Population Changes and Introduced Species
In 1859, pastoralist Thomas Austin released 24 wild rabbits near Geelong, Victoria; within 70 years the population had exploded to over 10 billion animals across the continent.
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Q1 · Q1: Why did rabbit numbers explode in Australia but not in Europe?
Q2 · Q2: What happens to native animals when a new predator arrives?
● Know
- populations change due to factors such as food, predators and disease
- introduced species can outcompete or prey on native species
- Australia has major problems with rabbits, cane toads, foxes and cats
● Understand
- introduced species often thrive because they lack natural predators
- competition for resources can push native species toward decline
- population data can show trends over time
● Can do
- interpret simple data tables showing population trends
- explain why introduced species are hard to remove
- evaluate claims about controlling invasive species using evidence
Watch a rock pool over the course of a month and you will notice the number of periwinkle snails there one week can be completely different from the number two weeks later, rain, temperature and how many predatory fish visited all left their mark on that count. The number of individuals of one species in an area shifts constantly, rising when conditions favour survival and falling when they do not.
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.
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.
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.
Introduced species such as rabbits and cane toads often thrive because they have few natural [blank] in Australia. Cane toads have [blank] skin that kills native . Foxes and cats are skilled [blank] that have caused the [blank] of many small native mammals.
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.
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-Evidence-Reasoning Frame
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.
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.
Wrong: Rabbits are harmless because they're small and only eat plants, they couldn't really damage the environment.
Right: Rabbits caused massive soil erosion, destroyed native vegetation and competed with native herbivores. They are one of Australia’s most damaging pests.
Wrong: Once a pest is found, scientists can just round them all up and remove them quickly.
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.
Diagram 1: Rabbit Population Growth Curve
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.
Today's hook told you that within 70 years of arriving in Australia, 24 rabbits had become 10 billion, the fastest population explosion in recorded history, because they had no natural predators here. That single difference explains almost everything.
Now that you've worked through the lesson, explain fully why rabbit numbers exploded in Australia but not in Europe. Use the concepts of predators, competition and available resources in your answer, and explain what this tells us about how ecosystems normally keep populations in check.
Q1. Define introduced species and native species, giving one Australian example of each.
1 mark for each definition, 1 mark for one correct example.Q2. 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.Q3. 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.Model answers (click to reveal)
Model Answers
+Multiple Choice
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.
Short Answer 1 (3 marks)
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).
Short Answer 2 (4 marks)
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).
Short Answer 3 (5 marks)
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).
Revisit Your Thinking
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?
Model answers (click to reveal)
Model Answers
+Multiple Choice
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.
Short Answer 1 (3 marks)
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).
Short Answer 2 (4 marks)
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).
Short Answer 3 (5 marks)
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).
● Big Idea
Populations change due to food, predators, disease and competition.
● Key Threat
Introduced species such as rabbits, cane toads, foxes and cats cause major harm because they lack natural controls.
● Why It Matters
Understanding population trends helps scientists protect native wildlife and design better control programs.
● Bridge Forward
This lesson completes Block E on ecosystems. You can now explain how matter cycles and how populations interact.