Biology • Year 11 • Module 4 • Lesson 8
Interspecific and Intraspecific Competition
Apply the competition framework to real Australian population data, an introduced-species scenario, and a resource-partitioning data set.
1. Analyse population data — spotted-tailed quolls and Tasmanian devils
A wildlife monitoring program tracked spotted-tailed quoll (Dasyurus maculatus) and Tasmanian devil (Sarcophilus harrisii) populations at three sites in Tasmania over five years. At Sites 1 and 2, devils were present throughout. At Site 3, all devils were removed at the start of the study as part of a devil facial tumour disease (DFTD) management trial. Both species eat carrion as a primary food source. 8 marks
| Year | Site 1 — quoll count (devils present) | Site 2 — quoll count (devils present) | Site 3 — quoll count (devils removed) |
|---|---|---|---|
| 1 | 12 | 9 | 10 |
| 2 | 9 | 7 | 14 |
| 3 | 7 | 5 | 19 |
| 4 | 5 | 4 | 24 |
| 5 | 4 | 3 | 31 |
1.1 Describe the trend in quoll population size at Sites 1 and 2 compared with Site 3 from Year 1 to Year 5. 2 marks
1.2 Using the concepts of interspecific competition and realised niche, explain the pattern shown in the data. 3 marks
1.3 Predict what would happen at Site 3 if devils were reintroduced at the start of Year 6. Justify your prediction using Gause’s Law. 2 marks
1.4 A student claims: “The quoll decline at Sites 1 and 2 is an example of intraspecific competition.” Identify the error and correct it. 1 mark
2. Interpret graph — kangaroo habitat use and resource partitioning
The graph below shows the percentage of feeding observations recorded in each habitat type for red kangaroos (Osphranter rufus) and eastern grey kangaroos (Macropus giganteus) in a region of south-western Queensland where their ranges overlap. 6 marks
Figure 2. Percentage of feeding observations per habitat type for two kangaroo species. Hypothetical data consistent with published ecology.
2.1 Identify the habitat where the two species show the greatest overlap in feeding activity, and estimate the difference in percentage observations. 2 marks
2.2 Name the type of resource partitioning shown in the graph and explain how it allows the two species to coexist in the same region. 2 marks
2.3 A drought makes the dense woodland and creek-line vegetation uninhabitable for both species. Predict the outcome for the eastern grey kangaroo population, using the concept of competitive exclusion. 2 marks
3. Apply to a new scenario — sheep introduction in western NSW
A sheep station in the far west of NSW runs 4 000 Merino sheep on native pasture. Red kangaroos and euros (Osphranter robustus) have historically coexisted on the property by using different habitats. When sheep numbers increase above 3 000, both kangaroo species decline. When a drought year is followed by a rainfall event, sheep numbers are maintained artificially by the farmer through supplementary feeding and bore-water, while kangaroo numbers fall sharply. 7 marks
3.1 Classify the competition between sheep and red kangaroos. Is it intraspecific or interspecific? Give two pieces of evidence from the scenario to support your answer. 2 marks
3.2 Explain why supplementary feeding of sheep during drought gives the sheep an artificial competitive advantage over kangaroos that rely solely on natural pasture. Link your answer to the concept of realised niche. 3 marks
3.3 Before sheep were introduced, red kangaroos and euros coexisted through resource partitioning. Explain why resource partitioning between native species may fail to protect them once a subsidised introduced competitor is added to the system. 2 marks
Q1 — Quoll population data
1.1 At Sites 1 and 2 (devils present), quoll counts declined steadily each year (Sites 1 and 2 fell from 12 and 9 to 4 and 3 respectively) [1]. At Site 3 (devils removed), quoll counts increased substantially each year, rising from 10 to 31 [1].
1.2 Devils and quolls are different species competing for the same carrion resource — this is interspecific competition [1]. Devils are larger, more aggressive, and are the superior competitor; they reduce the availability of carrion for quolls, shrinking the quolls’ realised niche [1]. Where devils are absent (Site 3), quolls can occupy a wider portion of their fundamental niche, so the population grows [1].
1.3 Quoll numbers at Site 3 would be expected to decline again [1]. Gause’s Law predicts that the superior competitor (devil) will exclude the inferior competitor (quoll) from the shared carrion resource when they occupy the same area — the competitive advantage of devils seen at Sites 1 and 2 would reassert itself [1].
1.4 Error: quolls and devils are different species, so competition between them is interspecific, not intraspecific. Intraspecific competition would only occur between two quolls (or two devils) competing with each other [1].
Q2 — Kangaroo graph
2.1 Sparse woodland shows the greatest overlap: reds 20%, greys 30%, a difference of only 10 percentage points [1 mark for identifying sparse woodland; 1 mark for quoting values / calculating difference]. Accept any valid reading within ±2 percentage points.
2.2 Spatial (habitat) resource partitioning [1]. Each species predominantly uses different habitat types (reds dominate open plains, greys dominate dense woodland), reducing direct competition for food and space by dividing the landscape between them [1].
2.3 Eastern greys rely predominantly on dense woodland and creek-line vegetation (combined ~62% of feeding observations). If these habitats become uninhabitable, grey kangaroos would be forced onto open plains where red kangaroos are the dominant competitor [1]. Competitive exclusion is likely: reds are better adapted to open, arid plains and would outcompete greys for the reduced pasture, causing grey numbers to decline or causing local exclusion [1].
Q3 — Sheep introduction scenario
3.1 Interspecific competition [0.5]. Evidence 1: sheep and kangaroos are different species [0.5]. Evidence 2: both use the same pasture resource but in a shared habitat, and both decline when the other increases — a hallmark of interspecific competition [1].
3.2 In a drought, native pasture collapses and both sheep and kangaroos compete for the same scarce grass [1]. Sheep are given supplementary feed and bore-water by the farmer, so their access to food and water is maintained independently of natural pasture conditions [1]. This shrinks the kangaroos’ realised niche further: kangaroos cannot exploit artificial food/water sources, so competition in the natural pasture zone becomes even more intense and the kangaroos are effectively excluded from a resource that sheep can still access [1].
3.3 Native species evolved to partition resources with other native species whose competitive abilities are matched by co-evolution over millennia [1]. An introduced competitor with artificial subsidies (supplementary feed, water points) removes the natural constraints that make resource partitioning effective — the introduced species can exploit the resource regardless of natural scarcity, bypassing the niche differences that normally prevent exclusion [1].