Keystone Species, Introduced Species and Ecological Disruption

Q1 — Sample Band 6 response (7 marks), annotated

A keystone species is a species whose impact on ecosystem structure and function is disproportionately large relative to its biomass or abundance; its removal causes ecosystem-wide restructuring. An introduced species is one moved to a new ecosystem, typically thriving because it has been released from co-evolved predators, competitors, and pathogens (the enemy release hypothesis). [1 — both definitions correct]

Both types of species have outsized effects on biodiversity, but in opposite directions. A keystone species such as the dingo maintains biodiversity by controlling herbivore and mesopredator populations. Dingo exclusion fence data show that without dingoes, kangaroo density is ~10× higher, ground cover halves, and native mammal abundance drops — a collapse in biodiversity that follows from removing a single species. By contrast, an introduced species such as the cane toad reduces biodiversity by eliminating native predators (quolls, goannas, snakes, crocodiles) that consume it and are killed by its toxin. [1 — effect on biodiversity criterion with named examples]

The mechanism of impact differs: keystone species act primarily through predation and competition within the existing food web, while introduced species often act through a novel weapon — in the cane toad’s case, a toxin that native species have had no evolutionary time to adapt to. [1 — mechanism of impact criterion]

The reversibility of change also differs. Removing a keystone species causes rapid ecosystem degradation that may be partially reversible if the species is reintroduced; the dingo fence provides proof-of-concept. Damage from an established introduced species is typically irreversible because the species cannot be eradicated once established — the cane toad now covers most of northern Australia with no prospect of full eradication. [1 — reversibility criterion]

The predictability of the impact also differs: the ecological role of a keystone species is observable and can be studied experimentally (e.g. dingo fence). The full impact of an introduced species is often unpredictable at the time of introduction — the cane toad was introduced as pest control and its ecological destruction was not anticipated. [1 — predictability criterion; 3 of 3 required criteria now covered]

Overall, introduced species tend to have the more destabilising effect, because their impacts are novel (toxins or competition that native species cannot cope with), permanent, and often cascade across multiple trophic levels simultaneously. Keystone species, by contrast, generate stability; it is their removal (often caused by human activity) that destabilises ecosystems. [1 — evaluative judgement, justified]

The dingo provides an ironic illustration of both categories: itself an introduced species (~4,000 years ago), it has become a keystone species, demonstrating that the introduced/keystone categories are not mutually exclusive on ecological timescales. [1 — precise Australian examples; bonus insight is not required but shows Band 6 depth]

Marking criteria:

• 1 mark — Correct definition of both keystone species and introduced species using lesson language.
• 1 mark — Effect on biodiversity criterion: keystone maintains it, introduced reduces it; at least one named example each.
• 1 mark — Mechanism of impact criterion (keystone: food web role; introduced: novel weapon / enemy release).
• 1 mark — Reversibility criterion (keystone removal more reversible; introduced establishment largely irreversible).
• 1 mark — Third valid comparison criterion (any of: predictability; geographic scale of impact; timescale; human intention).
• 1 mark — Evaluative judgement: which type is more destabilising, with justification linked to lesson evidence.
• 1 mark — Named Australian example used correctly for each category (dingo / cane toad / rabbit / prickly pear accepted).

Q2 — Sample Band 6 response (8 marks), annotated

The cane toad is ecologically destructive primarily because it is toxic to native predators that eat it. Unlike most invasive herbivores that compete for resources, the cane toad deploys a novel weapon — its skin toxin — that Australian predators (quolls, goannas, snakes, freshwater crocodiles) have no co-evolved resistance to. This is directly predicted by the enemy release hypothesis: in its South American native range, the toad is controlled by predators, parasites, and pathogens that have co-evolved with it; in Australia, these natural enemies are absent. [1 — enemy release and novel weapon identified]

Developing biocontrol for the cane toad is harder than the prickly pear case for several reasons. Prickly pear is a plant; Cactoblastis moth larvae eat it from the inside, and after host-specificity testing the moth was found to attack only Opuntia cacti in Australia. A biocontrol agent for an amphibian must be specific enough not to attack the ~200 native frog species that share Australian ecosystems with the cane toad. The lungworm parasite (Rhabdias pseudosphaerocephala) is not yet confirmed safe for native frogs, so the irreversibility risk — that a released agent cannot be recalled if it shifts hosts — is substantial. [1 — host specificity challenge identified and linked to lesson risk criteria]

Evaluating the lungworm: it has plausible effectiveness (the lesson notes that co-evolved parasites can regulate host populations); however, host-specificity is uncertain. If it attacks native frogs, the biocontrol agent would itself become an introduced species causing secondary harm — and once released, it cannot be recalled (irreversibility). This is precisely the risk the lesson flags. [1 — effectiveness evaluated; 1 — irreversibility risk applied to lungworm]

Evaluating gene-drive: this is a fundamentally different strategy. A gene-drive is a genetic construct that causes infertility to spread through a population over generations. It is not a living predator/pathogen and does not fit neatly into the lesson’s biocontrol framework, but shares the irreversibility concern (once in the wild population, a gene-drive also cannot be recalled). Its potential effectiveness is high in theory; its risk profile includes off-target effects if hybridisation with related toad populations elsewhere occurs. [1 — gene-drive evaluated against effectiveness + irreversibility]

Against the three criteria: effectiveness — both are theoretically effective; host specificity — lungworm is the greater short-term risk (frog non-target attack), gene-drive the greater long-term risk (spread beyond Australia); irreversibility — both are effectively irreversible once deployed. [1 — all three criteria explicitly applied to both strategies]

My recommendation: the lungworm parasite is closer to the lesson’s model of biocontrol and has precedent (the Cactoblastis success shows parasite/herbivore biocontrol can work spectacularly when host-specific). It should be pursued, but only after definitive host-specificity quarantine testing. Gene-drive technology is not yet ready for release and should remain a research avenue only, given its global irreversibility implications. [1 — justified recommendation integrating lesson criteria]

In sum, the cane toad problem illustrates the lesson’s core warning: the greatest risk of biological control is that a released agent cannot be recalled. Any strategy for the toad must clear the host-specificity bar that the prickly pear program met, and that the toad’s own 1935 introduction so catastrophically failed to consider. [1 — integrates lesson framework with stimulus context; shows Band 6 synthesis]

Marking criteria:

• 1 mark — Explains cane toad destructiveness via novel weapon / enemy release (no co-evolved resistance in Australian predators).
• 1 mark — Explains why biocontrol is harder than prickly pear: host specificity challenge for an amphibian in a frog-rich country; irreversibility risk.
• 1 mark — Evaluates lungworm: identifies effectiveness potential and host-specificity risk to native frogs.
• 1 mark — Applies irreversibility criterion to lungworm: once released, cannot be recalled.
• 1 mark — Evaluates gene-drive: effectiveness rationale and at least one risk (irreversibility, off-target spread, or novel-technology concern).
• 1 mark — Explicitly compares both strategies against all three lesson criteria (effectiveness, host specificity, irreversibility).
• 1 mark — Reaches a justified, context-aware recommendation using lesson terminology.
• 1 mark — Integrates the lesson’s cane toad origin story (failed biocontrol → ecological harm) as evidence for the importance of pre-release testing.

Q3 — Sample Band 6 response (6 marks)

The claim contains defensible elements but is largely overstated or incorrect. [1 — overall evaluative judgement]

What is defensible: Biological control does often have advantages over chemical pesticides — it can be self-sustaining (a successful agent multiplies with the target population), it may avoid the chemical toxicity and environmental persistence associated with synthetic pesticides, and when host-specific it can be highly targeted. The Cactoblastis success demonstrates these advantages. [1 — concedes the defensible core]

What is wrong:

• “Poses no risk to native species.” The lesson explicitly states that a biocontrol agent may attack non-target native species. The cane toad was itself a failed biocontrol attempt that became one of Australia’s most damaging invasive species. Any proposed agent must undergo years of host-specificity testing precisely because this risk is real and documented. [1 — refutes “no risk” claim with evidence]
• “The cane toad is the only biocontrol program that caused harm.” This is incorrect. The myxoma virus program, while not directly harmful to native species, demonstrates that a biocontrol program can face unintended evolutionary consequences — both the host and pathogen evolved, with rabbits developing genetic resistance and the virus evolving lower virulence, reducing effectiveness over time. More directly, the lesson explicitly warns that any biocontrol agent may attack non-target native species; this is precisely why years of host-specificity testing are required before release. Claiming the cane toad is an anomaly ignores the lesson’s own documented risks and the myxoma coevolution example. [1 — refutes “only harm” claim with lesson examples]
• “Always preferable.” Biological control is irreversible. The lesson states that once a biocontrol agent is released it cannot be recalled, and evolution can shift host preferences over time. In situations where rapid knockdown is needed (e.g. a new invasive species still in a containable stage), a targeted chemical treatment may actually be preferable to deploying an irreversible living agent. [1 — refutes “always preferable” using irreversibility and context-dependence]

Defensible reformulation: “Biological control can be an effective, self-sustaining, and environmentally targeted method for managing introduced species when the biocontrol agent has been confirmed host-specific through rigorous quarantine testing. However, because biological control is irreversible and carries the risk of non-target impacts — including becoming a destructive introduced species itself — it is not always preferable to chemical or other control methods. The choice must weigh effectiveness, host specificity, reversibility, and the ecological characteristics of both the target species and the native community it lives in.” [1 — biologically accurate reformulation]

Marking criteria:

• 1 mark — Overall evaluative judgement (partly correct but significantly overstated/incorrect).
• 1 mark — Correctly identifies the defensible element (biocontrol can be self-sustaining and targeted when host-specific; Cactoblastis is evidence).
• 1 mark — Refutes “no risk to native species”: lesson explicitly states non-target risk; cane toad itself is the primary example.
• 1 mark — Refutes “only anomaly”: uses lesson content to show the cane toad is not uniquely harmful — accepts the myxoma coevolution example (unintended evolutionary consequences reducing effectiveness) or the lesson’s stated risk that any biocontrol agent may attack non-target species.
• 1 mark — Refutes “always preferable”: correctly applies irreversibility from lesson; notes that context determines which approach is best.
• 1 mark — Reformulates the claim into a defensible alternative that centres host specificity, irreversibility, and context-dependence in precise lesson language.