Interspecific and Intraspecific Competition

Multiple Choice

MC answers and full explanations are shown inline as you complete each question.

Activity 1, Competition Analysis

(a) Intraspecific contest competition. Both are red kangaroos (same species) competing for a mate (same resource). Outcome: the stronger male wins access to the female. This drives sexual selection for larger body size and fighting ability in males.

(b) Interspecific competition. Sheep and kangaroos are different species competing for the same grass resource. Outcome: sheep are subsidised with supplementary feed and water, giving them an artificial competitive advantage. Kangaroos, relying entirely on native pasture, are outcompeted and decline in numbers.

(c) Interspecific competition. They coexist because of spatial resource partitioning: Chthamalus occupies the upper shore (drier, more exposed) while Semibalanus dominates the lower shore. In the overlap zone, Semibalanus outcompetes Chthamalus by growing faster and smothering it. However, Chthamalus tolerates desiccation better, so it survives where Semibalanus cannot.

(d) Interspecific competition with competitive exclusion. Devils are the superior competitor. Quolls are excluded from the carrion resource in areas with devils. On Maria Island (no devils), quolls occupy their full fundamental niche and population is stable. This demonstrates how the superior competitor reduces the realised niche of the inferior competitor.

Activity 2, Resource Partitioning

(a) Spatial partitioning: different feeding heights (upper canopy vs mid canopy vs understorey). Temporal partitioning: different peak activity times (dawn/dusk vs mid-morning vs afternoon). Morphological partitioning: different body sizes (25 g vs 15 g vs 12 g) may allow access to different flower sizes or nectar volumes. Each species exploits a different subset of the nectar resource, reducing direct competition.

(b) White-plumed honeyeaters rely on shrubs and small trees in the mid-canopy and understorey. If fire destroys these layers, their primary nectar sources are eliminated. They cannot easily shift to canopy eucalypt flowers because the New Holland honeyeater already occupies that niche and would outcompete them. Population would decline sharply or the species would be locally excluded.

(c) Gause's Law predicts that if all three species now depend on the same nectar source with identical timing and height, one will be competitively excluded. The New Holland honeyeater (largest at 25 g) is most likely to persist because larger birds can defend territories more effectively. The smaller species would be excluded because they cannot compete against a larger, more aggressive competitor. This illustrates why monocultures reduce biodiversity: they eliminate the resource heterogeneity that allows partitioning.

Short Answer Model Answers

Q1 (4 marks): (a) The two kangaroo species coexist across Australia because they occupy different habitats at the landscape scale (spatial partitioning): reds in open arid plains, greys in woodland. At the local scale, they rarely coexist because they have overlapping realised niches when forced into the same patch. Niche differentiation (reds tolerate drier conditions and poorer-quality grass) allows coexistence at the landscape scale but not locally [2 marks]. (b) Interspecific competition. Sheep are subsidised with water and supplementary feed, giving them an advantage over kangaroos that rely solely on natural pasture. Sheep also alter vegetation structure through selective grazing, reducing habitat quality for both kangaroo species. Both kangaroo species decline because introduced grazers increase the intensity of interspecific competition beyond what native ecosystems evolved to handle [2 marks]. Total: 4 marks.

Q2 (5 marks): Contest competition involves direct physical confrontation over a discrete resource. Australian example: male red kangaroos "box" using their forearms and hind legs to fight for access to females [1 mark]. Scramble competition involves indirect exploitation where all individuals deplete the shared resource, reducing availability for everyone. Australian example: locusts in a plague strip vegetation bare; all individuals suffer when food runs out [1 mark]. Intraspecific competition is the strongest density-dependent factor near K because members of the same species have identical resource requirements. As density increases, each individual has less food, less space and less access to mates. The effect intensifies proportionally with density, by definition, density-dependent. Contest competition removes the weakest individuals first, while scramble competition reduces survival across the entire population. This drives natural selection for traits that improve resource acquisition [3 marks]. Total: 5 marks.

Q3 (6 marks): Kangaroo-kangaroo partitioning: reds and greys partition resources by habitat, diet quality and water requirements. This evolved over millennia through natural selection [1 mark]. Kangaroo-introduced competition: sheep, cattle and rabbits eat the same grasses as kangaroos but are not limited by natural water availability (humans provide water points) or predator pressure. Introduced grazers also alter vegetation structure and soil chemistry [1 mark]. Why niche differentiation fails: native species evolved to partition resources with other native species. Introduced species often have different feeding strategies, reproductive rates or habitat tolerances that fall outside the native partitioning framework. Rabbits breed faster than any native herbivore and can survive in burrows that kangaroos cannot use [1 mark]. Conservation requires active management (fencing, culling, habitat restoration) in addition to relying on natural coexistence mechanisms [1 mark]. Evaluated conclusion: resource partitioning among native species is effective for maintaining biodiversity but is often insufficient to protect against introduced competitors because the new species exploit resources in ways that bypass native partitioning mechanisms [2 marks]. Total: 6 marks.