Biology • Year 11 • Module 4 • Lesson 8
Interspecific and Intraspecific Competition
Lock in the key vocabulary, the intra/inter distinction, Gause’s Law, and the three types of resource partitioning before moving to application tasks.
1. Classify the niche overlap diagram
The diagram below shows the resource niches of three pairs of species. For each pair, write the letter of the correct outcome from the list provided. Then answer the questions. 6 marks
Outcome bank: i. Coexistence with no competition ii. Interspecific competition; resource partitioning possible iii. Competitive exclusion likely (Gause’s Law)
| Pair | Outcome (i, ii or iii) | Reason |
|---|---|---|
| A | ||
| B | ||
| C |
1.4 For Pair B, name one mechanism that could reduce the overlap and allow coexistence. 1 mark
1.5 Explain why interspecific competition is generally less intense than intraspecific competition for the same resource. 2 marks
2. Term–definition match
The nine definitions below are shuffled. Write the matching term from the list: intraspecific competition, interspecific competition, competitive exclusion principle, ecological niche, resource partitioning, fundamental niche, realised niche, contest competition, scramble competition. 9 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 2.1 | Competition between individuals of the same species for the same limited resource. | |
| 2.2 | The full range of conditions and resources a species could theoretically use in the absence of competitors. | |
| 2.3 | Gause’s Law: two species competing for identical resources cannot coexist indefinitely. | |
| 2.4 | The actual range of resources and conditions a species occupies when biotic factors such as competition are present. | |
| 2.5 | Competition between individuals of different species for overlapping resources. | |
| 2.6 | The functional role and environmental requirements of a species — its “address and profession” in the ecosystem. | |
| 2.7 | Indirect competition in which all individuals deplete the same shared resource, with no single winner. | |
| 2.8 | The division of resources among species to reduce interspecific competition (spatial, temporal or morphological). | |
| 2.9 | Direct physical confrontation between individuals over a discrete resource; produces a winner and a loser. |
3. True or false — with correction
Circle T or F. If false, write the corrected statement. 8 marks (1 T/F + 1 correction where needed)
3.1 Intraspecific competition occurs between individuals of different species. T / F
3.2 Two species can coexist indefinitely even when they compete for overlapping resources, provided they differ in some aspect of resource use. T / F
3.3 The realised niche is always larger than the fundamental niche because competition expands what a species can use. T / F
3.4 Intraspecific competition is described as the strongest density-dependent limiting factor because same-species individuals have identical resource needs. T / F
4. Function recall
Answer each in 1–2 sentences using precise lesson terms. 10 marks (2 each)
4.1 What is the ecological function of intraspecific competition in regulating population size?
4.2 What is the evolutionary consequence of sustained intraspecific competition in a population?
4.3 What does resource partitioning allow two competing species to do?
4.4 What is the difference between contest and scramble competition? Give one Australian example of each.
4.5 What happens to a species’ realised niche when a superior interspecific competitor is introduced to its habitat?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase. Aim for at least 6 labelled arrows. 6 marks
Supplied terms: intraspecific competition · interspecific competition · competitive exclusion · resource partitioning · realised niche · coexistence
Q1 — Niche overlap classification
Pair A (no overlap) → outcome i — The two niches are fully separate; there is no resource overlap so no competition occurs.
Pair B (partial overlap) → outcome ii — Some resource overlap means interspecific competition occurs in the overlap zone. Species can coexist if they further differentiate through resource partitioning.
Pair C (complete overlap) → outcome iii — Niches are identical; Gause’s Law predicts that the superior competitor will eventually exclude the inferior competitor locally.
1.4 Any one of: spatial partitioning (use different microhabitats), temporal partitioning (use the resource at different times), morphological partitioning (use different sizes/types of the resource).
1.5 Interspecific competition is generally less intense than intraspecific because different species almost always differ in some aspect of their resource use, habitat preference, or activity time [1]. Their niches are rarely 100% identical, so competition occurs only in the overlap zone rather than across the full niche [1].
Q2 — Term–definition matches
2.1 — intraspecific competition • 2.2 — fundamental niche • 2.3 — competitive exclusion principle • 2.4 — realised niche • 2.5 — interspecific competition • 2.6 — ecological niche • 2.7 — scramble competition • 2.8 — resource partitioning • 2.9 — contest competition.
Q3 — True / false with correction
3.1 False. Intraspecific competition occurs between individuals of the same species. Competition between different species is called interspecific competition.
3.2 True. This is the basis of coexistence through niche differentiation: even partial differences in resource use allow two species to avoid Gause’s Law.
3.3 False. The realised niche is always smaller than (or equal to) the fundamental niche. Competition and other biotic factors restrict a species to a subset of what it could theoretically use — they shrink the realised niche.
3.4 True. Members of the same species have identical resource requirements, so every new individual added to the population competes directly with every other member, making intraspecific competition intensify proportionally with density.
Q4 — Function recall
4.1 Intraspecific competition acts as a density-dependent limiting factor: as population size increases, individuals have less food, space and access to mates, increasing mortality and reducing reproduction, which limits further growth and keeps the population near carrying capacity.
4.2 Natural selection. Individuals with traits that improve resource acquisition (stronger muscles, better foraging efficiency, more effective territorial defence) survive and reproduce more successfully. Over generations the population becomes better adapted.
4.3 Resource partitioning allows two competing species to reduce the overlap of their realised niches so they no longer compete for the same resource at the same place and time — enabling coexistence that would otherwise be prevented by competitive exclusion.
4.4 Contest competition: direct physical confrontation for a discrete resource; produces a clear winner (e.g. male red kangaroos boxing for access to females). Scramble competition: indirect exploitation where all individuals deplete the same resource with no winner (e.g. locusts in a plague stripping vegetation bare until it runs out for all).
4.5 The superior competitor forces the inferior species to use a smaller subset of its resources — its realised niche shrinks. In extreme cases (complete niche overlap) the inferior species may be competitively excluded from the habitat entirely.
Q5 — Sample concept map
A correct map should include arrows such as:
- interspecific competition — can lead to → competitive exclusion
- interspecific competition — can lead to → resource partitioning
- resource partitioning — enables → coexistence
- resource partitioning — shrinks → realised niche
- intraspecific competition — drives → natural selection (accept any evolution link)
- competitive exclusion — prevents → coexistence
Award full marks for at least 6 biologically valid, correctly directed arrows with linking phrases.