Why did rabbits take over Australia? What happens when a new predator arrives? This lesson examines how populations change and why introduced species can cause enormous problems for native wildlife.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Think about what Australia offered rabbits that Europe did not. Write your best explanation before reading on.
Consider a local Australian animal such as a bilby or a quoll. How might its life change if a fox or cat moves into its habitat?
A population is not a fixed number. It goes up and down depending on what is happening in the environment.
The size of a population depends on births, deaths, immigration and emigration. But these numbers are influenced by larger factors. Food availability matters: if there is plenty of grass, kangaroo numbers can rise. Predators matter: if dingo numbers increase, kangaroo numbers may fall. Disease and competition also play major roles.
Scientists track populations over time using surveys, trapping and aerial counts. The data often show curves: a rapid rise, a peak, then a crash or a stable level. Understanding these patterns helps managers protect native species and control pests.
When humans bring animals or plants to a new place, those species sometimes explode in number and cause serious harm.
Rabbits were brought to Australia in 1859. With mild climates, abundant food and almost no predators that could control them, their population exploded. By the 1920s there were an estimated ten billion rabbits across the continent. They ate native vegetation, competed with native herbivores such as bilbies, and caused massive soil erosion through overgrazing.
Cane toads were introduced in 1935 to control beetles in sugar cane fields. They failed at that job but succeeded at spreading. They have few predators in Australia because their skin is toxic. As they move across northern Australia, they poison goannas, quolls and snakes that try to eat them.
Foxes and cats are responsible for the decline and extinction of many small native mammals. They are skilled hunters and have spread across almost the entire continent. Unlike in their native ranges, they face limited competition and few diseases in Australia.
Scientists use data to tell the story of a population. A table or graph can show whether a species is thriving, stable or in trouble.
When reading population data, look for three things: the trend (going up, down or stable), the rate of change (fast or slow), and any patterns (such as seasonal cycles or sudden drops). A sudden drop might mean a new predator arrived, a disease spread, or a drought reduced food.
For example, data on brush-tailed rabbit-rat populations in northern Australia show steep declines after cane toads arrived in an area. The rats were not eaten by toads; instead, the goannas that controlled rat numbers died from eating toads. This is an indirect effect that only becomes clear when scientists examine the data.
Wrong: “Introduced species always die out.”
Right: Many introduced species thrive because they have no natural predators, plenty of food and few diseases. Rabbits, foxes and cane toads are all examples.
Wrong: “Cane toads eat everything.”
Right: Cane toads have specific diets, mainly insects. The problem is that they outcompete native species for food and poison the predators that try to eat them.
Right: Rabbits caused massive soil erosion, destroyed native vegetation and competed with native herbivores. They are one of Australia’s most damaging pests.
Right: Eradication is extremely difficult. Cane toads now number in the hundreds of millions, and foxes occupy almost all of mainland Australia. Control programs cost millions of dollars and require long-term commitment.
Annotated graph showing estimated rabbit population in Australia from 1860 to 1950, with labels for introduction point, exponential growth phase, peak and plateau after control measures.
Illustrated food web showing how foxes and cats prey on small native mammals, and how cane toads poison goannas, leading to indirect increases in prey species.
Populations change due to food, predators, disease and competition.
Introduced species in Australia include rabbits, cane toads, foxes and cats. They often thrive because they lack natural controls.
Do not assume introduced species are easy to remove. Eradication is expensive and rarely successful once a species is widespread.
Population data helps scientists decide where to focus conservation efforts and which control methods might work.
The table below shows estimated rabbit population in Australia at selected dates. Use the data to answer the questions.
| Year | Estimated population (millions) |
|---|---|
| 1860 | Less than 1 |
| 1870 | 10 |
| 1880 | 100 |
| 1900 | 1,000 |
| 1920 | 10,000 |
| 1950 | 5,000 |
a) Describe the overall trend from 1860 to 1920.
b) Suggest two reasons why the population grew so rapidly.
c) Suggest one reason why the population fell between 1920 and 1950.
A student says: “We should just release a virus to kill all the cane toads.” Use the Claim-Evidence-Reasoning frame below to evaluate this statement.
Claim: State whether you agree, disagree or partly agree.
Evidence: Use facts from the lesson about introduced species, ecosystems and unintended consequences.
Reasoning: Explain how the evidence supports your claim.
1. What is a population?
What is NOT a population?
2. Which of the following is an introduced species in Australia?
3. Why did rabbit numbers explode in Australia?
4. Foxes and cats have caused the decline of many small native mammals mainly because they:
5. According to the lesson, why is it so difficult to remove introduced species such as cane toads from Australia?
Define introduced species and native species, giving one Australian example of each. 1 mark for each definition, 1 mark for one correct example.
Explain why rabbit numbers exploded in Australia using the terms competition and predator. 2 marks for explaining lack of predators, 2 marks for explaining competition with native species.
Explain why introduced species such as foxes and cane toads are difficult to remove from Australian ecosystems. In your answer, refer to at least two factors discussed in the lesson. 2 marks for each factor explained, 1 mark for linking the factors to the difficulty of removal.
Look back at what you wrote in the Think First section. How has your understanding changed about why rabbits exploded in Australia and what happens to native animals when a new predator arrives?
1: B. A population is all the organisms of one species living in a particular area at the same time.
2: C. Cane toads were introduced to Australia. Kangaroos, wombats and emus are native.
3: A. Rabbits had abundant food and almost no natural predators that could control their numbers.
4: D. Foxes and cats are efficient predators, and in Australia they have few natural controls.
5: B. Introduced species such as cane toads are widespread, reproduce quickly and have few predators, making eradication extremely difficult.
Sample answer: An introduced species is one that humans have brought to an area where it does not naturally occur (1 mark). A native species is one that naturally occurs in an area without human intervention (1 mark). For example, the cane toad is an introduced species, while the kangaroo is a native species (1 mark).
Sample answer: Rabbit numbers exploded because in Australia there were very few predators that could hunt them effectively (1 mark). In Europe, predators and disease kept rabbit numbers in check, but these controls were missing in Australia (1 mark). Rabbits also competed with native herbivores such as bilbies for food and shelter (1 mark). This competition reduced resources for native species while allowing rabbit numbers to keep rising (1 mark).
Sample answer: Introduced species are difficult to remove because they often lack natural predators in their new environment (1 mark). For example, cane toads have toxic skin, so native predators that try to eat them die (1 mark). This means there is almost nothing to stop their spread (1 mark). They also reproduce very quickly and have now spread across vast areas of northern Australia (1 mark). Because they are so widespread, any control method would need to cover enormous areas, making eradication almost impossible (1 mark).
Populations change due to food, predators, disease and competition.
Introduced species such as rabbits, cane toads, foxes and cats cause major harm because they lack natural controls.
Understanding population trends helps scientists protect native wildlife and design better control programs.
This lesson completes Block E on ecosystems. You can now explain how matter cycles and how populations interact.
Jump through questions on population changes, introduced species and Australian ecosystems. See how well you know your natives from your invaders!