Human Impacts — Habitat Destruction, Fragmentation, Pollution and Introduced Species

Q1 — Sample Band 6 response (7 marks), annotated

Fragmentation divides continuous habitat into isolated patches, and its consequences exceed what the loss of area alone would predict [framing]. Effect 1 — reduced population size: each patch holds only a fraction of the original population, so populations are small and suffer genetic drift and inbreeding depression; small populations are far more likely to go extinct from random fluctuations once below their minimum viable population [1 — mechanism + extinction-risk link]. Effect 2 — edge effects: patch margins have a hotter, drier, windier microclimate that favours generalists and invaders and lets predators hunt more effectively, so the effective habitat area is smaller than the measured area, further shrinking the usable space for specialists [1 — mechanism + risk]. Effect 3 — reduced gene flow: isolated individuals cannot move between patches to breed, so each patch becomes a genetic island with reduced diversity and adaptive potential, losing the "genetic rescue" migration provides — raising extinction risk over time [1 — mechanism + risk].

A fourth effect, extinction debt, means some species persist in a fragment temporarily but are already committed to extinction because the fragment is below the viable size [1 — extinction debt]. Australian example: in the WA Wheat Belt, 93% of woodland has been cleared; remnants under 10 ha have lost most native birds, and even large remnants have lost specialists such as Carnaby's black-cockatoo that need continuous forest [1 — named Australian example].

Evaluation of wildlife corridors: corridors reconnect patches, restoring gene flow and allowing recolonisation, which counteracts the small-population and reduced-gene-flow effects [1 — benefit]. However, corridors do not fix edge effects or the underlying habitat loss, can also transmit disease, fire and invaders, and require sufficient width and quality to be effective — so they are a valuable but partial solution best combined with protecting large core areas [1 — balanced evaluation].

Marking criteria.

• 3 marks — Three distinct effects, each with mechanism and an explicit link to extinction risk (1 each).
• 1 mark — Correctly explains a fourth effect (e.g. extinction debt) or develops one effect in depth.
• 1 mark — Uses a named Australian example accurately (WA Wheat Belt / Carnaby's black-cockatoo).
• 1 mark — States a valid benefit of wildlife corridors.
• 1 mark — Gives a balanced evaluation (corridors are partial; note a limitation).

Q2 — Sample Band 6 response (8 marks), annotated

Definition. Multi-stressor synergy is where multiple human impacts act together and their combined effect exceeds the sum of their individual effects, because each stressor amplifies the damage of the others [1].

How stressors amplify each other (example 1). Fragmentation by dams and weirs blocks fish migration and isolates populations, so they are already small and below their viable size; the carp invasion then adds competition and increases sediment, and pollution adds nutrients, pesticides and salinity — a weakened, isolated population that could survive any one of these is far less able to survive all of them together [1 — identifies interaction]. The combined pressure pushes populations past thresholds that a single stressor would not [1 — explains amplification rather than addition].

How stressors amplify each other (example 2). Climate change reduces rainfall and raises temperatures, increasing drought; lower, warmer flows concentrate pollutants and salinity and lower dissolved oxygen, while carp thrive in the degraded, turbid water at the expense of natives — so climate change makes the pollution and invasive-species stressors worse than they would be alone [1 — second interacting pair].

Why single-stressor management fails. Because the stressors amplify one another, removing only pollution while ignoring water extraction, barriers to migration and carp would still leave native fish suppressed: the remaining stressors continue to interact and prevent recovery [1 — explains why one fix is insufficient]. The decline to under 10% of pre-European levels is the product of all five stressors together [1 — uses the data].

Evaluation of an effective response. An effective response must address several stressors simultaneously — e.g. restoring environmental flows (water and climate stress), installing fishways to restore connectivity (fragmentation), controlling carp (invasive species) and reducing runoff/salinity (pollution) — because only tackling the interacting causes together can break the synergy [1 — justified, integrated recommendation].

Marking criteria.

• 1 mark — Defines multi-stressor synergy (combined effect > sum of parts).
• 1 mark — Identifies a first interacting pair of named stressors.
• 1 mark — Explains amplification (not mere addition) for that pair.
• 1 mark — Identifies and explains a second interacting pair.
• 1 mark — Explains why addressing one stressor alone would fail.
• 1 mark — Uses the <10% data / scale of decline appropriately.
• 1 mark — Reaches a justified, integrated management recommendation.
• 1 mark — Sustains a coherent, evaluative analysis throughout.

Q3 — Sample Band 6 response (6 marks)

The claim is partly correct but largely flawed. [1 — evaluative judgement]

What is defensible: warming is genuinely a major driver of reef decline — rising sea temperatures cause coral bleaching, and the warming is global and largely beyond local control [1 — concedes the valid element].

What is wrong: The reef does not decline because of climate change "alone". The reef faces multiple stressors that act synergistically — warming, nutrient runoff and crown-of-thorns outbreaks [1 — refutes "alone" with synergy]. The claim that "nothing local" can help ignores the land–sea connection: about 7,000 tonnes of nitrogen per year from land clearing and agriculture feed phytoplankton blooms that fuel CoTS outbreaks, which have killed ~40% of GBR coral since 1985 [1 — refutes "nothing local" with GBR evidence]. Reducing farm runoff therefore would make a difference, because it removes a locally controllable stressor; with one stressor eased, heat-stressed coral has a better chance to recover from CoTS damage, since stressors amplify each other [1 — refutes "no difference" using synergy].

Defensible reformulation: “The Great Barrier Reef is declining because of several synergistic stressors — ocean warming, nutrient runoff and crown-of-thorns outbreaks. While warming requires global action, local management of land-based runoff can reduce a controllable stressor and improve the reef's capacity to recover, so local action is worthwhile alongside global emissions reduction.” [1 — biologically defensible reformulation]

Marking criteria.

• 1 mark — States an overall evaluative judgement (partly correct, largely flawed).
• 1 mark — Concedes the valid element (warming/bleaching is a real, global driver).
• 1 mark — Refutes "climate change alone" using multiple synergistic stressors.
• 1 mark — Refutes "nothing local" using the land–sea connection / nitrogen / CoTS evidence.
• 1 mark — Refutes "runoff makes no difference" using synergy (easing one stressor aids recovery).
• 1 mark — Reformulates the claim into a defensible statement combining local and global action.