HSC Exam Practice

Sample response. An ecosystem is a community of organisms (biotic component) interacting with each other and with their non-living (abiotic) environment, such as sunlight, temperature, water and soil minerals.

Marking notes. 1 mark for identifying the biotic component (community of organisms / living organisms interacting with each other). 1 mark for identifying the abiotic component (non-living physical or chemical environment). A response that mentions only organisms and not the physical environment scores 1/2.

Sample response. A community is all the populations of different species in a defined area and includes only biotic (living) components. An ecosystem is the community plus its abiotic environment. For example, in Moreton Bay, the community consists of all the fish, seagrasses, invertebrates and microbes living together; the ecosystem adds the abiotic factors such as water temperature, salinity, dissolved oxygen, pH and light penetration.

Marking notes. 1 mark for correctly defining community (all populations of different species, biotic only — accept equivalent phrasing). 1 mark for correctly defining ecosystem as community plus abiotic environment. 1 mark for a named Australian example that correctly illustrates the distinction. A response that names an ecosystem without linking biotic and abiotic components scores 2/3.

Sample response.

Marking notes. 1 mark per correct row (biotic/abiotic classification plus correct trophic role where required), maximum 3 marks. For abiotic components, the trophic role column is not required. Award 0 for any row where the biotic/abiotic classification is incorrect regardless of trophic role.

Sample response. Energy enters ecosystems as sunlight and is captured by producers via photosynthesis, converting light energy into chemical energy stored in glucose. This energy is transferred to consumers through feeding (consumption). At each trophic transfer, energy is lost irreversibly as heat via cellular respiration. Because energy is continuously lost and cannot be recovered or recycled, ecosystems require a constant external supply of solar energy to maintain biological activity. Carbon, by contrast, enters as carbon dioxide, is fixed into organic molecules by producers via photosynthesis, is transferred to consumers through feeding, and is returned to the abiotic environment as carbon dioxide by producers and consumers via respiration and by decomposers breaking down dead organic matter. The same carbon atoms cycle indefinitely between biotic organisms and the abiotic environment; no new carbon atoms need to enter the system.

Marking notes. 1 mark for describing the energy pathway (sunlight → photosynthesis by producers → transfer to consumers via feeding → lost as heat via cellular respiration). 1 mark for explaining why a continuous energy supply is needed (energy lost as heat at each step; cannot be recycled). 1 mark for describing the carbon pathway (CO₂ → photosynthesis → feeding → respiration/decomposition → CO₂ returns to environment). 1 mark for explaining why continuous carbon input is not needed (same atoms recycled, no net loss from the system).

Sample response. (i) Water temperature: reef-building corals survive only between approximately 18°C and 30°C. Above this threshold, corals expel their zooxanthellae (bleaching), lose their primary energy source, and may die, limiting coral distribution to thermally stable tropical and subtropical waters. (ii) Light intensity: zooxanthellae require sunlight for photosynthesis, which provides up to 90% of the coral's energy. Corals are therefore limited to the photic zone (generally less than 50 m depth) where sufficient light penetrates. Below this depth, insufficient light reaches the zooxanthellae and corals cannot calcify. (iii) pH: ocean acidification (lower pH) reduces the concentration of carbonate ions (CO₃²⁻) in seawater. Carbonate is required for coral polyps to build calcium carbonate skeletons. At lower pH, calcification rates decrease and existing skeletons may dissolve, restricting coral distribution to waters with sufficiently high pH.

Marking notes. 1 mark per factor (maximum 3). Each mark requires both identification of the specific effect and a mechanistic explanation linking the factor to coral biology. Naming the factor alone without a mechanism scores 0 for that point.

Sample response (a). Where dingoes are present, kangaroo density is low (8 km⁻²) and grass cover is high (73%). Where dingoes are absent, kangaroo density is markedly higher (42 km⁻², approximately five times greater) and grass cover is substantially lower (29%). The data show an inverse relationship between dingo presence and kangaroo density, and a positive association between dingo presence and grass cover.

Marking notes (a). 1 mark for identifying the inverse relationship between dingo presence and kangaroo density with supporting data. 1 mark for identifying the positive relationship between dingo presence and grass cover with supporting data. A response that describes only one of the two relationships scores 1/2.

Sample response (b). Dingoes are tertiary consumers (top carnivores). Kangaroos are primary consumers (herbivores) that eat grasses (producers). In the dingo-absent area, predation pressure on kangaroos is removed; the kangaroo population increases dramatically (from 8 to 42 km⁻²), and their increased herbivory reduces grass cover from 73% to 29%. Reduced grass cover is both a direct biological consequence (reduced producers) and an indirect abiotic consequence: grass roots bind soil, so reduced cover leads to increased soil erosion, altered water retention and reduced soil nutrient levels — abiotic changes that further impair plant re-establishment. This is a trophic cascade in which the removal of a top predator (biotic change) propagates through trophic levels to alter both biotic (grass cover) and abiotic (soil structure, nutrient availability) components of the ecosystem.

Marking notes (b). 1 mark for classifying dingo as tertiary consumer / top carnivore and kangaroo as primary consumer / herbivore and grass as producer. 1 mark for explaining the causal chain: dingo removal → kangaroo increase → increased herbivory → grass decline. 1 mark for identifying at least one abiotic change caused by reduced grass cover (e.g. increased soil erosion, reduced water retention, altered nutrient levels) and linking it back to the removal of a biotic component. 1 mark for using the term “trophic cascade” or equivalent concept (changes at one trophic level cascade through to other levels) in a biologically accurate context.

Sample response. The claim that biotic and abiotic components are independent is not supported by evidence and is scientifically inaccurate. Ecosystems are integrated systems in which biotic and abiotic components continuously influence each other in both directions.

In the Great Barrier Reef, two named abiotic factors directly shape the biotic community. First, water temperature determines which species can survive: reef-building corals require temperatures between approximately 18°C and 30°C because their symbiotic algae (zooxanthellae, the primary producers of the reef) require stable, warm conditions to photosynthesise efficiently. When temperature rises above 30°C, corals expel their zooxanthellae, lose up to 90% of their energy supply, cease calcification, and may die — restructuring the entire biotic community. This demonstrates that an abiotic factor (temperature) directly and profoundly alters the biotic component. Second, pH determines calcification rates: as ocean acidification lowers the concentration of carbonate ions (CO₃²⁻), corals cannot build calcium carbonate skeletons at normal rates. The loss of coral skeletal structure reduces the physical complexity of the reef habitat, which is itself an abiotic feature that determines the number of niches available to reef fish and invertebrates. A more complex reef supports greater biodiversity; a degraded one does not.

Biotic changes also alter abiotic conditions, directly refuting the second part of the claim. When mass bleaching kills coral, the structural complexity of the reef (an abiotic habitat feature) collapses: there are fewer crevices, overhangs and microhabitats. Reduced structural complexity increases light penetration to the seafloor and increases sedimentation from the unbound rubble, both of which are abiotic changes triggered by a biotic collapse. A terrestrial example reinforces this: in semi-arid Australian shrublands, overgrazing by kangaroos (following dingo removal) strips grass cover, exposing soil to wind and rain erosion. Grass roots stabilise soil, retain moisture and maintain nutrient cycling; their loss creates altered soil structure and moisture regimes — abiotic changes caused by a biotic change (reduced plant cover). Decomposers also illustrate bidirectional coupling: when dead organic matter (a biotic output) accumulates, it chemically alters the pH, dissolved oxygen and nutrient concentrations of the water or soil (abiotic factors), which in turn influence which microbial decomposer communities can survive.

In conclusion, the claim is unjustified. The evidence from the Great Barrier Reef demonstrates that abiotic factors (temperature, pH) continuously regulate the biotic community, and changes to the biotic community (coral death, vegetation removal) feed back to alter abiotic conditions (structural complexity, soil properties, water chemistry). Biotic and abiotic components do not operate independently; they are coupled through reciprocal interactions that define the ecosystem as an integrated system rather than a simple list of species and conditions.

Marking notes.

• 1 mark — States a clear evaluative position (e.g. claim is unjustified / incorrect) at the outset or conclusion, supported by reasoning.
• 1 mark — Identifies and explains how a first named abiotic factor shapes the biotic community of a named Australian ecosystem (e.g. temperature → coral bleaching on the Great Barrier Reef, with mechanism).
• 1 mark — Identifies and explains how a second named abiotic factor shapes the biotic community (e.g. pH → reduced calcification → altered structural complexity).
• 1 mark — Provides a specific example in which a biotic change alters the abiotic environment (e.g. coral death reduces reef structural complexity and increases sedimentation; or vegetation removal alters soil properties in a terrestrial ecosystem).
• 1 mark — Uses at least one named Australian ecosystem consistently throughout the response as the primary case study.
• 1 mark — Demonstrates understanding of reciprocal/bidirectional coupling: the response must show that causation runs both ways (abiotic → biotic AND biotic → abiotic), not only in one direction.
• 1 mark — Response reaches an explicit, justified evaluative conclusion that links the evidence to the claim, using appropriate ecology terminology (ecosystem, biotic, abiotic, trophic, integrated system, or equivalent).