HSC Exam Practice

Sample response. A trophic cascade is a top-down effect where removing an apex predator releases herbivore populations from predation pressure (direct effect), causing herbivores to consume more vegetation than it can regenerate (indirect effect), which reduces plant biomass, alters soil structure, and changes the habitat available to other species across multiple trophic levels.

Marking notes. 1 mark for defining trophic cascade as a top-down effect initiated by predator removal; 1 mark for the mechanism: predator removal → herbivore population increase (released from predation); 1 mark for the downstream consequence: increased herbivory → vegetation loss → ecosystem restructuring.

Sample response. A direct effect is an immediate, first-order interaction between two species. An indirect effect is a consequence that flows through one or more intermediate species. In the dingo removal case study: the direct effect is that dingo removal reduces predation mortality on kangaroos (dingoes and kangaroos interact directly). An indirect effect is that reduced kangaroo predation leads to increased kangaroo grazing, which reduces ground cover, which increases soil erosion — the vegetation and soil are not interacting directly with dingoes but are affected via the intermediate kangaroo population.

Marking notes. 1 mark for correctly defining direct effect (immediate, first-order interaction); 1 mark for correctly defining indirect effect (mediated through intermediate species); 1 mark for a correct dingo example of a direct effect (dingo kills kangaroo, so removing dingoes directly increases kangaroo survival); 1 mark for a correct dingo example of an indirect effect with the intermediate species named (e.g. dingo removal → kangaroos increase [intermediate] → vegetation lost [indirect effect on vegetation]).

Sample response. The predator population peak lags behind the prey peak because predators must first respond to increased prey availability through reproduction and improved survival, which takes time — a generation or more. Predators do not instantly appear in response to increased prey; births, maturation, and survival of new individuals are required before the predator population reaches its peak.

Marking notes. 1 mark for identifying that predator reproduction takes time to respond to increased prey (demographic lag); 1 mark for explaining the mechanism (births and maturation of new predators are needed, not an instantaneous response).

Sample response. (1) Herbivory shifts the competitive balance among plant species: palatable species are preferentially consumed, favouring species with physical or chemical defences (e.g. spines, tannins) that are not grazed as heavily. Over time this changes plant community composition toward defended species. (2) Herbivory alters disturbance regimes: overgrazing removes ground cover and litter, so fires burn hotter and more extensively because the reduced fuel continuity of defended species changes how fire spreads across the landscape. Accept also: alters nutrient cycling (herbivores accelerate nutrient cycling by excreting consumed nutrients); reduces structural complexity (removes understory shelter for ground-dwelling animals).

Marking notes. 2 marks per effect (1 for naming the effect, 1 for explaining the mechanism). Any two of the four lesson effects are acceptable: competitive balance shift; disturbance regime change; nutrient cycling alteration; structural complexity reduction. Half marks not awarded — each effect requires naming and mechanism for full 2 marks.

Sample response (a). Tiger sharks increased to 220% of the 2005 baseline, and groupers increased to 180% of baseline — both more than doubling. Small herbivorous fish declined to 62% of baseline (a 38% decrease). Seagrass cover recovered strongly, reaching 195% of baseline. The pattern shows large predatory species increased while their prey (herbivorous fish) decreased and the resource previously consumed by those herbivores (seagrass) recovered.

Marking notes (a). 1 mark for identifying that predatory species (sharks, groupers) increased and that herbivorous fish decreased, supported by figures; 1 mark for noting the seagrass increase and identifying the overall pattern (predators up, herbivores down, vegetation up).

Sample response (b). This data pattern is explained by a trophic cascade. Tiger sharks and groupers are apex and mid-level predators that prey on smaller fish, including herbivorous fish. As sharks and groupers recovered, their predation on small herbivorous fish intensified — a direct effect on the herbivorous fish population. With fewer herbivorous fish, grazing pressure on seagrass declined, allowing seagrass beds to recover — an indirect effect of shark/grouper recovery on seagrass. The chain of causation is: shark and grouper recovery (fishing ban) → increased predation on herbivorous fish (direct effect) → herbivorous fish population decreases → seagrass grazing pressure decreases → seagrass cover recovers (indirect effect). This is a trophic cascade operating in the positive direction (predator recovery → herbivore suppression → vegetation recovery).

Marking notes (b). 1 mark for identifying sharks/groupers as predators of herbivorous fish; 1 mark for identifying herbivorous fish decline as a direct effect of predator recovery; 1 mark for identifying seagrass recovery as an indirect effect (mediated via reduction in herbivorous fish grazing); 1 mark for providing a complete, correctly ordered chain of causation with all four steps.

Sample response (c). The second biologist is correct: the reduction in herbivorous fish is an indirect effect of shark recovery, not a direct one. Tiger sharks are large apex predators that primarily prey on large fish and marine mammals such as dugongs — they do not typically prey directly on small herbivorous fish. The herbivorous fish are reduced by groupers (mid-level predators that do prey directly on them) — so the chain is: tiger sharks suppress grouper competitors, groupers increase, groupers prey on herbivorous fish. Alternatively, if groupers are the direct predators, then herbivorous fish decline is a direct effect of grouper recovery but still an indirect effect of tiger shark recovery.

Marking notes (c). 1 mark for stating that the reduction in herbivorous fish is an indirect effect of shark recovery (mediated via groupers as an intermediate predator); 1 mark for explaining the mechanism: tiger sharks do not directly consume herbivorous fish, so their effect on herbivorous fish must pass through an intermediate predator. Accept: direct effect of grouper recovery on herbivorous fish.

Sample response. Predation and herbivory operate at different trophic levels but interact to shape both population structure and ecosystem function. Their effects can be analysed through the lens of direct and indirect effects and trophic cascade theory.

In the Shark Bay case study, the removal of tiger sharks (apex predator) had one direct effect — reduced predation mortality on dugongs, causing the dugong population to increase — and multiple indirect effects: dugong overgrazing collapsed seagrass meadows, which in turn eliminated the habitat and food resources for fish, turtles, and dolphins. Here, predation (apex level) structured herbivory (dugong level), and herbivory (dugong grazing) structured primary producer density (seagrass). The trophic cascade concept captures this: top-down predation control suppresses herbivore populations, limiting herbivory; remove the predator, and herbivory intensifies, fundamentally restructuring the ecosystem at the producer level.

In the dingo removal case study, the same logic applies across a terrestrial system: dingo removal released kangaroos and rabbits from predation, intensifying herbivory on ground cover, producing soil erosion, dryland salinity, weed invasion, and habitat loss for ground-dwelling species. The direct effect (reduced kangaroo mortality) was far smaller in magnitude than the cumulative indirect effects (ecosystem degradation across thousands of km²).

This interdependence has critical implications for ecosystem recovery. If only the predation pressure is restored (e.g. dingoes returned to grazing land) but herbivory pressure is not addressed (e.g. feral rabbits persist at high density without additional control), recovery is likely to be incomplete. Rabbits are herbivores that dingoes do not preferentially control; rabbit overgrazing would continue even if kangaroo densities dropped. Conversely, if herbivory alone is managed (e.g. culling kangaroos) but dingoes are absent, the management effort must be continuous because without natural predation to regulate kangaroo numbers, culling can only temporarily reduce grazing pressure. Sustained, system-wide recovery requires addressing both the predation structure (restoring apex predators) and the herbivory consequences (allowing vegetation to recover, controlling introduced herbivores).

In conclusion, predation shapes population structure by limiting herbivore numbers through top-down control; herbivory shapes ecosystem function by determining plant community composition, soil stability, and habitat availability for other species. Because the two processes are linked through trophic cascades, managing only one while the other remains unaddressed will not restore the full structure and function of the ecosystem.

Marking notes. 1 mark — defines or correctly applies trophic cascade to at least one named Australian case study, identifying the predation link. 1 mark — identifies a direct effect of apex predator removal in the chosen case study (e.g. reduced predation mortality on dugongs/kangaroos). 1 mark — identifies at least one indirect effect with a correctly ordered chain of causation. 1 mark — identifies a second indirect effect OR demonstrates that indirect effects are larger in magnitude than the direct effect. 1 mark — explains how predation and herbivory interact (predator suppresses herbivore, limiting herbivory; remove predator → herbivory intensifies → ecosystem restructured). 1 mark — evaluates whether restoring only predation OR only herbivory management alone is sufficient for ecosystem recovery, with a justified argument. 1 mark — reaches an explicit synthesis conclusion using precise lesson terminology (trophic cascade, direct/indirect effects, top-down control, population structure, ecosystem function) that integrates both processes.