Biodiversity — Measurement, Importance and Ecosystem Stability

Q1 — Sample Band 6 response (8 marks), annotated

Species diversity is the measure of the number and relative abundance of species in a community. It has two components: species richness (the total number of species present) and species evenness (how uniformly individuals are distributed across those species). A community can have high richness but low evenness if one species dominates numerically. [1 — species diversity defined with both components]

Functional redundancy is the situation where multiple species perform the same ecological function within an ecosystem. For example, in Australian woodlands multiple native bee, wasp, and butterfly species all contribute to pollination. When one species is reduced by pesticide use, others continue the function. Higher species diversity leads to greater redundancy because there are more species available to overlap in their ecological roles. [1 — functional redundancy defined; 1 — linked to species diversity]

Resilience is an ecosystem’s ability to resist disruption and return to its pre-disturbance state after a stress. Higher biodiversity increases resilience through two mechanisms: redundancy buffers the loss of any individual species, and the variety of life histories means that when one functional group is stressed, another with different tolerances can maintain energy flow and nutrient cycling. [1 — resilience defined; 1 — two mechanisms linking diversity to resilience]

The lesson’s Australian example is instructive. Since European settlement, the loss of digging mammals — the lesser bilby, bandicoots, and bettongs — has eliminated the ecosystem engineers that aerated soil and allowed water infiltration. With no species to replace this function (low redundancy), soil has become compacted and water runoff has increased across arid zones. The ecosystem has not recovered to its pre-disturbance state; resilience has been permanently reduced. [1 — named Australian example; 1 — links loss of species to reduced redundancy and reduced resilience]

In summary, as species diversity declines, functional redundancy falls and resilience contracts: each additional species lost removes a potential back-up for ecosystem functions, making the community increasingly fragile to further perturbation. [1 — explicit conclusion linking the three concepts]

Marking criteria.

• 1 mark — Defines species diversity and correctly identifies both richness and evenness as its components.
• 1 mark — Defines functional redundancy correctly (multiple species performing the same function).
• 1 mark — Links functional redundancy to species diversity (higher diversity → more overlap → more redundancy).
• 1 mark — Defines ecosystem resilience correctly (ability to resist/recover from disturbance).
• 1 mark — Explains at least one mechanism connecting biodiversity to resilience (redundancy or diverse response to disturbance).
• 1 mark — Names a specific Australian species, ecosystem, or event as a supporting example.
• 1 mark — Uses the example to show what happens to resilience when biodiversity declines (reduced redundancy → reduced resilience).
• 1 mark — Reaches an explicit conclusion integrating species diversity, redundancy, and resilience.

Q2 — Sample Band 6 response (7 marks), annotated

The claim is largely incorrect, with one defensible element.

Defensible element: 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 evolution does replace extinct lineages over geological timescales. [1 — concedes defensible element with context]

What is wrong: The current extinction rate in Australia is approximately 1,000 times the background rate, driven primarily by habitat destruction from land clearing, introduced predators (cats and foxes), and altered fire regimes — human-caused pressures rather than natural selection processes. Claiming these extinctions are “natural evolution” is misleading. [1 — contrasts current vs background rate and identifies human drivers]

The ecological consequences of losing digging mammals such as the lesser bilby directly contradict the claim that “the ecosystem will adjust.” The lesser bilby was an ecosystem engineer: its burrowing aerated soil, increased water infiltration, and cycled nutrients to the surface. Loss of this function has caused soil compaction and reduced water penetration in arid zones, degrading vegetation cover. This is a regulating ecosystem service — soil aeration and water infiltration — that has been lost and has not been replaced. [1 — explains bilby’s ecological role; 1 — links to regulating ecosystem service; 1 — shows service has been lost, not replaced]

Furthermore, bandicoots dispersed fungal spores that maintained mycorrhizal networks supporting plant health across Australian forests. Their extinction has weakened these networks, reducing plant reproductive success across a wide area. The ecosystem has not adjusted; it has degraded. [1 — additional example of ecological consequence with lesson content]

Evaluative conclusion: The claim that “the ecosystem will adjust” is not supported by the evidence. Decades after these extinctions, soils remain compacted, mycorrhizal networks weakened, and no native species has assumed the ecological functions the lost mammals performed. When redundancy was low (few species performing the same function), there was no ecological replacement. Conservation programs are therefore justified: they are not preventing a natural process but attempting to reverse the destruction of functional ecosystem services. [1 — justified conclusion that rejects the claim, uses evidence from the lesson]

Marking criteria.

• 1 mark — Identifies the one defensible element (extinction is a natural process; background rate context).
• 1 mark — Identifies that the current rate vastly exceeds the background rate and names human-caused drivers.
• 1 mark — Explains the ecological role of a named digging mammal (ecosystem engineering: soil aeration, water infiltration).
• 1 mark — Connects this to a specific ecosystem service category (regulating).
• 1 mark — Provides evidence that the ecosystem has not adjusted (soil compaction, reduced water infiltration, weakened mycorrhizal networks).
• 1 mark — Applies at least one additional lesson concept (low functional redundancy, loss of genetic diversity, intrinsic value, or IUCN threat status).
• 1 mark — Reaches a justified conclusion that explicitly addresses the “ecosystem will adjust” claim using lesson evidence.

Q3 — Sample Band 6 response (6 marks), annotated

How low genetic diversity caused vulnerability: Genetic diversity refers to the variation in allele frequencies within a population. Tasmanian devils experienced a historical bottleneck that reduced this variation severely. Because most individuals share nearly identical alleles at immune-system loci, the population lacks the variation in immune-system genes needed for some individuals to recognise DFTD cells as foreign. Nearly all individuals have the same susceptibility, so the disease can spread rapidly through the entire population once introduced. [1 — defines genetic diversity; 1 — explains mechanistically why low diversity caused uniform susceptibility]

This connects directly to the concept of resilience. A population with high genetic diversity would be more likely to contain some individuals with immune-gene alleles conferring at least partial resistance; natural selection could then increase resistance allele frequency over generations. With low genetic diversity, there is no such variation to select upon, so the population cannot evolve a response — resilience is severely limited. [1 — links to resilience concept correctly; 1 — explains why low diversity reduces adaptive potential]

Evaluation of the conservation breeding program: The program preserves disease-free individuals and prevents extinction in the short term. This satisfies a basic conservation goal (preventing total species loss) and maintains the devil’s role as an apex predator in the Tasmanian ecosystem. However, if the program simply maintains the same genetically uniform population in captivity without deliberately introducing new genetic variation (e.g. by identifying and selectively breeding rare resistant individuals), it does not address the underlying biological problem — low genetic diversity. A population released into the wild from such a program would still be uniformly susceptible. [1 — evaluates short-term benefit; 1 — identifies that the program does not inherently increase genetic diversity, limiting its long-term effectiveness]

Marking criteria.

• 1 mark — Defines genetic diversity correctly (allele variation within a population).
• 1 mark — Explains mechanistically why low genetic diversity caused uniform susceptibility to DFTD (shared immune-system alleles → disease recognised as self).
• 1 mark — Links this case to the concept of resilience (low diversity → reduced adaptive potential → reduced resilience).
• 1 mark — Explains why low diversity limits natural selection as a recovery mechanism.
• 1 mark — Evaluates the conservation program’s short-term benefit (prevents extinction; maintains ecological role).
• 1 mark — Evaluates the program’s long-term limitation: if genetic diversity is not deliberately increased, the underlying vulnerability persists.