Predation and Herbivory — Population Structure and Ecosystem Effects
In 1992, a CSIRO study documented the trophic cascade triggered by tiger shark removal from Shark Bay, Western Australia. Between 1963 and 1990, shark populations were reduced by targeted fishing. Without their main predator, dugong populations increased dramatically, shifting from patchy grazing to intensive cropping of seagrass meadows. Seagrass density fell 70%. The fish, turtles, and dolphins that depended on seagrass structure disappeared from areas that once supported them. A 300% increase in dugong grazing pressure, driven by the removal of one shark species, restructured a whole ecosystem across three trophic levels.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
Q1. In a national park, all dingoes are removed to protect livestock on neighbouring properties. Predict what will happen to kangaroo populations, grass cover, and soil erosion over the next 10 years. Explain the chain of causation at each step.
Q2. A farmer notices that caterpillars are eating his crop leaves. He sprays pesticide and kills 95% of the caterpillars. Predict what might happen to the crop over the following two years if the farmer continues spraying every season. Consider both direct and indirect effects.
Core Content
Predator-prey populations oscillate in cycles where the predator peak lags behind the prey peak due to delayed demographic responses
Between 1963 and 1990, tiger sharks were removed from Shark Bay, Western Australia, through targeted fishing. A 1992 CSIRO study found that dugong populations responded by shifting from cautious, patchy grazing to intensive continuous grazing on seagrass beds. Seagrass density fell 70%. The fish, turtles, and dolphins that used seagrass as feeding and nursery habitat disappeared. The shark did not eat the seagrass, and it did not eat the turtles directly — but its removal set off a chain that restructured the ecosystem across three trophic levels. That chain is called a trophic cascade, and it begins with understanding the dynamics of predation itself.
The Lotka-Volterra model (qualitative): The classic model describes a cyclical relationship with a time lag:
Step 1: Prey increase
With abundant food and few predators, the prey population grows exponentially.
Step 2: Predators respond
More prey means more food for predators. Predator survival and reproduction increase.
Step 3: Prey decline
Increased predation pressure reduces the prey population.
Step 4: Predators decline
With fewer prey, predators starve or fail to breed. The cycle repeats.
Lotka-Volterra cycle: prey increase → predator increase → prey decrease → predator decrease → repeat. The predator peak lags the prey peak because predator reproduction takes time to respond to more food. The cycle is driven by delayed demographic responses — predators do not appear instantly when prey increase.
Pause — copy the highlighted cycle sequence and the reason for the time lag into your book.
The predator peak lags behind the prey peak. Predators do not immediately suppress prey; their population response takes time through births and immigration. This time lag produces the characteristic oscillating pattern seen on predator-prey graphs.
Australian example: In the Snowy Mountains, foxes prey on bush rats. During years of heavy snow, rats shelter under deep snowpack where foxes cannot dig, acting as a refuge that allows rat populations to persist even at high fox density. In mild winters, rat populations crash because the refuge is lost.
In the Lotka-Volterra predator-prey model, why does the predator population peak lag behind the prey population peak?
Predator removal causes effects that cascade downward through the food web — reshaping vegetation, altering soil, changing which species survive
We just saw that predator-prey populations oscillate in coupled cycles. That raises a question: what happens when the predator is removed entirely — not just temporarily suppressed? This card answers it → trophic cascades show how apex predator removal reshapes entire ecosystems across multiple trophic levels.
When a predator is removed, the effect does not stop at its prey. The impact cascades downward through the food web, reshaping vegetation, altering soil chemistry, and changing which species can survive. These are called trophic cascades.
When predators are removed: (1) herbivore population increases due to reduced predation; (2) herbivores consume more plants; (3) plant biomass declines; (4) plant diversity may decrease as palatable species are eaten first; (5) soil erosion increases as ground cover is lost.
When an apex predator is removed, mid-level predators (mesopredators) increase — this is called mesopredator release. Mesopredators then suppress herbivores, but they also prey on birds, reptiles, and small mammals, causing collateral damage beyond the original cascade.
Trophic cascade: top-down effect where predator removal releases herbivores → overgrazing → vegetation loss → soil erosion → ecosystem restructuring. Mesopredator release: removal of apex predator increases mid-level predator populations, causing collateral damage to birds, reptiles and small mammals.
Copy the cascade definitions and both Australian examples before the check below.
Dingoes were systematically removed from grazing land to protect sheep and cattle. The result was a massive trophic cascade:
- Direct effect: Kangaroo and rabbit populations increased without predation control.
- Indirect effect 1: Overgrazing removed native ground cover, reducing habitat for ground-nesting birds and reptiles.
- Indirect effect 2: Soil became exposed to wind and water erosion, increasing dryland salinity.
- Indirect effect 3: Vegetation composition shifted from perennial grasses to unpalatable woody weeds.
The dingo exclusion fence in South Australia provides a natural experiment: inside the fence (no dingoes), kangaroo density is 10× higher and ground cover is 50% lower than outside (dingoes present).
In the 1960s, overfishing removed tiger sharks from Shark Bay, WA. Without shark predation, dugong populations exploded. Dugongs grazed seagrass meadows to bare sediment. The seagrass meadows — which had supported fish, turtles, and dolphins — collapsed. This four-level cascade (shark → dugong → seagrass → fish/turtles/dolphins) shows that marine apex predators are equally critical.
What is a trophic cascade?
We just saw that removing a predator reshapes ecosystems through herbivore release. That raises a question: how powerful is herbivory itself as an ecological force — independent of predator control? This card answers it → herbivory actively shapes plant communities, nutrient cycling, and disturbance regimes.
Herbivory is not passive consumption. It is a powerful force that selects for plant defences, reshapes vegetation communities, and can transform landscapes from forest to grassland or from grassland to desert.
1. Shifts competitive balance
Heavy grazing removes palatable plant species first, favouring plants with physical defences (spines, tough leaves), chemical defences (toxins), or rapid regrowth. The plant community shifts toward defended species.
2. Reduces structural complexity
When herbivores remove understory vegetation, ground-dwelling animals lose shelter. When they browse tree seedlings, forest regeneration stops and canopy gaps fail to close.
3. Alters nutrient cycling
Herbivores accelerate nutrient cycling by consuming plant biomass and excreting nutrients in concentrated form. Overgrazing can export nutrients through erosion or reduce decomposition by removing the litter layer.
4. Changes disturbance regimes
Overgrazed landscapes have less ground cover, so fires burn hotter. In contrast, moderate grazing can reduce fuel loads and prevent catastrophic wildfire.
Herbivory four effects: (1) shifts plant competitive balance toward defended species; (2) reduces structural complexity — less shelter, less forest regeneration; (3) alters nutrient cycling — accelerates or depletes depending on grazing intensity; (4) changes fire regimes — overgrazing increases fire intensity.
Copy the four herbivory effects and the Gascoyne River restoration example before moving on.
Sheep and cattle introduced to arid zones: hooves compacted soil reducing water infiltration; perennial grasses replaced by annual weeds; soil erosion stripped topsoil exposing saline subsoils; native marsupial herbivores (bilbies, bandicoots) declined due to competition and habitat loss. In the Gascoyne River catchment (WA), where cattle grazing was removed and fencing installed, native perennial grasses recovered within five years, soil erosion declined, and small mammal diversity increased.
HSC exam questions often test your ability to distinguish first-order from second- and third-order effects. Band 6 responses trace effects to at least the third order.
We just saw the full trophic cascade from dingo removal and the herbivory effects on vegetation. That raises an exam question: how do you correctly classify which effects are "direct" and which are "indirect"? This card answers it → the distinction is about how many intermediate species stand between the cause and the outcome.
Direct effects (first-order)
An immediate interaction between two species.
- A dingo kills a kangaroo
- A dugong eats seagrass
- A caterpillar chews a leaf
Direct effects are the simplest to observe and measure. They involve physical contact or immediate consumption.
Indirect effects (second- and third-order)
Consequences that flow through intermediate species.
- Dingo removal → more kangaroos → less grass → more soil erosion
- Shark removal → more dugongs → less seagrass → fewer fish
- Caterpillar removal (pesticide) → fewer parasitic wasps → caterpillar outbreak next season
Direct effect: immediate first-order interaction between two species (predator kills prey). Indirect effect: downstream consequence flowing through one or more intermediate species. Band 6 rule: always trace effects to at least the third order — don't stop at "kangaroos increase."
Copy the definitions and add one example of your own for each category.
Which of the following is an INDIRECT effect of removing tiger sharks from Shark Bay?
The graph below (described) shows the population cycles of snowshoe hares and Canada lynx over 20 years.
- Describe the phase lag between the hare peak and the lynx peak. What does this tell you about the direction of energy flow? (2 marks)
- Explain three reasons why the hare population does not crash to zero during the trough. (3 marks)
- A disease wipes out 80% of the lynx population in year 12. Predict what happens to hare numbers in years 13–15 and explain your reasoning. (2 marks)
- Would this same cycling pattern occur in Australia's dingo-kangaroo system? Justify your answer. (2 marks)
In a Victorian grassland, kangaroo grazing reduces grass biomass. This indirectly increases wildflower abundance but also increases soil erosion during drought.
- Identify one direct effect and one indirect effect of kangaroo grazing in this ecosystem. (2 marks)
- Explain how the indirect effect on wildflowers occurs, naming the intermediate trophic level. (2 marks)
- A land manager proposes culling kangaroos to reduce soil erosion. Evaluate this strategy by considering at least two unintended ecological consequences. (3 marks)
The removal of dingoes from Australian grazing land led to increased kangaroo and rabbit populations. This is best described as:
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
ApplyBand 4(4 marks) 1. Describe the Lotka-Volterra predator-prey cycle. In your answer, explain why the predator population peak lags behind the prey population peak, and identify one real-world factor that could disrupt this cycle.
AnalyseBand 4(4 marks) 2. Explain the trophic cascade that occurred when dingoes were removed from Australian grazing land. Your answer should include at least one direct effect and two indirect effects, with a clear chain of causation for each indirect effect.
EvaluateBand 5–6(6 marks) 3. In the 1960s, overfishing removed tiger sharks from Shark Bay, Western Australia. (a) Predict and explain the direct effects of tiger shark removal on the dugong population. (b) Predict and explain two indirect effects of tiger shark removal on the Shark Bay ecosystem. For each effect, trace the chain of causation from shark removal to the final outcome.
Show all answers
Short Answer Model Answers
Q1 (4 marks): The Lotka-Volterra cycle describes oscillating predator and prey populations. When prey are abundant, predators have plentiful food and their population increases. As predator numbers rise, predation pressure on prey intensifies, causing the prey population to decline. With fewer prey, predators starve or fail to reproduce, and their population falls. Reduced predation allows prey to recover, restarting the cycle (1 mark). The predator peak lags behind the prey peak because predator reproduction and survival take time to respond to increased food availability. Predators do not instantly appear when prey increase; their population grows through births and immigration, which requires a generation or more (1 mark). One real-world factor that disrupts the cycle is environmental disturbance such as drought, fire, or flood, which can reduce prey populations independently of predation, breaking the feedback loop (1 mark). Alternatively, a refuge habitat where prey hide from predators can stabilise prey numbers and dampen oscillations (1 mark).
Q2 (4 marks): Direct effect: dingo removal directly reduces predation on kangaroos and rabbits, causing their populations to increase because mortality from predation is reduced (1 mark). Indirect effect 1: more kangaroos and rabbits consume more vegetation, reducing ground cover and plant biomass. With less vegetation, soil becomes exposed to wind and water erosion, leading to land degradation and altered hydrology (1.5 marks). Indirect effect 2: reduced ground cover eliminates habitat for ground-nesting birds, reptiles, and small mammals, causing their populations to decline. Vegetation composition shifts from palatable perennial grasses to unpalatable woody weeds, further reducing forage quality (1.5 marks).
Q3 (6 marks): (a) The direct effect of tiger shark removal was an increase in the dugong population because dugong mortality from shark predation dropped sharply (1 mark). With fewer predators, dugong survival and reproduction improved, allowing the population to grow toward a higher carrying capacity (1 mark). (b) Indirect effect 1: more dugongs grazed more heavily on seagrass meadows, consuming shoots faster than they could regrow. This reduced seagrass cover and biomass, converting meadow areas to bare sediment (1 mark). Seagrass meadows are nursery habitat for fish and feeding grounds for turtles; their loss caused declines in these populations (1 mark). Indirect effect 2: the loss of seagrass reduced the physical structure that trapped sediment and stabilised the seafloor. Without seagrass roots, sediment resuspended more easily, reducing water clarity and further inhibiting seagrass recovery through light limitation (1 mark). This created a positive feedback loop where seagrass loss promoted further seagrass loss, making ecosystem recovery difficult even if sharks returned (1 mark).
Five timed questions integrating predator-prey dynamics, trophic cascades, and direct vs indirect effects. Beat the boss to bank a tier.
Enter the arenaThe 1992 CSIRO study of Shark Bay found that tiger shark removal produced a 300% increase in dugong grazing intensity, which reduced seagrass density by 70%, eliminating fish and turtle nursery habitat. This is a textbook trophic cascade: tiger shark → dugong → seagrass → fish, turtles, dolphins. The predator did not touch the seagrass or the turtles directly — the effect moved through two intermediate steps and restructured the whole ecosystem.
Return to your Think First response. Write one complete cascade chain using the Shark Bay data: tiger shark removed → [step 1] → [step 2] → [step 3] → final ecosystem outcome. Then identify which effects are direct and which are indirect.