Interspecific and Intraspecific Competition
A 2019 CSIRO 'State of the Climate' report documented that the grey-headed flying fox has extended its range 500 km southward since 1990, while the alpine ash tree has retreated 300 m upslope in Victoria since 1980 as mean temperatures rose 1.4°C. These range shifts are not random — they are the direct result of abiotic conditions changing and species tracking the conditions that suit them. Distribution patterns reveal which abiotic and biotic factors are controlling where each species can live.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
Interspecific competition — the overlap zone between two species' niches is where competition occurs.
Q1. Two species of parrots live in the same forest and both eat eucalypt seeds. A student claims that because they eat the same food, one species must eventually drive the other to extinction. Is this claim always true? If not, what else could happen?
Q2. Two male kookaburras fight over a territory containing good hunting grounds. Is this an example of interspecific or intraspecific competition? Explain your reasoning.
Core Content
The most intense form of competition — identical resource requirements
CSIRO's 2019 'State of the Climate' report recorded that the alpine ash tree's distribution retreated 300 m upslope in Victoria since 1980 as temperatures rose 1.4°C. Where the alpine ash retreated, shrubs moved in. Where the grey-headed flying fox expanded 500 km south, it entered territory already occupied by other nectar feeders. In both cases, shifting distributions created new competition — individuals of the same species (intraspecific) and individuals of different species (interspecific) suddenly competing for the same resource in the same space.
Intraspecific competition: same species competing for the same resource. Most intense because requirements are identical. Primary density-dependent factor limiting population growth near K.
Intraspecific competition is the primary density-dependent factor that limits population growth as a population approaches carrying capacity. When a kangaroo population grows, each individual has less grass, less water and less shelter. The weakest individuals — juveniles, the elderly, or those in poor condition — die first, while the strongest survive and reproduce.
Forms of intraspecific competition:
- Contest competition: Direct physical confrontation over a discrete resource. Male red kangaroos "box" for access to females. Kookaburras defend territorial boundaries through vocal displays and fighting.
- Scramble competition: Indirect competition where all individuals exploit the same resource, and the resource is depleted for everyone. Locusts in a plague strip vegetation bare; all individuals suffer when food runs out.
Contest competition: direct physical confrontation (e.g. kangaroos boxing). Scramble competition: indirect depletion of shared resource (e.g. locusts stripping vegetation). Both intensify as density increases.
Evolutionary consequence: Intraspecific competition drives natural selection. Individuals with traits that improve resource acquisition leave more offspring. Over generations, the population becomes better adapted to its environment.
Pause — copy the highlighted definitions into your book, with one Australian example for each type.
Two male kangaroos fight for access to a single female. What type of competition is this?
Different species, same resource — leads to exclusion or niche differentiation
We just saw that intraspecific competition is the most intense because requirements are identical. That raises a question: what happens when different species share a resource? This card answers it → interspecific competition, governed by Gause's Law.
When individuals of different species compete for the same resource, the competition is interspecific (inter = between). This occurs when two species have overlapping ecological niches.
Gause's Law (competitive exclusion principle): two species with identical niches cannot coexist indefinitely in the same location. The superior competitor excludes the inferior one.
The competitive exclusion principle (also called Gause's Law) states that two species competing for identical resources cannot coexist indefinitely. The superior competitor — the one that extracts the resource more efficiently, reproduces faster, or tolerates the environment better — will eventually exclude the inferior competitor from that area.
However, complete competitive exclusion is rare in nature because species rarely compete for identical resources in identical ways. Instead, species evolve to reduce competition through niche differentiation — adjusting their resource use so their niches no longer fully overlap.
Two species with overlapping niches compete in the overlap zone.
Gause's Law applies to local coexistence at a single site. Two species can coexist at the landscape scale if they occupy different patches — but not in the same spot if their niches are identical.
Pause — copy the highlighted Gause's Law statement and the landscape-scale nuance into your book.
Interspecific competition occurs between:
Spatial, temporal and morphological partitioning reduce niche overlap
We just saw that Gause's Law predicts competitive exclusion when niches are identical. That raises a question: why do ecosystems contain hundreds of coexisting species? This card answers it → resource partitioning — species evolve to use different subsets of available resources.
If competitive exclusion were inevitable, every ecosystem would eventually contain only one species per resource type. The fact that ecosystems contain hundreds of coexisting species tells us that species have evolved ways to reduce competition. This is called resource partitioning or niche differentiation.
There are three main types:
- Spatial partitioning: Species use different habitats or microhabitats. In Australian eucalypt forests, the yellow-tailed black cockatoo feeds in the canopy while the superb lyrebird forages on the forest floor. They do not compete because they use different vertical spaces.
- Temporal partitioning: Species use the same resource at different times. The sugar glider is nocturnal; the squirrel glider is also nocturnal but has a slightly different activity peak.
- Morphological partitioning: Species evolve different body structures to use different resource sizes. Darwin's finches on the Galapagos Islands have beaks of different sizes — large-beaked finches crack hard seeds, small-beaked finches eat small soft seeds.
Resource partitioning: spatial (different habitats), temporal (different times), morphological (different sizes). Reduces niche overlap → allows coexistence.
Realised niche vs fundamental niche: A species' fundamental niche is the full range of conditions it could theoretically tolerate. Its realised niche is the smaller range it actually occupies when competitors are present. Competition shrinks the realised niche.
Fundamental niche = full theoretical range. Realised niche = actual range with competitors present (always ≤ fundamental). Competition shrinks the realised niche.
Pause — copy the highlighted partitioning types and niche definitions into your book.
Three species of honeyeater coexist in an Australian woodland by feeding at different heights and different times of day. Which ecological principle does this demonstrate?
Three large kangaroo species dominate the Australian landscape: the red kangaroo (Osphranter rufus) in the arid interior, the eastern grey kangaroo (Macropus giganteus) in the woodlands of the east, and the western grey kangaroo (Macropus fuliginosus) in the south and west. All three are grazers that eat grasses and herbs. How do they coexist?
Resource partitioning in action:
- Spatial partitioning: Red kangaroos prefer open plains and arid zones; eastern greys prefer woodlands with tree cover.
- Temporal partitioning: Both are crepuscular, but reds extend activity into hotter parts of the day, while greys retreat to shade earlier.
- Dietary partitioning: Reds are more selective grazers, preferring green shoots; greys are less selective and will eat drier, coarser grasses.
Evolutionary consequence: Competition between kangaroo species has driven selection for habitat specialisation. Reds have evolved exceptional water conservation (they can concentrate urine to levels higher than camels) and heat tolerance, allowing them to exploit arid zones that greys cannot use. This is a classic example of how interspecific competition shapes species distributions and drives adaptive evolution.
For each scenario, identify whether the competition is intraspecific or interspecific. Explain your reasoning and predict the likely outcome.
A student argues that Gause's Law is wrong because five bird species eat insects in the same tree at the same time. Which response best evaluates this argument?
The following data were collected from a eucalypt woodland where three nectar-feeding bird species coexist:
| Species | Body mass (g) | Feeding height | Peak activity | Primary nectar source |
|---|---|---|---|---|
| New Holland honeyeater | 25 | Upper canopy (8–15 m) | Dawn and dusk | Eucalyptus flowers |
| White-plumed honeyeater | 15 | Mid canopy (3–8 m) | Mid-morning | Shrubs and small trees |
| Eastern spinebill | 12 | Understorey (0–3 m) | Afternoon | Tubular flowers (e.g. correas) |
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
ApplyBand 4(4 marks) 1. Red kangaroos and eastern grey kangaroos both graze on native grasses in Australian rangeland. The following observations have been made: Red kangaroos dominate open plains; eastern greys dominate wooded areas; Red kangaroos can survive on lower-quality, drier grass; Eastern greys require more frequent access to water; Where sheep are introduced, both kangaroo species decline. (a) Explain why the two kangaroo species can coexist across Australia but rarely in the same small patch of grassland. Use the concepts of niche differentiation and realised niche. (b) Identify whether competition between kangaroos and sheep is intraspecific or interspecific, and explain why both kangaroo species decline when sheep are introduced.
AnalyseBand 4–5(5 marks) 2. Explain the difference between contest competition and scramble competition, using a specific Australian example for each. Then explain why intraspecific competition is described as the strongest density-dependent limiting factor as a population approaches carrying capacity.
EvaluateBand 5–6(6 marks) 3. Using the Australian kangaroo case study, evaluate whether resource partitioning is sufficient to protect native species from the impacts of introduced competitors such as sheep, cattle and rabbits. In your answer, compare how kangaroos partition resources with each other versus how they respond to introduced grazers, and explain why niche differentiation may fail when a new competitor is introduced.
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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.
Five timed questions on intraspecific vs interspecific competition, Gause's Law, resource partitioning and niche concepts. Beat the boss to bank a tier.
Enter the arenaCSIRO's 2019 climate data showed the grey-headed flying fox moving 500 km south and the alpine ash retreating 300 m upslope as temperatures rose 1.4°C. These range shifts are distribution patterns changing in real time, driven by abiotic conditions pushing species into new areas where competition dynamics shift too. When the flying fox entered new territory, it encountered resident nectar feeders — the same resource, the same habitat — creating interspecific competition that previously did not exist.
Return to your Think First response. Could you now distinguish what type of competition (intraspecific or interspecific) would occur between the flying fox and resident nectar feeders — and explain why the flying fox's fundamental niche may be larger than its realised niche in the new territory?