Drive across the Australian outback and you will see red kangaroos in the arid centre and eastern grey kangaroos in the woodlands — but you rarely see both in the same small patch of grassland. Why? The answer lies in competition: the invisible force that shapes who lives where, who eats what, and how species evolve to coexist.
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Before you read, commit to a prediction. You will revisit these at the end.
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.
Competition between individuals of the same species for the same limited resources (food, territory, mates, nesting sites). The strongest density-dependent factor as populations approach K.
Competition between individuals of different species for the same or overlapping resources. Can lead to competitive exclusion or drive niche differentiation.
Gause's Law: two species competing for identical resources in the same niche cannot coexist indefinitely. The superior competitor will locally exclude the inferior one.
The functional role and environmental requirements of a species — its "address" and "profession" in the ecosystem. Includes habitat, diet, activity time, temperature range and interactions.
The division of resources among species to reduce interspecific competition. Can be temporal (different times), spatial (different places) or morphological (different food sizes).
Fundamental niche = the full range of conditions a species could theoretically occupy. Realised niche = the actual range it occupies when competition and other biotic factors are present (usually smaller).
When individuals of the same species compete for the same limited resource, the competition is intraspecific (intra = within). This is the most intense form of competition because members of the same species have identical resource requirements.
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:
Evolutionary consequence: Intraspecific competition drives natural selection. Individuals with traits that improve resource acquisition — stronger muscles, better foraging efficiency, more effective territorial defence — leave more offspring. Over generations, the population becomes better adapted to its environment.
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 — they need the same food, the same shelter, or the same breeding sites.
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.
Important nuance: Competitive exclusion applies to local coexistence at a single site. Two species can coexist at the landscape scale if they occupy different patches — this is not a violation of Gause's Law, because they are not competing at the same place at the same time.
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 of resource partitioning:
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.
[COMMON ERROR] "If two species eat the same food, one must always go extinct."
Correction: Competitive exclusion only applies when species compete for identical resources in the same place at the same time (complete niche overlap). In reality, species almost always differ in some aspect of their resource use, habitat preference, activity time or body size. These differences allow coexistence through resource partitioning. Even subtle differences — eating seeds of slightly different sizes, foraging at slightly different heights, or breeding at slightly different times — are sufficient to prevent exclusion.
[COMMON ERROR] "Competition only happens when resources are scarce."
Correction: Competition occurs whenever two individuals attempt to use the same limited resource, regardless of overall resource abundance. Even in a rainforest with abundant food, two birds competing for the same nesting hollow are competing. However, the intensity of competition increases dramatically as resources become scarce — this is why competition is a density-dependent limiting factor.
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?
At the local scale: They rarely coexist in the same small patch of grassland. Ecological research shows that when eastern grey and red kangaroos occur together, they partition resources:
At the landscape scale: Where their ranges overlap, the two species use different habitats — reds on the open flats, greys in the wooded ridges. This is not a violation of competitive exclusion; it is niche differentiation in action. Where habitat is uniform and both species are forced together, one usually dominates: eastern greys outcompete reds in wetter, more productive areas, while reds dominate in drier, more open country.
Evolutionary consequence: Competition between kangaroo species (and with sheep and cattle) 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.
Same species, same resource. Contest (fighting) or scramble (resource depletion). Drives natural selection. Strongest density-dependent factor.
Different species, same resource. Can lead to competitive exclusion (Gause's Law) or niche differentiation.
Spatial (different places), temporal (different times), morphological (different sizes). Reduces overlap and allows coexistence.
Fundamental = full theoretical range. Realised = actual range when competitors present (usually smaller). Competition shrinks the realised niche.
| 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) |
1. Two male kangaroos fight for access to a single female. What type of competition is this?
2. According to the competitive exclusion principle (Gause's Law), what is required for two species to coexist indefinitely in the same habitat?
3. In a Tasmanian forest, spotted-tailed quolls and Tasmanian devils both eat carrion. Devils are larger and more aggressive. Where devils are abundant, quoll populations decline. In areas without devils (such as Maria Island), quoll populations are stable. Which statement best explains this pattern?
4. 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?
5. A student argues: "Gause's Law is wrong because I can see five different bird species eating insects in the same tree at the same time. If competitive exclusion were real, only one insect-eating bird species would exist." Evaluate this argument using your knowledge of resource partitioning, niche differentiation and the distinction between fundamental and realised niches.
6. Red kangaroos and eastern grey kangaroos both graze on native grasses in Australian rangeland. The following observations have been made:
(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. 2 MARKS
(b) Identify whether competition between kangaroos and sheep is intraspecific or interspecific, and explain why both kangaroo species decline when sheep are introduced. 2 MARKS
7. 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. 5 MARKS
8. 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. 6 MARKS
Return to your Think First responses at the start of this lesson.
(a) Intraspecific contest competition [0.5 marks]. Both are red kangaroos (same species) competing for a mate (same resource). Outcome: the stronger male wins access to the female; the loser is excluded from mating but survives [0.5 marks]. This drives sexual selection for larger body size and fighting ability in males.
(b) Interspecific competition [0.5 marks]. 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 [0.5 marks]. This is a case where human intervention alters the natural competitive balance.
(c) Interspecific competition [0.5 marks]. They coexist because of spatial resource partitioning: Chthamalus occupies the upper shore (drier, more exposed) while Semibalanus dominates the lower shore (submerged more often) [0.5 marks]. In the overlap zone, Semibalanus outcompetes Chthamalus by growing faster and smothering it. However, Chthamalus tolerates desiccation better, so it survives where Semibalanus cannot [0.5 marks]. This is Connell's classic experiment — a foundational study in ecology.
(d) Interspecific competition with competitive exclusion [0.5 marks]. Devils are the superior competitor: larger body size, stronger jaws, more aggressive behaviour. Quolls are excluded from the carrion resource in areas with devils [0.5 marks]. On Maria Island (no devils), quolls occupy their full fundamental niche and population is stable [0.5 marks]. This demonstrates how the superior competitor reduces the realised niche of the inferior competitor.
(a) Spatial partitioning: different feeding heights (upper canopy vs mid canopy vs understorey) [1 mark]. Temporal partitioning: different peak activity times (dawn/dusk vs mid-morning vs afternoon) [1 mark]. Morphological partitioning: different body sizes (25 g vs 15 g vs 12 g) may allow access to different flower sizes or nectar volumes [0.5 marks]. 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 [0.5 marks]. They cannot easily shift to canopy eucalypt flowers because the New Holland honeyeater already occupies that niche and would outcompete them [0.5 marks]. Population would decline sharply or the species would be locally excluded [0.5 marks].
(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 [0.5 marks]. The New Holland honeyeater (largest at 25 g) is most likely to persist because larger birds can defend territories more effectively and access more nectar per visit [0.5 marks]. The smaller species (white-plumed and spinebill) would be excluded because they cannot compete for the limited resource against a larger, more aggressive competitor [0.5 marks]. This illustrates why monocultures reduce biodiversity: they eliminate the resource heterogeneity that allows partitioning.
1. B — Same species competing for a mate = intraspecific contest competition.
2. C — Gause's Law: coexistence requires niche differentiation.
3. A — Devils outcompete quolls for carrion, excluding quolls from their realised niche.
4. D — Different heights and times = resource partitioning.
5. B — The student misunderstands Gause's Law. Coexisting species partition resources; their realised niches are smaller than fundamental niches.
Q6 (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 [0.5 marks]. At the local scale, they rarely coexist because they have overlapping realised niches when forced into the same patch — both eat green grass and require water [0.5 marks]. Niche differentiation (reds tolerate drier conditions and poorer-quality grass) allows coexistence at the landscape scale but not locally [0.5 marks]. Where they do overlap, one usually dominates depending on habitat type [0.5 marks]. Total: 2 marks. (b) Interspecific competition [0.5 marks]. 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 [0.5 marks]. Both kangaroo species decline because introduced grazers increase the intensity of interspecific competition beyond what native ecosystems evolved to handle [0.5 marks]. Any valid explanation of sheep impact: [0.5 marks]. Total: 2 marks.
Q7 (5 marks): Contest competition involves direct physical confrontation over a discrete resource [0.5 marks]. Australian example: male red kangaroos "box" using their forearms and powerful hind legs to fight for access to females; the winner mates, the loser is excluded [0.5 marks]. Scramble competition involves indirect exploitation where all individuals deplete the shared resource, reducing availability for everyone [0.5 marks]. Australian example: locusts in a plague strip vegetation bare; all individuals suffer when food runs out, with no individual "winning" the resource [0.5 marks]. Intraspecific competition is the strongest density-dependent factor near K because members of the same species have identical resource requirements [0.5 marks]. 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 [0.5 marks]. Contest competition removes the weakest individuals first, while scramble competition reduces survival across the entire population [0.5 marks]. This drives natural selection for traits that improve resource acquisition [0.5 marks]. Clear structure: [1 mark]. Total: 5 marks.
Q8 (6 marks): Kangaroo-kangaroo partitioning: reds and greys partition resources by habitat (open plains vs woodland), diet quality (reds eat drier grass) and water requirements (reds tolerate aridity better) [1 mark]. This evolved over millennia through natural selection, allowing both species to exploit different subsets of the Australian landscape [0.5 marks]. 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 [1 mark]. Introduced grazers also alter vegetation structure and soil chemistry, changing the resource base itself rather than just competing for it [0.5 marks]. 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 [0.5 marks]. For example, rabbits breed faster than any native herbivore and can survive in burrows that kangaroos cannot use, giving them access to refugia where competition is reduced [0.5 marks]. 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. Conservation requires active management (fencing, culling, habitat restoration) in addition to relying on natural coexistence mechanisms [1.5 marks]. Total: 6 marks.