Interspecific and Intraspecific Competition

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

Intraspecific competition is competition between individuals of the same species for the same limited resource. Interspecific competition is competition between individuals of different species for overlapping resources. [1 — both definitions with the same/different species distinction]

Intensity: intraspecific competition is generally more intense than interspecific because members of the same species have identical resource requirements, so every new individual competes directly with every other. In interspecific competition, niches only partially overlap, so competition is limited to the overlap zone. [1 — intensity comparison with explanation]

Density-dependence: intraspecific competition is the primary density-dependent factor limiting population growth. As a population approaches carrying capacity (K), food, territory and mates become limiting; the effect intensifies proportionally with density. Interspecific competition can also be density-dependent but is typically weaker because niche differentiation reduces its per-capita effect. [1 — density-dependence and K link]

Outcome: intraspecific competition most commonly results in reduced survival and reproduction for individuals in poor condition, shifting the population toward better-adapted individuals. Interspecific competition results in one of two outcomes: (a) competitive exclusion of the inferior competitor (Gause’s Law) or (b) niche differentiation, where the inferior competitor shifts its resource use to reduce overlap and achieve coexistence. [1 — outcome comparison including Gause’s Law]

Australian examples: Intraspecific — male red kangaroos (Osphranter rufus) engage in contest competition (boxing) for access to females; the loser is excluded from mating but not from the population [1]. Interspecific — Tasmanian devils and spotted-tailed quolls both eat carrion; where devils are abundant, quolls are excluded from the highest-quality carrion patches (their realised niche shrinks), demonstrating interspecific competitive exclusion at the resource level. [1 — both examples, correctly classified]

Evolutionary consequence: intraspecific competition is the principal driver of natural selection within a species. Traits that improve resource acquisition (kangaroo body size, fighting ability, foraging efficiency) are favoured, progressively adapting the population to its environment. Interspecific competition drives niche differentiation and, over evolutionary time, character displacement — gradual divergence in traits (e.g. beak sizes in sympatric Darwin’s finches) that reduces overlap and allows coexistence. [1 — evolutionary consequence for both]

Conclusion: intraspecific competition has the greater short-term effect on population dynamics because it is the dominant density-dependent regulator at carrying capacity. Interspecific competition has a greater effect on long-term species distribution and evolution: it shapes where species live (by determining the realised niche) and drives the diversification of traits that characterises adaptive radiation. Both forces are essential, and neither operates in isolation in real ecosystems. [1 — justified, nuanced conclusion]

Marking criteria (7 marks):

• 1 mark — Both definitions correctly stated, with same/different species distinction.
• 1 mark — Intensity comparison: intraspecific more intense than interspecific, with correct reasoning (identical resource needs).
• 1 mark — Density-dependence and link to K for intraspecific competition.
• 1 mark — Outcome comparison: competitive exclusion OR niche differentiation for interspecific; survival of fittest for intraspecific.
• 1 mark — At least one named Australian example per type, correctly classified.
• 1 mark — Evolutionary consequence correctly linked to each type (natural selection / niche differentiation).
• 1 mark — Explicit, justified conclusion that distinguishes short-term vs long-term / population dynamics vs evolution, referencing both types.

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

Before the monoculture was planted, the three honeyeater species coexisted through resource partitioning: spatial (different feeding heights — canopy, mid-canopy, understorey), temporal (different peak activity times — dawn/dusk, mid-morning, afternoon) and morphological (different body sizes, 25 g vs 15 g vs 12 g, enabling access to different flower sizes). [1 — three types of partitioning identified with supporting detail from stimulus]

Each species occupied a realised niche smaller than its fundamental niche: for example, the New Holland honeyeater could theoretically feed at all heights (fundamental niche), but competition restricts it to the upper canopy (realised niche) [1]. This niche differentiation meant competition occurred only in the small overlap zones, not across each species’ full resource range. [1 — fundamental vs realised niche applied]

After the monoculture is planted, the single winter-flowering eucalypt collapses the three distinct resource niches into one: all species now compete for the same nectar at the same height at the same time. This satisfies the conditions for Gause’s Law (identical resource, same place, same time). [1 — explains how monoculture eliminates partitioning]

Gause’s Law predicts competitive exclusion: the superior competitor will exclude the inferior ones from the resource. The New Holland honeyeater (25 g) is most likely to persist because larger body size confers a competitive advantage: larger birds defend territories more effectively, access more nectar per visit, and can physically displace smaller competitors [1]. The smaller species (white-plumed, 15 g, and eastern spinebill, 12 g) would be progressively excluded from the monoculture nectar source. [1 — identifies dominant species with justified reasoning]

However, the smaller species may not go locally extinct — they would likely retreat to their alternative food sources elsewhere (if those still exist), demonstrating that competitive exclusion applies locally, at the monoculture patch, not necessarily at the landscape scale. [1 — nuance: local vs landscape exclusion]

Evaluation: the scenario does not contradict Gause’s Law. Rather, it illustrates both sides of the same law: when resource heterogeneity allows partitioning, coexistence is possible (pre-monoculture); when heterogeneity is eliminated, competitive exclusion follows (post-monoculture). The scenario is a direct experimental demonstration of Gause’s Law in action. [1 — clear evaluative conclusion]

This also illustrates why monocultures reduce biodiversity: they replace a diverse mosaic of resource types that support niche differentiation with a single homogeneous resource, forcing all species to compete directly. [1 — ecological implication / broader reasoning]

Marking criteria (8 marks):

• 1 mark — Identifies all three types of resource partitioning (spatial, temporal, morphological) from the stimulus data.
• 1 mark — Correctly applies fundamental vs realised niche distinction to explain coexistence before the monoculture.
• 1 mark — Explains how the monoculture collapses the three distinct niches into one identical resource, triggering Gause’s Law conditions.
• 1 mark — States Gause’s Law correctly and applies it to predict competitive exclusion.
• 1 mark — Identifies the New Holland honeyeater as the likely dominant competitor with justified reasoning (body size advantage).
• 1 mark — Notes that exclusion is local (at the monoculture patch) not necessarily at landscape scale, or that smaller species may retreat to alternative resources.
• 1 mark — Explicit evaluative conclusion: the scenario supports (not contradicts) Gause’s Law.
• 1 mark — Broader ecological implication stated (e.g. monocultures reduce biodiversity by removing resource heterogeneity) or similar insight.

Q3 — Sample Band 6 response (6 marks)

The claim contains one partially correct element and three significant errors. [1 — overall evaluative judgement]

What is defensible: Competition does sometimes lead to local exclusion of the inferior competitor — this is the core prediction of Gause’s Law. So the claim is correct that competition can be a powerful force that eliminates some species from a given area. [1 — identifies the correct element]

Error 1 — “competition always leads to extinction.” Gause’s Law applies only when two species compete for identical resources in the same place at the same time — complete niche overlap. In reality, species almost always differ in at least one aspect of resource use, allowing niche differentiation and coexistence. Hundreds of herbivore species coexist in Australian grasslands precisely because they partition resources (different plants, different heights, different times, different body sizes), so competition does not lead to mass extinction. [1 — refutes “always extinction” using niche differentiation]

Error 2 — “coexisting herbivores disprove Gause’s Law.” This misunderstands the law. Gause’s Law is not falsified by coexistence — coexistence is only possible because the species are not competing for identical resources. The law predicts exclusion when niches are identical; it says nothing about species with different niches. The coexistence of hundreds of herbivores is entirely consistent with Gause’s Law. [1 — refutes the “Gause’s Law is wrong” claim]

Error 3 — “competition does not limit populations.” This is the most serious error. Intraspecific competition is the primary density-dependent factor limiting population growth as populations approach carrying capacity: less food, space and mates per individual as density increases directly reduces survival and reproduction. This is a fundamental principle of population ecology, supported by observation across all taxa. [1 — refutes the “competition does not limit” claim]

Defensible reformulation: “Competition is a powerful force that limits population growth and shapes species distributions. Gause’s Law correctly predicts that two species competing for identical resources in the same place and time cannot coexist indefinitely. However, in real ecosystems most species differ in some aspect of their niche (habitat, diet, activity time, body size), allowing coexistence through resource partitioning. Far from disproving Gause’s Law, the coexistence of hundreds of herbivore species demonstrates the effectiveness of niche differentiation.” [1 — biologically defensible reformulation]

Marking criteria (6 marks):

• 1 mark — States an overall evaluative judgement on the claim.
• 1 mark — Correctly identifies the one defensible element (competition can lead to local exclusion).
• 1 mark — Correctly refutes “always leads to extinction” using niche differentiation / resource partitioning.
• 1 mark — Correctly refutes the claim that coexisting species disprove Gause’s Law (Gause’s Law only applies to identical niches; coexistence confirms it, not disproves it).
• 1 mark — Correctly refutes “competition does not limit populations” with reference to intraspecific competition as a density-dependent factor.
• 1 mark — Reformulates the claim into a biologically defensible statement that integrates Gause’s Law, niche differentiation and competition as a limiting factor.