HSC Exam Practice

Sample response. Intraspecific competition is competition between individuals of the same species for the same limited resource (food, territory, mates or nesting sites). It is the strongest density-dependent factor because every individual added to the population competes directly with every other member: as density increases, food, space and access to mates decrease proportionally for all individuals, increasing mortality and reducing reproduction, keeping the population near carrying capacity.

Marking criteria. 1 mark — correct definition linking ‘same species’ and ‘same limited resource’. 1 mark — explains density-dependence: as population density rises toward K, competition intensifies because resource availability per individual falls, limiting population growth.

Sample response. Contest competition involves direct physical confrontation over a discrete resource, producing a clear winner and loser; e.g. male red kangaroos (Osphranter rufus) ‘box’ for access to females — the winner mates, the loser does not. Scramble competition is indirect: all individuals exploit the same resource simultaneously, depleting it for everyone with no single winner; e.g. a plague of locusts strips vegetation bare, reducing food for every individual in the swarm.

Marking criteria. 1 mark — contest: direct confrontation / winner and loser identified, plus a valid named Australian example. 1 mark — scramble: indirect / all individuals deplete the same resource with no winner, plus a valid named example (Australian preferred but not required).

Sample response. Gause’s Law (competitive exclusion principle) states that two species competing for identical resources cannot coexist indefinitely; the superior competitor will eventually exclude the inferior one from that location. The principle applies only when the two species have complete niche overlap — that is, they compete for the same resource in the same place at the same time.

Marking criteria. 1 mark — correct statement of the principle: two species / identical resources / cannot coexist / one excludes the other. 1 mark — states the condition: complete (or identical) niche overlap / same resource, same place, same time.

Sample response. A species’ fundamental niche is the full range of conditions and resources it could theoretically use in the absence of other species. Its realised niche is the actual, usually smaller, range of conditions and resources it occupies when biotic factors such as competition are present. The main biotic factor that contracts the realised niche relative to the fundamental niche is interspecific competition: a superior competitor excludes the species from parts of its potential range, forcing it to use only a subset of available resources.

Marking criteria. 1 mark — fundamental niche correctly defined as the theoretical maximum range in the absence of competitors; realised niche correctly defined as the actual, smaller range when biotic factors are present. 1 mark — identifies interspecific competition (accept ‘competition’ from a superior competitor) as the main biotic factor that reduces the realised niche.

Sample response. Resource partitioning is the division of resources among competing species so that each uses a different subset of the available resources, reducing the overlap between their realised niches and preventing competitive exclusion. Spatial partitioning occurs when species use different habitats or microhabitats; for example, in Australian eucalypt forests, the New Holland honeyeater feeds in the upper canopy while the eastern spinebill forages in the understorey, so they rarely compete for the same nectar. Temporal partitioning occurs when species use the same resource at different times; for instance, cheetahs hunt during the day while lions hunt at dawn and dusk, reducing direct competition for prey. (Accept also: morphological partitioning — species evolve different body structures to use different sizes or types of a resource; e.g. Darwin’s finches with beaks of different sizes.)

Marking criteria. 1 mark — correct explanation of how resource partitioning allows coexistence (reduces niche overlap / prevents competitive exclusion). 1 mark — first type correctly named and described with any valid example. 1 mark — second type correctly named and described (example not required for the second type but must be correct). Maximum 3 marks; any two of the three types accepted.

Sample response (a) — Describe. In the mid shore zone, Chthamalus cover drops from 61% (alone) to 11% (together with Semibalanus), a reduction of 50 percentage points. In the low shore zone, cover falls from 55% (alone) to just 3% (together), a reduction of 52 percentage points. In contrast, Chthamalus cover on the high shore is barely affected (58% alone vs 52% together, a difference of only 6 percentage points). The data therefore show that the presence of Semibalanus dramatically reduces Chthamalus in the mid and low shore zones but has little effect on the high shore.

Marking criteria (a). 1 mark — correctly describes that Chthamalus cover is substantially reduced in the mid and low shore in the mixed condition; 1 mark — supports with at least two specific data values from the table (e.g. mid shore 61% → 11%, low shore 55% → 3%). Do not award if no data are quoted.

Sample response (b) — Account for. The fundamental niche of Chthamalus spans the full tidal range (high, mid and low shore), as shown by its high cover across all zones when alone. When Semibalanus is present, interspecific competition occurs because both species compete for the same attachment space on the rock surface. Semibalanus is the superior competitor in the mid and low shore: it grows faster, physically undercuts and smothers Chthamalus, excluding it from those zones. As a result, Chthamalus’s realised niche is contracted to the high shore, the only zone where Semibalanus cannot survive (due to excessive desiccation), allowing Chthamalus to persist there. This is a classic demonstration of interspecific competition shrinking the realised niche relative to the fundamental niche.

Marking criteria (b). 1 mark — correctly identifies that Chthamalus fundamental niche covers the full shore range (shown by the alone condition). 1 mark — correctly states that interspecific competition with Semibalanus (superior competitor on the mid/low shore) shrinks Chthamalus’s realised niche to the high shore only. 1 mark — explains the mechanism: Semibalanus physically outcompetes / excludes Chthamalus from the lower shore where Semibalanus is better adapted; accept also ‘Chthamalus survives on high shore because Semibalanus cannot tolerate desiccation there’.

Sample response (c) — Identify and justify. The data partially support the competitive exclusion principle. Semibalanus competitively excludes Chthamalus from the mid and low shore zones, consistent with Gause’s Law. However, complete mutual exclusion does not occur: the two species coexist at the landscape scale because they occupy different zones (high vs mid/low shore), which represents spatial resource partitioning rather than a violation of Gause’s Law.

Marking criteria (c). 1 mark — states that the data support the principle (with Gause’s Law applied correctly to one species excluding the other from shared zones); accept also ‘partial support’ if the response notes that coexistence at the landscape scale is enabled by spatial partitioning, not a contradiction of the law. Do not award if the student claims the data contradict Gause’s Law without valid justification.

Sample response.

The claim contains a defensible observation but reaches two incorrect conclusions. It is correct that both red kangaroos (Osphranter rufus) and eastern grey kangaroos (Macropus giganteus) are grazers that eat grasses and herbs across overlapping parts of the Australian continent — so interspecific competition between them is real. However, the inference that one must drive the other to extinction, and that their coexistence disproves Gause’s Law, are both scientifically incorrect.

Gause’s Law applies only when two species have complete niche overlap — competing for identical resources in the same place at the same time. Red and eastern grey kangaroos do not satisfy this condition because they have differentiated their niches through resource partitioning. Spatial partitioning: red kangaroos predominate in open, arid plains and semi-desert zones of the interior; eastern grey kangaroos predominate in wetter woodlands and grassy forests of the east coast. Each species exploits a different segment of the Australian landscape, reducing direct competition for the same patch of grass. Dietary partitioning (a form of morphological/resource partitioning): reds are more selective, preferring green, high-moisture shoots and demonstrating exceptional water conservation (concentrated urine); eastern greys are less selective and will consume drier, coarser grasses when green vegetation is unavailable. These differences mean that even where ranges overlap, the two species use different subsets of the available resources.

The realised niche of each species is therefore smaller than its fundamental niche. In the absence of competition, each species could theoretically occupy the full Australian grassland spectrum (its fundamental niche). In practice, interspecific competition has contracted each species’ realised niche to the habitat and dietary range where it is the superior competitor. Where the two ranges overlap spatially, one species tends to dominate: eastern greys outcompete reds in wetter, more productive woodland; reds dominate in drier, hotter open plains.

The evolutionary consequence of sustained interspecific competition is character displacement and niche differentiation. Over evolutionary time, natural selection has favoured traits in each species that improve competitive performance in its preferred habitat and reduce overlap with the other. In red kangaroos, selection has driven exceptional heat tolerance, water conservation and openland locomotion efficiency. In eastern greys, selection has favoured browsing flexibility and exploitation of the more productive woodland zone. This divergence reduces the intensity of competition between the two species and is itself evidence that interspecific competition has shaped their evolution.

In conclusion, the coexistence of red and eastern grey kangaroos does not contradict Gause’s Law — it demonstrates it. Gause’s Law predicts competitive exclusion only when niches are identical. Because the two kangaroo species have evolved resource partitioning over time, their niches are no longer identical, and coexistence is precisely what the law predicts. The claim fundamentally misunderstands the scope of the competitive exclusion principle.

Marking criteria (8 marks):

• 1 mark — Correctly identifies the defensible element: both species are grazers with overlapping ranges, so interspecific competition is real.
• 1 mark — Correctly identifies and refutes Error 1: Gause’s Law only applies to complete niche overlap / identical resources; the kangaroos do not satisfy this condition.
• 1 mark — Names and correctly describes spatial resource partitioning between the two kangaroo species with supporting detail (e.g. reds on open plains / arid interior; greys in woodland / wetter east).
• 1 mark — Names and correctly describes a second type of resource partitioning (accept dietary / morphological: reds prefer green shoots / better water conservation; greys use coarser dry grass; or temporal: reds extend activity into hotter parts of the day).
• 1 mark — Correctly applies the fundamental vs realised niche distinction to the kangaroo case: fundamental niche spans the full grassland spectrum; interspecific competition has contracted each species’ realised niche to the habitat where it is the superior competitor.
• 1 mark — Correctly explains the evolutionary consequence of interspecific competition: natural selection / character displacement drives trait divergence (e.g. water conservation, heat tolerance in reds) that reduces niche overlap over time.
• 1 mark — Correctly refutes Error 2: the coexistence of both species does not contradict Gause’s Law; rather, it confirms the law because coexistence is only possible due to niche differentiation.
• 1 mark — Reaches an explicit, justified evaluative conclusion: the kangaroo example supports (not contradicts) Gause’s Law because the law’s conditions (identical niches) are not met where resource partitioning is effective.