Why is Australia so vulnerable to introduced species?

Australia evolved in long isolation, producing many endemic species that grew up with no cats, foxes, rabbits or hoofed grazers. Having never faced such predators and competitors, native species developed no defences against them, so introduced species can spread and cause severe harm.

Why do introduced species often explode in number?

Largely because of "enemy release" — in a new land a species escapes the predators, competitors, parasites and diseases that controlled its numbers at home, so its population grows unchecked. Successful invaders also tend to breed fast and eat widely, and human disturbance (clearing, altered water and fire) creates the conditions they exploit. About two dozen rabbits released in 1859 became hundreds of millions within decades.

What is the difference between biotic and abiotic effects of introduced species?

Biotic effects fall on other living things — predation (foxes and cats eating natives), competition (rabbits, carp) and disease or poisoning (the toxic cane toad). Abiotic effects change the non-living environment — overgrazing causing soil erosion, carp muddying water and reducing light, and altered fire, water and nutrient cycles. One species can cause both at once.

How do introduced species cause native Australian species to decline or go extinct?

Mainly through predation on "naive" native prey that never evolved defences, competition for food and shelter, habitat alteration such as rabbit-driven erosion, and disease or poisoning. Australia has the world’s worst mammal extinction record, with foxes and feral cats the leading cause; declines usually result from several of these pressures acting together, with human disturbance favouring the invaders.

Introduced species

Tens of millions of years of isolation made Australia's wildlife unique — and uniquely defenceless. When humans brought in rabbits, foxes, cats and cane toads, they met native species with no evolved defences, and the results were catastrophic. This powerbook follows why invaders explode, how they harm the living and non-living world, how they drive natives to extinction, and the hard choices they force.

effect types: biotic & abiotic

mammal extinctions worldwide

short sections

Introduced speciesSection 1 of 5

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This book has 5 short sections. Read each one, answer the check, then use Next to move on — the bar above tracks how far you've come.

A continent with no defences

Australia spent tens of millions of years as an island continent, cut off from the rest of the world. In that long isolation its plants and animals evolved alone, producing an extraordinary collection of species found nowhere else — marsupials, monotremes and countless endemic plants. Crucially, they evolved with no cats, no foxes, no rabbits, no hoofed grazers. They never had to defend against such things, so they never developed the means to.

That is what makes Australia so spectacularly vulnerable to introduced species — organisms brought here by humans from somewhere else. Most introductions cause little harm. But some spread out of control and wreak havoc, and these we call invasive species. A defenceless continent and a stream of newcomers with no local enemies is a recipe for disaster, and Australia has suffered some of the worst of it on Earth.

This chapter answers how those introduced species affect the Australian environment: why a newcomer so often explodes in numbers, how it harms both the living and the non-living world, how it drives native species toward extinction, and how we study and respond to it — including the differing views people hold about the value and harm of a given species.

New words

introduced species

A species brought by humans into a region it did not naturally reach.

invasive species

An introduced species that spreads widely and causes serious harm.

endemic/en-DEM-ik/

Found naturally in one place only — as much of Australia’s isolated wildlife is.

🔒 Lock it in

Why is Australian wildlife unusually vulnerable to introduced predators like foxes and cats?

Answer the check to continue

Quick reference & sources

The interactive book above is the lesson. What follows is a plain recap you can revise from, plus the sources and how this maps to the syllabus.

Why invaders succeed here

Australia’s long isolation left native species endemic and defenceless against introduced predators and competitors. Newcomers explode through enemy release — escaping the predators, competitors and diseases that controlled them at home (24 rabbits → hundreds of millions). Human activity upsets the balance and favours them by clearing and disturbing land, altering water and fire, and transporting them.

Biotic and abiotic effects

Biotic (on living things): predation (foxes, cats), competition (rabbits, carp), disease and poisoning (cane toad). Abiotic (on the environment): overgrazing → erosion, carp muddying water and cutting light, altered fire, water and nutrient cycles. The same species can do both — a rabbit competes (biotic) and bares the soil to erosion (abiotic).

Decline, control and differing views

Introduced species drive native extinction through predation on naive prey, competition, habitat change and poisoning — Australia leads the world in mammal extinctions, mostly from foxes and cats. Control includes biological control (myxoma virus, calicivirus for rabbits), baiting, fencing and predator-free reserves, at a cost of billions a year. Views on a species’ value or harm differ across farmers, hunters and communities, including the diverse perspectives of Aboriginal and Torres Strait Islander peoples.

For HSC students

This powerbook covers the Effects of Introduced Species thread of Module 4 (Human Impacts), framed by the inquiry question how do introduced species affect the Australian environment and ecosystems? Working through it, you should be able to:

outline the biotic and abiotic effects of introduced species;
analyse how human activity can upset the balance of ecosystems and favour introduced species;
describe how introduced species contribute to the decline or extinction of native Australian species;
frame an investigation of a local introduced species — reason introduced, effects, area affected, human impacts that favour it, control or mitigation, economic impact, and differing views (including those of Aboriginal and Torres Strait Islander peoples).

This chapter completes Module 4 and the Year 11 course. Explanations here are original and are a study aid, not a copy of the syllabus, and references to Aboriginal and Torres Strait Islander peoples reflect that views are diverse rather than uniform.

Sources

Australian Government Department of Climate Change, Energy, the Environment and Water — invasive species, pest animals and weeds.
CSIRO — biological control of rabbits (myxomatosis, calicivirus), the cane toad, and invasion ecology.
Published research on Australia’s mammal extinctions and the roles of foxes and feral cats.
Indigenous ranger and land-management programs — feral animal and weed control combining traditional knowledge and modern methods.
NSW NESA Earth and Environmental Science Stage 6 Syllabus (2017) — used only to scope the Module 4 content; explanations above are original.

Keep going

A continent that evolved alone had no locks on its doors. Many of its worst invaders we let in on purpose — and learning why they took hold is the first step to holding the line.