HSC Exam Practice

Sample response. Biodiversity is the total variety of life on Earth. It is measured at three levels: genetic diversity (variation in alleles within a population), species diversity (the number and relative abundance of species in a community), and ecosystem diversity (the variety of habitat types and ecological processes across a region).

Marking notes. 1 mark for defining biodiversity as the variety of life (accept “variety of life at multiple levels” or similar); 1 mark for correctly naming all three levels (genetic, species, ecosystem). Award 0 if fewer than three levels are named or any level is incorrect.

Sample response. Species richness is the total number of different species present in a community. Species evenness is how uniformly individuals are distributed across those species — how similar the proportional abundance of each species is. A community can have high richness but low evenness when many species are present but one or a few species dominate numerically. For example, a plantation forest might contain 20 tree species (high richness) but 95 % of individuals belong to a single pine species; the other 19 species are present but rare, so evenness is low.

Marking notes. 1 mark for correctly defining species richness (total number of species); 1 mark for correctly defining species evenness (uniform / proportional distribution of individuals); 1 mark for providing an appropriate example showing high richness with low evenness (one species dominant, others rare).

Sample response. Provisioning services supply resources directly used by humans — for example, food (crops, fish), freshwater, timber, or medicines derived from native plants. Regulating services maintain environmental conditions — for example, pollination of crops by native bees, carbon storage by forests, water purification by wetland vegetation, or flood mitigation by coastal mangroves. Cultural services include recreation, aesthetic value, Indigenous heritage, and scientific research conducted in national parks.

Marking notes. 1 mark per correctly named category paired with a valid named example (max 2). Accept any two of: provisioning, regulating, or cultural. Award 0 if a category is named without an example, or an example is given without its category.

Sample response. Tasmanian devils experienced a historical population bottleneck that severely reduced their genetic diversity — the variation in allele frequencies within the population. With most individuals sharing nearly identical alleles at immune-system loci, the population lacks individuals with immune-gene variants capable of recognising DFTD cells as foreign. The tumour cells are instead recognised as “self,” so the immune response is not triggered. Because all individuals share this vulnerability, the disease can spread through virtually the entire population once introduced.

Marking notes. 1 mark for correctly defining or applying genetic diversity (variation in alleles within a population, reduced by bottleneck); 1 mark for explaining the immune mechanism (shared immune-system alleles → tumour recognised as self → no immune response); 1 mark for linking low genetic diversity to uniform susceptibility across the whole population (no variation for natural selection to act upon).

Sample response. The lesser bilby and similar digging mammals were ecosystem engineers whose burrowing aerated soil, increased water infiltration, and brought nutrient-rich subsoil to the surface. Their extinction has led to soil compaction and reduced water penetration in arid zones, increasing surface runoff and degrading vegetation cover. No native species has assumed this ecological function, so the regulating service of soil aeration has been permanently diminished.

Marking notes. 1 mark for correctly describing the ecological role of digging mammals (burrowing → soil aeration, water infiltration, nutrient cycling); 1 mark for describing a specific consequence of their loss (soil compaction, reduced water infiltration / increased runoff, vegetation degradation). Accept reference to bandicoot dispersal of mycorrhizal spores as an alternative consequence.

Sample response (a). Extinct. The thylacine has not been confirmed since 1936 and no viable population is known to exist; it meets the IUCN criterion for extinction (no reasonable doubt that the last individual has died).

Sample response (b). Vulnerable (accept Endangered). The greater bilby is experiencing significant decline in range and abundance, meeting the threshold for a threatened category. If declines continue it may be reclassified as Endangered.

Marking notes. 1 mark per correct category with a justified sentence (max 2). Do not award the mark if the category is correct but the justification is absent or factually wrong. Accept Vulnerable or Endangered for the greater bilby with appropriate reasoning.

Sample response (a). Both sites have the same species richness (4 species each). However, Site X has substantially higher species evenness: each species represents exactly 25 % of individuals, so individuals are distributed equally across all four species. At Site Y, one species (common starling) makes up 85 % of all individuals; the remaining three species together account for only 15 %, giving very low evenness. Overall species diversity is therefore higher at Site X despite identical richness.

Sample response (b). Site X would be more resilient to an avian influenza outbreak. Functional redundancy refers to the situation where multiple species perform the same ecological function, buffering the ecosystem against the loss of any one species. At Site X, four roughly equal species occupy waterbird niches (foraging, seed dispersal, filter-feeding); if the disease eliminates one species, the others can maintain core ecological functions. At Site Y, the ecosystem relies overwhelmingly on the common starling (85 % of individuals). If that species is decimated, there are too few individuals of the remaining species to compensate, causing functional collapse. High evenness at Site X therefore underpins higher functional redundancy and greater resilience.

Marking notes. Part (a) — 1 mark for correctly comparing richness (equal, 4 species each); 1 mark for correctly comparing evenness, using supporting data (Site X equal distribution 25 % each vs Site Y starling dominance at 85 %). Part (b) — 1 mark for selecting Site X with correct reasoning; 1 mark for correctly defining or applying functional redundancy; 1 mark for linking the uneven distribution at Site Y to low redundancy and vulnerability to collapse.

Sample response. The claim contains one defensible element but is largely incorrect when examined against biological evidence.

It is true that extinction has always been a feature of life on Earth: the background extinction rate is approximately one species per million species per year, and over geological timescales evolution does replace extinct lineages. To this limited extent the claim is defensible.

However, Australia’s current extinction rate is approximately 1,000 times the background rate. The drivers of Australia’s 34 mammal extinctions since European settlement are not natural selection processes but human activities: habitat destruction through land clearing, introduced predators (cats and foxes), altered fire regimes, and invasive species competition. These are human-caused pressures, not the evolutionary processes the claim implies. Describing these extinctions as “natural evolution” is therefore biologically misleading.

The ecological consequences of losing digging mammals directly refute the claim that “the ecosystem will adjust.” The lesser bilby was an ecosystem engineer whose burrowing aerated soil, increased water infiltration, and cycled nutrients to the surface — a regulating ecosystem service. Similarly, bandicoots dispersed fungal spores that maintained mycorrhizal networks supporting plant health across Australian forests. Both of these services have been lost. No native species has assumed these functions because functional redundancy was low — only a small number of species performed the soil-engineering and spore-dispersal roles, and when those species disappeared there was no ecological back-up.

Decades after these extinctions, soils remain compacted, water infiltration is reduced, and mycorrhizal networks are weaker. The ecosystem has not self-adjusted; it has shifted to a degraded state. This is precisely what low functional redundancy predicts: when the only species performing a critical function is lost, the function ceases and recovery requires intervention.

The claim that human intervention is unnecessary is therefore rejected. Because Australia’s extinctions are human-caused and have reduced functional redundancy below the threshold at which ecosystems self-repair, conservation programs are not interfering with a natural process — they are attempting to restore functional diversity that human activity has destroyed.

Marking notes. 1 mark — Identifies the defensible element (extinction is natural; background rate context). 1 mark — Contrasts current Australian rate with background rate (~1,000× higher) and names human-caused drivers. 1 mark — Names a digging mammal (lesser bilby or bandicoot) and describes its ecosystem engineer role (soil aeration / water infiltration). 1 mark — Links the loss of this role to a specific regulating ecosystem service (soil aeration, water filtration, or nutrient cycling). 1 mark — Correctly applies functional redundancy to explain why the ecosystem has not self-adjusted (few species performed these functions → no back-up when lost). 1 mark — Provides a second named example (bandicoot + mycorrhizal spore dispersal, or Murray-Darling wetland collapse, or Tasmanian devil + low genetic diversity) that further undermines the claim. 1 mark — Reaches a clear evaluative conclusion that explicitly rejects the claim, using lesson evidence, and addresses whether ecosystems recover without intervention.