Biology • Year 11 • Module 4 • Lesson 13
Predation and Herbivory
Apply predator-prey dynamics and trophic cascade concepts to real population data and ecosystem scenarios drawn from Australian case studies.
1. Interpret dingo fence data
A research team compared key ecological measurements on either side of the dingo exclusion fence in South Australia. Inside the fence no dingoes are present; outside the fence dingoes are present. The data below represent averages across five survey sites. 8 marks
| Ecological variable | Inside fence (no dingoes) | Outside fence (dingoes present) |
|---|---|---|
| Kangaroo density (per km²) | 22 | 2 |
| Ground cover (%) | 28 | 62 |
| Soil erosion rate (tonnes/ha/yr) | 4.8 | 0.9 |
| Native ground-nesting bird species (no.) | 3 | 11 |
| Introduced weed cover (%) | 41 | 12 |
1.1 Describe the differences in kangaroo density and ground cover between the two sides of the fence. Support your description with specific figures from the data. 2 marks
1.2 Using the lesson concept of trophic cascade, explain why ground cover is lower inside the fence than outside. Trace the chain of causation. 3 marks
1.3 The data show fewer native bird species inside the fence. Identify whether this is a direct or indirect effect of dingo absence, and explain the chain of causation. 3 marks
2. Interpret a predator-prey cycle graph
The graph below shows stylised population cycles of snowshoe hares (prey) and Canada lynx (predator) over 30 years. Use the graph to answer the questions below. 6 marks
Stylised representation of the classic Lotka-Volterra predator-prey cycle — values are relative units for comparison only.
2.1 Identify the approximate time lag (in years) between the hare population peak and the lynx population peak in the first cycle. What does this tell you about the direction of energy flow? 2 marks
2.2 At year 15, the lynx population is near its peak. Predict what will happen to the hare population in years 15–18. Explain your reasoning using the Lotka-Volterra mechanism. 2 marks
2.3 A disease kills 90% of the lynx population at year 8. Predict what happens to hare numbers in years 9–12 and explain two reasons why the hare population may not continue to grow indefinitely even without lynx. 2 marks
3. Apply to a new scenario — Snowy Mountains fox and bush rat
In the Snowy Mountains of New South Wales, red foxes prey on bush rats. During years of heavy snowfall, bush rats shelter under deep snowpack where foxes cannot dig. In mild winters, there is no effective refuge and fox predation is the primary population control on bush rats. 7 marks
3.1 In a year with heavy snowfall, explain whether the bush rat population is likely to follow the classic Lotka-Volterra cycle prediction. Use the concept of ‘prey refuge’ in your answer. 2 marks
3.2 In a mild winter, the snow refuge is absent and bush rat numbers crash to very low levels due to fox predation. Using lesson concepts, explain how the absence of a prey refuge changes the Lotka-Volterra cycle prediction for the bush rat population compared to a heavy-snowfall year. 2 marks
3.3 After three consecutive mild winters, fox numbers are very high and bush rat numbers are very low. A bushfire then destroys 70% of the fox population. Predict the likely trend in bush rat numbers over the following two years and explain the mechanism. 3 marks
Q1.1 — Describe the fence data (2 marks)
Inside the fence (no dingoes), kangaroo density is 22 per km², which is 11 times higher than the 2 per km² outside the fence where dingoes are present [1]. Ground cover inside (28%) is less than half that outside (62%), a difference of 34 percentage points [1].
Q1.2 — Trophic cascade chain (3 marks)
Removing dingoes directly removes predation pressure on kangaroos, causing kangaroo density to increase from 2 to 22 per km² [1]. The high density of kangaroos causes overgrazing, consuming vegetation faster than it can regenerate [1]. This reduces ground cover from 62% to 28% — a classic trophic cascade where the predator’s absence restructures the plant layer via its herbivore prey [1].
Q1.3 — Bird species decline: direct or indirect? (3 marks)
The decline in native ground-nesting bird species is an indirect effect of dingo absence [1]. Dingoes do not directly prey on ground-nesting birds; the chain runs: dingo removal → kangaroo increase → ground cover loss → habitat destruction for ground-nesting birds [1]. Because the effect on birds is mediated through two intermediate steps (kangaroo overgrazing, then vegetation loss), it is a second- or third-order indirect effect [1].
Q2.1 — Time lag and energy flow (2 marks)
The hare peak occurs at approximately year 3 and the lynx peak at approximately year 5, giving a time lag of about 2 years [1]. This time lag indicates that energy flows from hare to lynx (hares are the energy source that allows lynx populations to grow), and that predator population growth lags because births and maturation of new lynx take time after the prey increase occurs [1].
Q2.2 — Hare prediction years 15–18 (2 marks)
With the lynx population near its peak at year 15, predation pressure on hares is very high, so hare numbers will decline over years 15–18 [1]. This is because the rate of hare mortality from lynx predation exceeds the hare birth rate, causing a net population decrease — consistent with step 3 of the Lotka-Volterra cycle [1].
Q2.3 — Disease kills lynx; hare prediction (2 marks)
With lynx drastically reduced at year 8, hare numbers will likely increase rapidly in years 9–12 due to reduced predation pressure [1]. However, hare numbers cannot grow indefinitely for at least two reasons: (i) food resources (vegetation) will become depleted as the hare population increases, limiting further growth through food shortage; (ii) crowded hare populations suffer disease outbreaks that reduce numbers — a real-world complexity the lesson describes as density-dependent disease acting before predators do [1].
Q3.1 — Bush rat with heavy snowfall (2 marks)
In a heavy snowfall year, bush rats have access to a prey refuge (deep snowpack) that fox predation cannot penetrate [1]. The classic Lotka-Volterra cycle would not apply as predicted because the refuge decouples predation from prey abundance, stabilising bush rat numbers at a higher level than the cycle predicts and damping the amplitude of oscillations [1].
Q3.2 — Absence of refuge changes Lotka-Volterra prediction (2 marks)
In a heavy-snowfall year, the prey refuge decouples predation from prey abundance, stabilising bush rat numbers and damping oscillations [1]. In a mild winter, with no refuge available, fox predation operates fully without any protection for the rats. The Lotka-Volterra cycle now proceeds as the classic model predicts: rising fox predation pressure reduces bush rat numbers sharply, producing a deeper trough in the prey population than the refuge year would show — consistent with the lesson’s point that refuges stabilise prey numbers [1].
Q3.3 — Bushfire kills foxes; bush rat trend (3 marks)
With fox numbers reduced 70% by the bushfire, predation pressure on bush rats will fall dramatically, releasing them from their primary mortality source [1]. Bush rat numbers will likely increase rapidly over the following two years as the birth rate now exceeds the reduced predation mortality [1]. However, because the fox population itself is depleted, it will take some time (1–2 reproductive seasons) before fox numbers recover sufficiently to suppress bush rats again — creating a period where bush rat growth is resource-limited rather than predator-limited [1].