Biology • Year 11 • Module 4 • Lesson 13
Predation and Herbivory
Lock in the core vocabulary, the predator-prey cycle model, trophic cascades, and direct vs indirect effects — the foundations of this lesson.
1. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: predation, herbivory, trophic cascade, mesopredator release, Lotka-Volterra model, direct effect, indirect effect, apex predator, prey refuge, time lag. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | A qualitative model describing cyclical oscillations in predator and prey population sizes, where the predator peak lags behind the prey peak. | |
| 1.2 | The consumption of plant material by animals; a form of biological interaction that shapes plant communities. | |
| 1.3 | A top-down effect where predator removal causes changes that ripple through multiple trophic levels, altering vegetation and soil. | |
| 1.4 | An immediate, first-order interaction between two species — for example, a dingo killing a kangaroo. | |
| 1.5 | A consequence that flows through one or more intermediate species — for example, dingo removal leading to increased soil erosion via kangaroo overgrazing. | |
| 1.6 | The species at the top of a food web that is not prey to any other species in that system. | |
| 1.7 | The increase in mid-level predator populations that occurs when their apex predator is removed. | |
| 1.8 | A habitat feature (e.g. deep snowpack, dense vegetation) where prey can shelter from predators, stabilising prey numbers. | |
| 1.9 | The delay between an increase in prey numbers and the subsequent increase in predator numbers, due to the time required for predator reproduction. | |
| 1.10 | The killing and consumption of one animal by another animal. |
2. Sequence the Lotka-Volterra cycle
The four steps of the predator-prey cycle are listed below in the wrong order. Number them 1–4 to show the correct sequence, and for each step write one sentence explaining the biological mechanism involved. 8 marks (1 sequence + 1 explanation per step)
Order: ______ Predator population declines due to food scarcity.
Mechanism:
Order: ______ Prey population increases with abundant food and few predators.
Mechanism:
Order: ______ Increased predation pressure reduces prey population.
Mechanism:
Order: ______ Predator population grows in response to more available prey.
Mechanism:
3. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version on the line below. 8 marks (1 for T/F, 1 for each correction needed)
3.1 In a predator-prey cycle, the predator population peak occurs at the same time as the prey population peak. T / F
3.2 A trophic cascade is a bottom-up effect where plant removal causes changes that ripple upward through the food web. T / F
3.3 When dingoes were removed from Australian grazing land, kangaroo populations increased because the direct predation pressure on kangaroos was removed. T / F
3.4 Herbivory has no effect on fire frequency in a landscape. T / F
4. Trophic cascade direction table
Complete the table below for the dingo removal case study and the Shark Bay case study. For each row, state the species affected, whether the population increases or decreases, and whether it is a direct or indirect effect. 12 marks (2 per row)
| Cause | Species directly/indirectly affected | Population change (increase/decrease) | Direct or indirect effect? Explain why. |
|---|---|---|---|
| Dingo removal from grazing land | Kangaroos | ||
| Kangaroo population increases (from above) | Native ground cover / vegetation | ||
| Vegetation cover declines (from above) | Soil erosion | ||
| Tiger shark removal from Shark Bay | Dugongs | ||
| Dugong population increases (from above) | Seagrass meadows | ||
| Seagrass meadows decline (from above) | Fish, turtles, dolphins |
5. Connect the concepts
Draw labelled arrows between the five terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “removes predation pressure on”, “leads to”, “is an example of”). Aim for at least 5 labelled arrows. 5 marks
Supplied terms: apex predator removal · herbivore population increase · trophic cascade · vegetation loss · indirect effect.
Q1 — Term–definition matches
1.1 Lotka-Volterra model • 1.2 herbivory • 1.3 trophic cascade • 1.4 direct effect • 1.5 indirect effect • 1.6 apex predator • 1.7 mesopredator release • 1.8 prey refuge • 1.9 time lag • 1.10 predation.
Q2 — Sequence and mechanisms
Step 1: Prey population increases with abundant food and few predators. Mechanism: with low predation mortality and sufficient resources, birth rate exceeds death rate, so the prey population grows exponentially.
Step 2: Predator population grows in response to more available prey. Mechanism: more prey means more food, improving predator survival and reproduction — but this demographic response takes time (births, maturation), creating the time lag.
Step 3: Increased predation pressure reduces prey population. Mechanism: rising predator numbers increase the rate of prey mortality, which eventually exceeds the prey birth rate, causing the prey population to decline.
Step 4: Predator population declines due to food scarcity. Mechanism: fewer prey means predators cannot find sufficient food; predator death rate rises and birth rate falls, shrinking the predator population until the prey can recover.
Q3 — True / false with correction
3.1 False. Correction: the predator population peak lags behind the prey peak because predator reproduction takes time to respond to the increase in prey availability.
3.2 False. Correction: a trophic cascade is a top-down effect where predator removal causes changes that ripple downward through the food web to herbivores and plants.
3.3 True. The direct effect of dingo removal is reduced predation on kangaroos, so kangaroo populations increase.
3.4 False. Correction: herbivory does affect fire regimes — overgrazing reduces ground cover, which can cause fires to burn hotter and more extensively because dry, exposed soil and reduced litter alter fuel load and fire behaviour.
Q4 — Trophic cascade direction table
Row 1 — Dingo removal / Kangaroos: Increase. Direct effect — dingoes directly preyed on kangaroos, so removing dingoes immediately reduces kangaroo mortality.
Row 2 — Kangaroo increase / Vegetation: Decrease. Indirect effect — the vegetation does not interact with dingoes; it is affected only via the intermediate step of increased kangaroo grazing.
Row 3 — Vegetation decline / Soil erosion: Increases. Indirect effect — soil erosion increases because vegetation root systems and ground cover are lost; this is two steps removed from dingo removal.
Row 4 — Tiger shark removal / Dugongs: Increase. Direct effect — tiger sharks directly preyed on dugongs; removing sharks reduces dugong mortality.
Row 5 — Dugong increase / Seagrass: Decrease. Indirect effect — seagrass is affected via increased dugong grazing, not directly by shark removal.
Row 6 — Seagrass decline / Fish, turtles, dolphins: Decrease. Indirect effect — these species depend on seagrass as habitat or food; their decline is three trophic steps from shark removal.
Q5 — Sample concept map
A correct map should include arrows such as:
- apex predator removal — initiates → trophic cascade
- apex predator removal — removes predation pressure on, causing → herbivore population increase
- herbivore population increase — leads to overgrazing, causing → vegetation loss
- vegetation loss — is a consequence of the → trophic cascade
- vegetation loss — is an → indirect effect (of apex predator removal)
- trophic cascade — produces multiple → indirect effects
Any biologically valid linking phrases are accepted. Award full marks for at least 5 correctly labelled arrows that respect causal direction.