In the 1960s, overfishing removed tiger sharks from the waters around Shark Bay, Western Australia. Without their predator, dugong populations surged. The dugongs grazed seagrass meadows to bare sand. Fish, turtles, and dolphins that depended on the seagrass disappeared. One species lost changed everything. This is the power of predation — and the danger of ignoring it.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Before you read, commit to a prediction. You will revisit these at the end.
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.
Predation is not simply a matter of one animal eating another. It is a dynamic interaction that shapes population cycles, drives evolution, and structures entire ecosystems. Understanding these dynamics is essential for predicting what happens when a predator is added or removed.
The classic model of predator-prey interaction 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.
The simple model assumes only two species interact. In reality, multiple factors modify the cycle:
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.
Predator → Herbivore → Plant
When predators are present, they suppress herbivore populations. With fewer herbivores, plants thrive. This is called a trophic cascade because the predator’s effect “cascades” down to the plant level.
When predators are removed:
Apex Predator → Mesopredator → Herbivore → Plant
When an apex predator is removed, mesopredators (mid-level predators) increase in number. This is called mesopredator release. The mesopredators then suppress herbivores, which could allow plants to recover — but mesopredators also prey on birds, reptiles, and other small animals, causing collateral damage.
Australian case study — Dingo removal:
In southern and eastern Australia, dingoes were systematically removed from grazing land during the 19th and 20th centuries to protect sheep and cattle. The result was a massive trophic cascade:
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 the fence (dingoes present). This demonstrates that apex predators structure ecosystems far beyond simply killing prey.
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 species with physical defences (spines, tough leaves), chemical defences (toxins, tannins), or rapid regrowth. Over time, 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. However, 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 and more extensively. In contrast, moderate grazing can reduce fuel loads and prevent catastrophic wildfire.
Introduced grazers in arid Australia:
Sheep and cattle were introduced to Australia in the 1800s. In arid zones, their impacts were catastrophic:
The Gascoyne River catchment in Western Australia shows this pattern: where cattle grazing was removed and fencing installed, native perennial grasses recovered within five years, soil erosion declined, and small mammal diversity increased.
When one species changes, some effects are immediate and obvious. Others ripple through the food web in ways that are harder to trace but equally important. HSC exam questions often test your ability to distinguish these levels of effect.
An immediate interaction between two species.
Direct effects are the simplest to observe and measure. They involve physical contact or immediate consumption.
Consequences that flow through intermediate species.
Indirect effects are often larger in magnitude than direct effects and can be unexpected.
Trace the cascading effects of species removal through a food web. Distinguish direct from indirect effects at each step.
The following food web occurs in a temperate woodland: Eucalypt trees → koalas → dingoes. A new government policy mandates dingo removal from all grazing land adjacent to the woodland.
A marine reserve is established where all fishing is banned. Tiger sharks, groupers, and other large predators recover. Predict three changes that would occur in the reserve over 10 years, and justify each prediction using concepts from this lesson.
Predator-prey dynamics
Prey increase → predators increase (more food) → prey decline (more predation) → predators decline (less food). The predator peak lags behind the prey peak due to delayed population response.
Trophic cascade
A top-down effect where removing an apex predator releases herbivores from predation pressure, causing overgrazing, vegetation loss, and ecosystem restructuring.
Australian example: dingo removal
Removing dingoes from grazing land caused kangaroo/rabbit population explosions, overgrazing, vegetation degradation, soil erosion, and dryland salinity.
Australian example: Shark Bay
Removing tiger sharks caused dugong overgrazing of seagrass meadows, collapsing habitat for fish, turtles, and dolphins.
Direct vs indirect effects
Direct: immediate interaction (dingo kills kangaroo). Indirect: downstream consequences through intermediate species (dingo removal → more kangaroos → less grass → more erosion).
Syllabus link
ACSBL052, ACSBL053, ACSBL060: Analyse the effect of predation and herbivory on population structure and ecosystems; predict change in one population given information about another.
Now that you have completed the lesson, review your initial answers. What did you get right? What surprised 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.
In this lesson you learned: