HSC Exam Practice

Sample response. Habitat fragmentation is the division of continuous habitat into smaller, isolated patches surrounded by an inhospitable matrix (such as farmland or roads). It differs from habitat destruction, which removes habitat entirely; fragmentation leaves patches of habitat but isolates them, producing effects (edge effects, reduced gene flow) beyond simple area loss.

Marking notes. 1 mark for defining fragmentation (continuous habitat divided into isolated patches). 1 mark for distinguishing it from destruction (habitat remains but is isolated, vs entirely removed).

Sample response. Effect 1 — reduced population size in each patch, leading to genetic drift and inbreeding depression. Effect 2 — reduced gene flow between isolated patches. (Also accept: edge effects degrading the patch margins; extinction debt.)

Marking notes. 1 mark per valid effect (max 2). Naming the effect is sufficient at Band 3; the effect must be a genuine consequence of fragmentation.

Sample response. Extinction debt refers to species that are committed to eventual extinction because of habitat loss, even though they persist temporarily in a fragment that is too small to support a viable population. The species is effectively already doomed, but its final disappearance may take decades.

Marking notes. 1 mark for "committed to eventual extinction due to habitat loss". 1 mark for "persists temporarily / final disappearance delayed (fragment below viable size)".

Sample response. The common carp competes with native fish for food and space, and feeds by stirring up bottom sediment, which reduces water clarity. The increased turbidity degrades habitat and reduces light for aquatic plants, disadvantaging native species so their abundance falls.

Marking notes. 1 mark for a competition/predation/disruption mechanism. 1 mark for a second mechanism or a clear link to reduced native abundance (e.g. sediment/turbidity degrading habitat). Accept any two valid mechanisms.

Sample response. Excess nitrogen and phosphorus enter the water from agricultural fertiliser or sewage. These nutrients cause explosive growth of algae (an algal bloom). When the algae die, bacteria decompose them, and this decomposition consumes large amounts of dissolved oxygen. The water becomes hypoxic (low in oxygen), so fish, crustaceans and other aerobic organisms suffocate and die.

Marking notes. 1 mark for nutrient input causing an algal bloom. 1 mark for algal death and bacterial decomposition consuming oxygen. 1 mark for oxygen depletion (hypoxia) directly causing fish death. The lethal step must be oxygen depletion, not direct algal toxicity.

Sample response. DDT is fat-soluble and chemically stable, so it is stored in body tissues rather than excreted, and accumulates in an organism over its lifetime. When organisms at one trophic level are eaten by those at the next, the stored DDT is passed on and concentrated, because a predator consumes many contaminated prey. This biomagnification means the concentration rises at each successive trophic level, so apex predators carry the highest concentration — high enough to cause harm such as eggshell thinning and reproductive failure, even though the original application concentration was low.

Marking notes. 1 mark for DDT being fat-soluble/stable and stored rather than excreted (bioaccumulation). 1 mark for concentration increasing at each trophic level because predators consume many contaminated prey (biomagnification). 1 mark for explaining that apex predators therefore reach lethal/harmful concentrations.

Sample response (a). DDT concentration increases at each higher trophic level: from 0.04 ppm in plankton, to 0.5 ppm in small fish, to 5 ppm in large fish, to 24 ppm in the fish-eating bird. The increase is large and accelerates toward the top of the food chain.

Sample response (b). 24 ÷ 0.04 = 600. The DDT concentration in the fish-eating bird is 600 times greater than in the plankton.

Sample response (c). DDT is fat-soluble and stable, so it is stored in tissues rather than excreted and accumulates over an organism's life. At each trophic level a consumer eats many contaminated prey, so the stored DDT is concentrated further (biomagnification), producing the rising trend. This is most dangerous for apex predators because they sit at the top of the chain and accumulate the highest concentration, which can reach harmful or lethal levels causing effects such as eggshell thinning and reproductive failure — even though the concentration in the water and plankton was very low.

Marking notes. Part (a) — 1 mark for the increasing trend; 1 mark for quoting at least two specific values. Part (b) — 1 mark for the method (24 ÷ 0.04); 1 mark for the answer (600 times). Part (c) — 1 mark for the mechanism (fat-soluble/stable, stored, concentrated at each level); 1 mark for explaining why apex predators are most affected (highest concentration / harmful effects).

Sample response. The claim that a single human impact is sufficient to explain the decline of most Australian ecosystems must be rejected. While each impact is individually damaging, real ecosystems usually decline through several stressors acting together, and their combined effect is often worse than the sum of the parts — a phenomenon called multi-stressor synergy. The Murray-Darling Basin demonstrates this clearly. Native fish have fallen to less than 10% of pre-European levels under at least five simultaneous stressors: habitat destruction (about half the wetlands drained and riparian vegetation cleared), fragmentation (dams and weirs blocking fish migration and isolating populations), pollution (agricultural runoff adding nitrogen, phosphorus and pesticides, plus rising salinity), the invasive common carp (outcompeting natives and increasing turbidity), and climate change (lower rainfall and higher temperatures increasing drought). These amplify each other: fragmented, isolated fish populations are already small and vulnerable, so the added pressure of carp, pollution and drought pushes them past thresholds that any single stressor would not. Addressing only pollution while ignoring water extraction, barriers to migration and carp would not recover the ecosystem, because the remaining interacting stressors continue to suppress native fish. The Great Barrier Reef provides a second case study of the same principle and of the land–sea connection. The reef declines not from warming alone but from the synergy of ocean warming (causing bleaching), nutrient runoff from land clearing and agriculture (about 7000 tonnes of nitrogen per year), and crown-of-thorns starfish outbreaks fuelled by that nutrient enrichment, which have killed roughly 40% of GBR coral since 1985. Heat-stressed coral recovers poorly from CoTS damage, so the stressors compound one another. Reducing one stressor (such as farm runoff) helps but cannot, on its own, reverse the decline while warming continues. In both cases, a single-impact explanation is incomplete and would lead to ineffective management. Therefore the claim is not supported: most Australian ecosystem decline is best explained by multiple, synergistic human impacts, and effective conservation must address several interacting stressors simultaneously rather than any one in isolation.

Marking criteria. 1 mark — Makes an explicit evaluative judgement rejecting the single-impact claim. 1 mark — Defines/uses multi-stressor synergy (combined effect exceeds the sum of parts). 1 mark — Uses the Murray-Darling Basin accurately as a named case study (multiple stressors; <10% decline). 1 mark — Explains how stressors amplify each other in that case (not just lists them). 1 mark — Uses the Great Barrier Reef accurately as a second named case study (warming + runoff + CoTS / land–sea connection). 1 mark — Explains why addressing one stressor alone would fail to recover the ecosystem. 1 mark — Reaches a sustained conclusion that integrated, multi-stressor management is required.