HSC Exam Practice

Sample response. Carrying capacity (K) is the maximum population size that a given environment can sustain at a particular time, given available resources, space, predation and competition. K is not fixed because it changes whenever the resources that define it change — for example, drought reduces food and water, lowering K, while improved rainfall can raise it.

Marking criteria. 1 mark — correctly defines K as the maximum sustainable population size (linked to available resources). 1 mark — gives one mechanistically valid reason K is not fixed (e.g. drought, predator removal, disturbance, resource pulse; not just “it changes over time”).

Sample response. Shelford’s Law of Tolerance predicts that a species can only survive within a defined range of a physical or chemical factor (e.g. temperature, salinity): performance is greatest at the optimum and falls to zero below the minimum or above the maximum. The limiting factor is any physical factor that moves outside the tolerance range. Liebig’s Law of the Minimum predicts that, even when most resources are adequate, it is the single scarcest resource — the “minimum” — that determines how large the population can grow. The two laws differ in that Shelford’s concerns survival thresholds across a factor range, while Liebig’s concerns the bottleneck resource among many adequate ones.

Marking criteria. 1 mark — Shelford’s Law: species survive only within a tolerance range; die outside minimum/maximum. 1 mark — Liebig’s Law: the single scarcest (limiting) resource sets the ceiling on population growth. 1 mark — clearly distinguishes the two laws (not just restating each).

Sample response. (a) Interspecific competition (specifically interference competition): the starling benefits by gaining a nesting hollow; the rosella is harmed by losing it (+ / −). (b) Mutualism: both species benefit — the zooxanthellae gain shelter and CO₂ for photosynthesis; the coral gains photosynthate and oxygen (+ / +). (c) Parasitism: the myxoma virus benefits by replicating; the rabbit host is harmed by reduced survival (+ / −).

Marking criteria. 1 mark per interaction correctly named with direction of effect on each organism (1 mark per row; accept minor variations in wording).

Sample response. Step 1 identifies abiotic thresholds using Shelford’s Law: if temperature, salinity or another physical factor falls outside the species’ tolerance range, it cannot survive in that habitat. Step 2 maps biotic interactions (competition, predation, mutualism, parasitism) using Liebig’s Law: even within the abiotic tolerance range, a species may be excluded or suppressed by a superior competitor or depleted by a predator. Step 3 models population dynamics: the population grows logistically toward K, but K is not fixed — it changes as resources, disturbance and biotic pressures change. The three steps combine to generate a conditional prediction: a species will be found in viable numbers only where all three sets of constraints are simultaneously satisfied.

Marking criteria. 1 mark — correctly describes Step 1 (abiotic thresholds / Shelford). 1 mark — correctly describes Step 2 (biotic interactions; a species within abiotic tolerance can still be excluded). 1 mark — correctly describes Step 3 (population dynamics / K) and states that all three must be integrated.

Sample response. Competitive exclusion (Gause’s principle) states that two species with identical niches cannot coexist indefinitely — the inferior competitor will be driven to local exclusion in that habitat. However, this is local, not global: the inferior competitor can persist in microhabitats where the superior competitor is absent or in lower densities, or it can evolve niche differentiation (resource partitioning) that reduces overlap enough to allow coexistence. In Australia, red kangaroos and eastern grey kangaroos coexist across much of eastern Australia even though both species compete for grass. They avoid competitive exclusion because red kangaroos prefer open arid grasslands and graze lower to the ground, while eastern grey kangaroos use woodland edges and eat a higher proportion of leaves — spatial and dietary partitioning reduces competition intensity below the exclusion threshold.

Marking criteria. 1 mark — correctly states that competitive exclusion causes local, not global, exclusion. 1 mark — explains a valid mechanism that prevents global extinction (microhabitat refuge, resource partitioning, or niche differentiation). 1 mark — uses a named Australian example with a specific mechanism (not just species names).

Sample response (a) — describe relationship. 2 marks. In the pre-drought period (1998–2001), water temperature was relatively low (~23°C) and Murray cod population index was at its highest (~100). During the drought peak (2006–2008), temperature reached its maximum (~29°C) and cod index fell to its lowest trough (~28). In the post-drought recovery (2010–2012), temperature declined to ~25°C and cod index partially recovered to ~55. The two variables show a clear inverse relationship: as temperature increased, cod population index decreased, and vice versa.

Marking criteria (a). 1 mark — identifies the inverse relationship (temperature up, cod down; temperature down, cod recovers). 1 mark — quotes at least one specific value from each panel to support the description (e.g. temperature ~29°C at drought peak coincides with cod index ~28).

Sample response (b) — Shelford’s Law. 3 marks. Shelford’s Law of Tolerance states that Murray cod can only survive and reproduce within a defined temperature range. Murray cod spawn only when water temperature drops below 17°C in spring, and juvenile survival is severely reduced above 28°C. Between 2004 and 2008, mean summer temperatures rose from ~27°C to ~29°C. At 27°C, summer temperatures are already elevated but may still allow some adult survival; at 29°C they exceed the 28°C juvenile survival threshold, compressing cod into cooler upstream refuges and eliminating most juvenile recruitment in the affected reach. Combined with spring temperatures that increasingly exceeded the 17°C spawning trigger, reproductive output fell toward zero. The population index declined steeply because adult survival declined and recruitment essentially ceased: the population was pushed toward the upper boundary of its tolerance range and could not replace itself.

Marking criteria (b). 1 mark — correctly states Shelford’s Law (species survive within a tolerance range; outside = zero performance). 1 mark — links the rising temperature (from the graph) to a specific cod thermal threshold (spawning <17°C and/or juvenile mortality >28°C). 1 mark — explains the population consequence: reduced or zero recruitment / reproductive failure / restriction to refuges, causing the observed index decline.

Sample response (c) — interspecific competition and thermal tolerance. 3 marks. Common carp tolerate a much broader temperature range (4–35°C) than Murray cod, meaning the drought-induced warming that pushed cod beyond their tolerance did not affect carp survival or reproduction. As cod numbers fell, carp experienced competitive release: they occupied the ecological space and resources (benthic invertebrates, spawning habitat, shelter) previously used by cod without the suppression of interspecific competition from a healthy cod population. Carp populations therefore grew rapidly during the drought (index rising from ~80 to ~118). After the drought, as temperatures moderated and cod began recovering, interspecific competition was partially restored and carp declined back toward ~80. The opposing trends are explained by asymmetric thermal tolerance combined with interspecific competition: the same abiotic stress that eliminated cod’s competitive ability gave carp a competitive advantage they exploited to expand their population.

Marking criteria (c). 1 mark — explains that carp’s broader thermal tolerance (4–35°C) means they were not stressed by the conditions that exceeded cod’s tolerance. 1 mark — applies the concept of interspecific competition: cod decline released resources previously contested by both species, allowing carp to expand. 1 mark — integrates both concepts into an explanation of the opposing trend (carp rose as cod fell; this is not coincidence but a competitive release caused by differential thermal tolerance).

Sample response. A 3°C warming alters at least one critical abiotic threshold for snow gums in the Australian Alps: mean growing-season temperature at the current treeline (~1,800 m) rises above the ~6°C minimum required for cambial growth and photosynthesis. Shelford’s Law predicts that warmth previously limiting growth at the treeline now extends the suitable thermal range upward by approximately 150–200 m, potentially allowing snow gum colonisation of previously sub-optimal terrain. Considered in isolation, warming would be expected to shift the treeline upward.

However, applying Liebig’s Law of the Minimum, when the combined scenario is considered, fire frequency becomes the single most limiting factor — not temperature. A 40% increase in fire frequency means adult snow gums, which are the seed source for upward colonisation, are killed before they can set sufficient seed. Even if temperature conditions above 1,800 m become physiologically suitable, there are no adult trees surviving long enough to disperse seeds into those new areas. Fire frequency thus caps recruitment to near zero regardless of thermal improvement: it is the scarce “resource” (or rather the absent essential process of seed production) that limits population expansion.

The biotic interaction of competition further compounds this outcome. After fire, faster-establishing grasses, sedges, and alpine shrubs rapidly colonise disturbed soil. These low-growing competitors shade out the slow-germinating snow gum seedlings that do establish, suppressing recruitment even in the rare fire-free windows. Snow gum seedlings require several snow-free seasons to reach a size where they can compete for light; frequent fire prevents this multi-year establishment period from being completed.

Consequently, the carrying capacity (K) for snow gum recruitment above the current treeline remains near zero under the combined scenario. K cannot increase because both the adult seed source (fire-killed) and seedling establishment (biotic competition after fire) are simultaneously suppressed. Rather than advancing upward, the treeline may remain static or even retreat as adult trees at the existing margin are killed by fire and not replaced from below.

Conditional prediction: if mean temperature rises by 3°C and summer fire frequency increases by 40%, then the alpine treeline for snow gums will not advance upward despite the thermal improvement, because fire frequency will emerge as the Liebig minimum — eliminating the seed source and preventing recruitment — while competition from grasses will suppress any seedlings that do establish; in the worst case, the treeline will retreat as standing adults are killed and not replaced.

Marking criteria. 1 mark — applies Shelford’s Law: identifies a specific abiotic threshold altered by warming (e.g. growing-season temperature now exceeds the minimum for cambial growth / growing season extended) and explains why this alone would favour upward advance. 1 mark — applies Liebig’s Law: identifies fire frequency as the single most limiting factor under the combined scenario, overriding the thermal benefit, because it eliminates the adult seed source. 1 mark — describes how increased fire frequency kills adult snow gums, preventing seed dispersal to higher elevations. 1 mark — describes at least one biotic interaction that further limits recruitment (competition from grasses / shrubs shading seedlings after fire). 1 mark — correctly explains that K for snow gum recruitment collapses to near zero under the combined scenario (not just falls). 1 mark — gives a justified prediction of treeline direction: static or retreating (not advancing) under the combined scenario with mechanism. 1 mark — expresses the final prediction in explicit conditional (“if…then…”) language that integrates at least two factors from the framework.

Sample response. The claim is partly correct but substantially incomplete and misleading in its conclusion. Overall evaluative judgement: the abiotic component is defensible as a necessary condition, but the claim incorrectly treats it as a sufficient condition for a species to be present in viable numbers.

What is defensible. Abiotic conditions do set the fundamental niche — the physical and chemical range within which a species can physiologically survive. Shelford’s Law confirms that outside the tolerance range, no population can persist regardless of biological conditions. Temperature, rainfall and soil type are real constraints: a species outside its abiotic tolerance cannot live in that habitat. In this sense, abiotic suitability is a necessary condition for presence.

What is incorrect. The claim that suitable abiotic conditions guarantee the species “will always be found there in viable numbers” is wrong. Biotic interactions can prevent a species from occupying a habitat that is abiotically suitable, reducing its realised niche to a subset of its fundamental niche. For example, crimson rosellas in south-east Australia remain within their abiotic tolerance for temperature and rainfall across large areas of open woodland. However, European starlings aggressively monopolise tree hollows — the critical limiting resource for rosella nesting — in cleared landscapes. As a result, rosellas are absent from or at very low density in many habitats that are abiotically suitable for them, not because of temperature or rainfall, but because interspecific competition has compressed their realised niche. A similar pattern is seen with snow gum seedlings at the alpine treeline: even at altitudes and temperatures within the snow gum’s abiotic tolerance, competition from faster-establishing grasses prevents recruitment after fire, so snow gums are absent from areas they could physically tolerate.

The extreme case. In the most severe outcome, biotic pressure (competitive exclusion, predation, or parasitism) can drive a local population to extinction within an abiotically suitable habitat. Competitive exclusion is not merely a reduction in numbers — it can result in complete local absence, demonstrating that abiotic suitability alone does not guarantee presence.

Defensible reformulation. “Suitable abiotic conditions are necessary but not sufficient for a species to be found in viable numbers. Species distribution and abundance are determined by the integration of abiotic factors (which set the fundamental niche through tolerance limits), biotic interactions (which compress the realised niche within those limits through competition, predation, mutualism and parasitism), and population dynamics (which determine whether a population can persist given both sets of constraints). A species will only be found in viable numbers where its abiotic tolerance is met and its biotic interactions permit establishment, reproduction and population maintenance simultaneously.”

Marking criteria. 1 mark — states a clear overall evaluative judgement (e.g. “partly correct but incomplete / misleading”; not just “wrong”). 1 mark — correctly identifies the defensible element: abiotic tolerance sets the fundamental niche and is a necessary condition for survival. 1 mark — refutes the claim using a specific named example where a species is absent from an abiotically suitable habitat due to biotic exclusion (rosellas vs starlings; snow gums vs grasses; or equivalent). 1 mark — correctly uses fundamental niche vs realised niche logic to explain why abiotic suitability is not sufficient for presence. 1 mark — identifies and explains the extreme case: biotic interactions can cause complete local absence (competitive exclusion / local extinction), not merely reduced numbers. 1 mark — integrates population dynamics (K, logistic growth, or minimum viable population) into the explanation. 1 mark — provides a biologically accurate reformulation of the claim that correctly incorporates abiotic tolerance, biotic interactions, and population dynamics as jointly necessary conditions.