | Myxoma virus; calicivirus (RHDV) |
| Cane toad | 1935 | Toxic to native predators (quolls, goannas, snakes, freshwater crocodiles) that eat them; causes predator population declines and trophic cascades | No effective biological control agent yet released; ongoing research |
| Prickly pear | 1800s | Covered 24 million ha; smothered pasture and native ground cover | Cactoblastis cactorum moth (released 1925; reduced prickly pear by >90% within 10 years) |