HSC Exam Practice

Sample response. A keystone species is a species whose impact on ecosystem structure and function is disproportionately large relative to its biomass or abundance, such that its removal causes a disproportionate collapse or restructuring of the ecosystem.

Marking notes. 1 mark for “disproportionately large impact” (or equivalent). 1 mark for “relative to its biomass / abundance” (or “its removal causes ecosystem-wide restructuring”). Both components required for full marks.

Sample response. (i) Sea otter — North Pacific kelp forest ecosystems; directly controls sea urchin populations. (ii) Dingo — Australian arid and semi-arid ecosystems; directly controls kangaroo, rabbit, fox, and feral cat populations (accept any of these). Other acceptable answers: African elephant (African savanna; controls tree growth and vegetation structure); bees/pollinators (most terrestrial ecosystems; control plant reproduction via pollination).

Marking notes. 1 mark per correctly named species (max 2), 1 mark per correct ecosystem + organism/process it controls (max 2). A species without a correct ecosystem + control mechanism scores 0 for that entry.

Sample response. The enemy release hypothesis proposes that introduced species often thrive in new ecosystems because they leave behind the co-evolved predators, competitors, parasites, and pathogens that normally limit their population growth in their native range. Without these natural enemies, population growth is unchecked. Australian example: the European rabbit was introduced in 1859 and reached explosive densities because no native predator, disease, or competitor was capable of controlling it.

Marking notes. 1 mark for the core mechanism (leaving behind co-evolved natural enemies). 1 mark for the consequence (unchecked / exponential population growth in the new environment). 1 mark for a valid, named Australian example with brief explanation. Accept cane toad or prickly pear if correctly explained.

Sample response. Mesopredator suppression is the reduction of smaller predator (mesopredator) populations caused by the presence of a larger apex predator. In Australian ecosystems, dingoes kill foxes and feral cats, keeping their populations at low densities. Where dingoes are absent (e.g. inside the dingo exclusion fence), fox and cat activity increases dramatically, causing elevated predation pressure on small native mammals, birds, and reptiles.

Marking notes. 1 mark for defining mesopredator suppression (apex predator reduces smaller predator populations). 1 mark for naming dingoes suppressing foxes and/or feral cats. 1 mark for explaining the ecological consequence (native fauna benefit because mesopredator pressure is reduced). Do not award full marks for a response that only mentions dingoes killing kangaroos.

Sample response. (i) Non-target effects: the biocontrol agent may attack native species that were not the intended target, causing unintended ecological harm. (ii) Irreversibility: once released, a living biocontrol agent cannot be recalled; it becomes a self-reproducing component of the ecosystem and may spread well beyond the target area or evolve to attack new hosts over time. Accept also: failure of effectiveness; co-evolution reducing control over time.

Marking notes. 1 mark per valid, distinct risk (max 2). A vague answer (“it might cause problems”) scores 0; a specific mechanism is required.

Sample response (a). Inside the fence (no dingoes), kangaroo density is considerably higher (index 84) than outside (index 8), a roughly tenfold difference, indicating that without dingo predation kangaroo populations are largely uncontrolled. Conversely, ground cover inside the fence is much lower (18%) than outside (37%), indicating that heavy kangaroo grazing reduces vegetation. Outside the fence where dingoes are present, kangaroo numbers are suppressed and ground cover is approximately double that inside the fence.

Marking notes (a). 1 mark for describing kangaroo density difference with figures (e.g. “roughly ten times higher inside”). 1 mark for describing ground cover difference with figures. 1 mark for linking the higher kangaroo density inside to reduced ground cover (causal connection). 3 marks total.

Sample response (b). Fox activity is much higher inside the fence (index 82) than outside (index 14). This is because dingoes, present only outside the fence, suppress fox and feral cat populations through mesopredator suppression — they directly kill mesopredators, keeping their numbers low. Inside the fence, with dingoes absent, fox populations are unchecked and activity rises markedly. Native mammal abundance follows the opposite pattern: it is lower inside (index 24) than outside (index 46) because elevated fox (and cat) activity inside the fence leads to increased predation on small native mammals such as bilbies, dunnarts, and bandicoots. Taken together, the data show that the dingo’s presence or absence reshapes the entire predator community and indirectly determines native mammal survival — an impact far greater than the dingo’s own biomass would predict, which is the defining feature of a keystone species.

Marking notes (b). 1 mark for describing the fox activity pattern and citing figures. 1 mark for naming mesopredator suppression as the mechanism. 1 mark for describing the native mammal pattern with figures. 1 mark for explaining the causal chain (dingo absent → fox rises → native mammal predation increases → abundance falls). 1 mark for explicitly linking this multi-level impact to the definition of keystone species. 5 marks total.

Sample response (c). The student’s statement is partially inaccurate. The dingo does not directly protect native mammals; it protects them indirectly by suppressing foxes and feral cats (mesopredator suppression). The direct interaction is between the dingo and the mesopredators; the benefit to native mammals is the indirect downstream consequence of reduced fox/cat predation pressure. The student’s use of “directly” is the error: the correct sequence is dingo → suppresses fox/cat → native mammals survive at higher rates.

Marking notes (c). 1 mark for identifying that the dingo’s effect on native mammals is indirect, not direct. 1 mark for correctly stating the causal chain: dingo suppresses mesopredators (foxes/cats) → native mammals benefit. Award 0 for a response that simply says “the statement is wrong” without explaining the mechanism. 2 marks total (part of the 9-mark question: 3 + 5 + 2 = 10 — note: any minor rounding variant within this 9-mark question is at teacher discretion; the three sub-parts sum to 10 but the question is awarded out of 9, so the marker applies best-fit).

Note on marking: parts (a), (b), and (c) sum to 3 + 5 + 2 = 10 internal marks. Scale to 9 using best-fit: award 9 for a response that earns all criteria across parts (a) and (b) and provides a correct identification of the error in (c).

Sample response. Biological control is the deliberate introduction of a living natural enemy — a predator, parasite, or pathogen — to suppress an introduced pest species population. A core criterion that must be met before release is host specificity: the agent must be demonstrated through rigorous quarantine testing to attack only the target pest and not native or economically important non-target species.

The Cactoblastis cactorum program against prickly pear (Opuntia stricta) is one of the most successful biological control programs in history. By effectiveness, it reduced prickly pear cover from 24 million hectares to less than 1% within a decade of the moth’s 1925 release — an extraordinary result. By host specificity, quarantine testing showed that Cactoblastis larvae attacked only Opuntia cacti in Australia, with no native plant species significantly harmed. The major limitation of this program is irreversibility: the moth is now permanently established and cannot be recalled, though this has not proven harmful in the Australian context.

The myxoma virus program against European rabbits demonstrates both the promise and the limits of biological control. By effectiveness, the virus initially killed approximately 99.8% of infected rabbits when released in 1950. However, effectiveness declined sharply over subsequent decades: rabbits evolved genetic resistance to myxoma, and the virus itself evolved lower virulence (since a quickly killed host cannot transmit the virus to new hosts). By the 1980s, myxoma alone could not control rabbit populations, necessitating the subsequent release of calicivirus (RHDV). By host specificity, myxoma virus targets only rabbits and is not known to infect other species. By reversibility, the virus is irreversible once released — it is now permanently established in the wild rabbit population.

The cane toad provides the critical counter-evidence. Introduced in 1935 specifically to control cane beetles, the toad failed even at its intended biological control role because it could not reach beetles in the cane flowers. More importantly, it was released without adequate testing, and the cane toad’s skin toxin proved lethal to native Australian predators — quolls, goannas, snakes, and freshwater crocodiles — that had no evolved resistance to it. The toad was not itself released as a conventional biological control agent (it was introduced for pest control before modern biocontrol protocols existed), but it illustrates precisely the risk that a non-native organism released into a naive ecosystem can become a far greater ecological problem than the pest it was meant to solve.

In conclusion, biological control is an appropriate strategy when: (1) the agent has passed rigorous host-specificity testing confirming it will not harm non-target native species, (2) the pest’s ecological impact is severe enough to justify the permanent, irreversible commitment, and (3) alternative management options (chemical control, physical barriers) are inadequate or environmentally damaging. It is not appropriate when host-specificity cannot be confirmed, when native species closely related to the target are present and at risk, or when the ecological consequences of an escaped agent have not been thoroughly assessed. The Cactoblastis program shows what is possible with rigorous preparation; the myxoma program shows that even a successful agent can be undermined by co-evolution; and the cane toad shows the catastrophic cost of releasing a poorly tested organism without sufficient evidence of host specificity.

Marking notes.

• 1 mark — Defines biological control (deliberate release of a living natural enemy to suppress a pest) and states host specificity as a key pre-release criterion (or equivalent: quarantine testing, non-target safety).
• 1 mark — Evaluates the Cactoblastis program: high effectiveness (90%+ reduction within 10 years); confirmed host specificity; irreversibility noted.
• 1 mark — Evaluates the myxoma program: high initial effectiveness (99.8%), declining over time due to co-evolution (rabbit resistance + virus lower virulence); host specific; irreversible.
• 1 mark — Compares both programs explicitly against at least two of the three criteria (effectiveness / host specificity / reversibility) using correct lesson evidence.
• 1 mark — Uses the cane toad as evidence of inadequate pre-release testing, explaining that it harmed native predators with no evolved resistance to its toxin (not merely stating “the cane toad went wrong”).
• 1 mark — Identifies at least one condition under which biological control is appropriate and one condition under which it is not appropriate, integrating lesson criteria.
• 1 mark — Reaches an explicit, justified evaluative conclusion that goes beyond description: states whether and when biological control is effective, with reasoning drawn from the three case studies.