Cane toads were introduced in 1935 to control cane beetles, but the beetles were largely unaffected while the toads spread across northern Australia and harmed native predators. This lesson uses that case study to examine how biodiversity is threatened by invasive species, habitat fragmentation, overexploitation, pollution, climate change and the loss of genetic diversity itself.
IQ4~50 minLesson 16 of 185 MC + 3 short answer
⚠️
Choose how you work — type your answers below or write in your book.
Use digital mode if you want to compare threat categories and work through the cane-toad and genetic-erosion examples on-screen. Switch to book mode if you want to organise the threats into a cause-effect table first, then return here to compare your summary.
Printable worksheet
Download this lesson's worksheet
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Lock in your first explanation before the categories are formalised.
1. Why can an introduced species damage biodiversity even if humans originally brought it in for a useful purpose?
2. If a population becomes very small but does not go extinct immediately, why can it still be at growing long-term risk?
Write your starting answer now. We will revisit it after the cane-toad and genetic-erosion sections.
Write your initial answer in your book, then return later to compare it with your final explanation.
Write this in your book, then revisit it later.
Saved locally
📚 Know
Key facts and definitions for Threats to Biodiversity
Relevant terminology and conventions
🔗 Understand
The concepts and principles underlying Threats to Biodiversity
How to explain the reasoning behind key ideas
✅ Can Do
Apply concepts from Threats to Biodiversity to exam-style questions
Justify answers using appropriate biological reasoning
Key Terms
examine how biodiversitythreatened by invasive species, habitat fragmentation, overexploitation, pollution, climate change and the loss of genet
What genetic erosionand why it matters
Bacterialiving cells; viruses are non-living particles that require host cells to reproduce
Understanding how systems interactessential for HSC success
Cane toadstoxic, and many native predators such as quolls, goannas and freshwater crocodiles are vulnerable when they attempt to e
The resultnot just the survival of the toad population itself, but disruption across food webs and population declines in native s
Know
The major categories of biodiversity threat.
The cane toad and feral cat case studies.
What genetic erosion means and why it matters.
Understand
How a single introduced species can disrupt both biotic and abiotic factors.
Why fragmentation links directly to reduced gene flow and smaller vulnerable populations.
Why biodiversity threats often interact rather than act alone.
Can Do
Explain the impact of an introduced species using evidence.
Compare the main threat categories clearly.
Describe how low genetic diversity makes populations less resilient.
Key Terms — scan these before reading
Definition relevant to Threats to Biodiversity.
Definition relevant to Threats to Biodiversity.
Definition relevant to Threats to Biodiversity.
Definition relevant to Threats to Biodiversity.
Definition relevant to Threats to Biodiversity.
Definition relevant to Threats to Biodiversity.
Misconceptions to Fix
✗
Wrong: Bacteria and viruses are the same thing.
✗
Right: Bacteria are living cells; viruses are non-living particles that require host cells to reproduce.
Core Content
Key Point
Connect this concept to the broader biology framework. Understanding how systems interact is essential for HSC success.
01
Introduced Species and the Cane Toad Case Study
Why a biological “solution” can become a biodiversity disaster
Major threats to biodiversity
Introduced species can alter ecosystems through predation, competition, disease transmission, hybridisation and indirect changes to ecological relationships.
Cane toads were introduced to Australia in 1935 to control cane beetles, but the plan failed because the beetles were not effectively controlled while the toads spread through tropical Queensland and the Northern Territory. Cane toads are toxic, and many native predators such as quolls, goannas and freshwater crocodiles are vulnerable when they attempt to eat them. The result is not just the survival of the toad population itself, but disruption across food webs and population declines in native species.
Predation RiskNative predators are poisoned when they attack cane toads.
Food-Web EffectsLosing predators changes the balance of the wider ecosystem.
Human CauseThe species was introduced intentionally, but the ecological outcome was harmful.
Image placeholder: this lesson is designed to support a future cane-toad spread map or photo asset. For now, treat this as the visual anchor for the 1935 introduction and northward spread case study.
Australian context: feral cats are another major introduced-species threat and are estimated to kill about 1.4 billion native animals each year in Australia. They are also considered a primary driver of the extinction of 25 Australian mammal species.
02
Major Categories of Biodiversity Threat
Different pathways, same result: lower resilience and higher extinction risk
Biodiversity declines for many reasons, but the major threats can be organised into a small set of recurring categories.
Threat
What It Does
Examples
Habitat loss and fragmentation
Reduces available space, isolates populations and lowers gene flow
Land clearing for agriculture, mining, urban expansion
Introduced / invasive species
Adds new predators, competitors, diseases or hybridisation pressures
Changes chemical conditions and harms organisms directly or indirectly
Eutrophication, pesticides, heavy metals, plastics
Climate change
Shifts habitats, temperatures and ecological conditions faster than some species can track
Bleaching on the Great Barrier Reef, range shifts, increased fire frequency
Fragmentation is especially important because it links directly to the evolutionary ideas from earlier lessons. When habitat patches become isolated, populations become smaller and more cut off from one another. That reduces gene flow, increases the importance of drift, and raises the risk that local populations cannot recover from disturbance.
Common misconception: biodiversity threats do not act one at a time in neat isolation. Habitat loss can amplify invasive-species effects, climate change can intensify fire or disease, and small populations can be hit hardest by every other threat.
03
Genetic Erosion and Long-Term Vulnerability
Why a small population can still be in danger even if it is not extinct yet
As population size decreases, genetic diversity usually decreases as well. This process is called genetic erosion.
Small populations are more likely to lose rare alleles by chance and more likely to breed with close relatives. That can produce inbreeding depression and lower the population's ability to adapt to future environmental change. A population might appear to persist for a time, but its evolutionary resilience is shrinking.
Smaller population -> less genetic diversity -> fewer rare alleles + more inbreeding -> lower adaptive potential
The Tasmanian devil case study helps show why this matters. Low genetic diversity is one reason devil facial tumour disease has been such a serious threat, because genetically similar populations may be less able to resist or contain emerging disease pressures.
Loss of Rare AllelesChance events remove variation more easily in small populations.
Inbreeding RiskClose relatives are more likely to breed when populations are small and isolated.
Lower AdaptabilityFuture environmental change becomes harder to survive genetically.
Big idea: a threat to biodiversity is not only a threat to the number of organisms alive right now. It is also a threat to the future evolutionary potential of that population.
Introduced Species
Can harm biodiversity through predation, competition, disease and food-web disruption.
Cane toads are a major Australian case study.
Major Threats
Habitat loss, invasive species, overexploitation, pollution and climate change.
These threats often interact and amplify one another.
Fragmentation Link
Fragmentation reduces gene flow and can isolate populations.
That increases drift and long-term extinction risk.
Genetic Erosion
Small populations lose genetic diversity.
That reduces their ability to adapt and can increase inbreeding depression.
Activities
ApplyBand 3-4
Activity 01
Assess the Cane Toad Introduction
Pattern B - Assess and connect
Explain why the cane toad introduction should be assessed as ecologically harmful rather than neutral. Include at least two different ecological effects in your answer.
A strong answer should go beyond “it spread a lot” and explain what changed in the ecosystem.
Draft your assessment in your book first, then record the final answer here.
Write the assessment in your book, then condense it here.
EvaluateBand 4-5
Activity 02
Why Small Populations Stay At Risk
Pattern B - Explain and evaluate
A conservation student argues that once a species stops declining in number, the biodiversity threat has basically ended. Explain why genetic erosion shows that this conclusion can be too optimistic.
A strong answer should mention rare alleles, inbreeding and reduced adaptive capacity.
Write the longer explanation in your book, then summarise it here.
Write the longer explanation in your book, then condense it here.
Revisit Your Thinking
The strongest way to think about biodiversity threats is as pressures on both present ecosystems and future evolutionary potential. A species can be alive now while still becoming more vulnerable every generation.
If your first answer treated biodiversity loss as just “fewer animals”, the upgrade is to include ecosystem interactions, gene flow, and genetic diversity.
Assessment
MC
Check Your Understanding
Answer first, then read the explanation
1. Which is the best description of habitat fragmentation?
2. Why are cane toads considered a major biodiversity threat in Australia?
3. Which threat category best matches nutrient runoff causing algal blooms and oxygen depletion?
4. What is genetic erosion?
What is NOT genetic erosion?
5. Why does genetic erosion increase long-term biodiversity risk?
Short Answer - 10 marks
1. Explain how an introduced species can affect both biotic and abiotic factors in an ecosystem. (4 marks)
SA1: An introduced species can affect biotic factors by preying on native species, competing with them for resources, spreading disease, or disrupting food-web structure. It can also affect abiotic conditions indirectly by changing vegetation cover, soil chemistry or fire regimes through its ecological effects. The key point is that the introduced species alters both living relationships and the broader environmental conditions that shape the ecosystem.
SA2: Habitat fragmentation is the splitting of a continuous habitat into smaller isolated patches, often reducing movement and gene flow between populations. Genetic erosion is the loss of genetic diversity as a population becomes small and loses alleles over time. Fragmentation can lead to genetic erosion because isolated populations become smaller and more cut off from one another.
SA3: Low genetic diversity makes a species more vulnerable to disease because individuals are genetically similar, so fewer may carry resistant variants. That means a pathogen can spread more effectively through the population. Low diversity is also linked to inbreeding and lower adaptive potential, making it harder for the species to respond to future disease pressures.
AR
Rapid Recall
Say each answer aloud before moving to the next prompt
What makes an introduced species a biodiversity threat?
What are the major categories of biodiversity threat?
Why does fragmentation reduce resilience?
What is genetic erosion?
Why can low genetic diversity increase disease risk?
Why do biodiversity threats often interact with one another?
