Since European settlement, Australia has lost 34 mammal species — the highest mammal extinction rate of any country in the world. Each loss was not just a name struck from a list. The lesser bilby, gone. The toolache wallaby, gone. The desert rat-kangaroo, gone. These species were pollinators, seed dispersers, soil engineers, and prey for larger animals. Their extinction has weakened the ecosystems they left behind. This lesson asks why biodiversity matters and how we measure what we are losing.
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. A rainforest has 200 tree species, each represented by roughly the same number of individuals. A plantation has 10 tree species, with one species making up 90% of all individuals. Predict which forest would recover faster after a cyclone, and explain your reasoning using the concept of redundancy.
Q2. Australia has lost 34 mammal species since 1788. Predict three ecological consequences of these extinctions, other than the simple absence of those animals. Consider their roles in food webs and ecosystems.
Biodiversity is not a single number. It operates at three nested scales — from the variation within a single population to the variety of ecosystems across a landscape. Understanding all three is essential for conservation.
Definition: The variation in allele frequencies within a population or species.
Why it matters: High genetic diversity means a population has more raw material for natural selection to act upon. When the environment changes — new disease, climate shift, habitat loss — genetically diverse populations are more likely to contain individuals with traits that allow survival.
Low genetic diversity: Tasmanian devils have low genetic diversity due to a historical population bottleneck. When devil facial tumour disease emerged, almost all individuals were susceptible because their immune systems recognised the tumour as “self.” A more genetically diverse population might have contained resistant individuals.
Definition: The number and relative abundance of species in a community.
Two components:
Simpson’s Diversity Index: HSC Biology requires conceptual understanding, not calculation. A higher Simpson’s D reflects higher diversity with greater evenness — communities where many species are present and no single species dominates score highest.
Definition: The variety of habitat types, communities, and ecological processes within a region.
Why it matters: A landscape with wetland, forest, and grassland has higher ecosystem diversity than a monoculture plantation. Each ecosystem provides different services and supports different species. Losing an entire ecosystem type is more devastating than losing a few species from a remaining ecosystem.
Australian example: The Murray-Darling Basin once contained floodplain forests, wetlands, rivers, and dryland woodlands. Clearing and water extraction have reduced wetland area by over 50%, collapsing the ecosystem diversity of the region and eliminating habitat for waterbirds, frogs, and native fish.
Biodiversity is not merely an ethical concern. It is the foundation of ecosystem function, human wellbeing, and planetary stability. The arguments for conserving biodiversity fall into three categories.
Stability hypothesis
Higher biodiversity increases ecosystem resilience — the ability to resist disruption and recover after disturbance. Diverse ecosystems have more pathways for energy flow and nutrient cycling.
Functional redundancy
When multiple species perform the same ecological function, the loss of one is buffered by others. If one pollinator disappears, another may fill the gap. Low-redundancy ecosystems collapse when any key species is lost.
| Category | Examples |
|---|---|
| Provisioning | Food, freshwater, timber, fibre, medicine |
| Regulating | Carbon storage, water purification, pollination, pest control, flood mitigation |
| Cultural | Recreation, aesthetics, Indigenous heritage, scientific research |
Some argue that species have a right to exist independent of their usefulness to humans. The extinction of the thylacine in 1936 was not just a loss of a predator — it was the erasure of 4 million years of unique evolutionary history. This ethical argument underpins much of modern conservation policy, including the IUCN Red List.
The Earth is experiencing its sixth mass extinction event. The current extinction rate is approximately 1,000 times the background rate. Australia, despite its relatively small human population, has been disproportionately affected.
The numbers:
Many of the lost mammals were ecosystem engineers. Bilbies and bettongs dug burrows that aerated soil and allowed water infiltration. Their extinction has contributed to soil compaction and reduced water retention in arid zones. Bandicoots dispersed fungi that supported plant health. Their loss weakened the mycorrhizal networks that underpin Australian forests.
Use data to compare biodiversity between ecosystems and evaluate the consequences of species loss.
Two sites were surveyed for bird species:
| Site | Species | Individuals |
|---|---|---|
| A (Regenerating forest) | Eastern yellow robin | 12 |
| Superb fairy-wren | 15 | |
| White-throated treecreeper | 8 | |
| Crimson rosella | 5 | |
| B (Pine plantation) | Common starling | 45 |
| Australian magpie | 8 | |
| Galah | 7 |
Australia has lost 34 mammal species since European settlement, including the toolache wallaby, lesser bilby, and desert rat-kangaroo.
Three levels
Genetic diversity: variation in alleles within a population. Species diversity: number and relative abundance of species (richness + evenness). Ecosystem diversity: variety of habitats and ecological processes in a region.
Measurement
Species richness = total number of species. Species evenness = how evenly individuals are distributed across species. Simpson’s Diversity Index reflects both (higher D = more diverse and more even).
Why it matters
Stability: higher biodiversity = greater resilience and redundancy. Ecosystem services: provisioning, regulating, and cultural benefits. Intrinsic value: ethical right of species to exist.
Australia’s record
34 mammal species extinct since European settlement — highest mammal extinction rate globally. Over 1,800 species currently threatened. Key drivers: habitat destruction, introduced predators, altered fire regimes, climate change.
IUCN Red List
Categories: Least Concern → Near Threatened → Vulnerable → Endangered → Critically Endangered → Extinct in the Wild → Extinct.
Syllabus link
ACSBL053, ACSBL054, ACSBL060, ACSBL061: Define and measure biodiversity; explain why biodiversity matters; model effects of biodiversity loss on ecosystem health.
Now that you have completed the lesson, review your initial answers. What did you get right? What surprised you?
Q1. A rainforest has 200 tree species, each represented by roughly the same number of individuals. A plantation has 10 tree species, with one species making up 90% of all individuals. Predict which forest would recover faster after a cyclone, and explain your reasoning using the concept of redundancy.
Q2. Australia has lost 34 mammal species since 1788. Predict three ecological consequences of these extinctions, other than the simple absence of those animals. Consider their roles in food webs and ecosystems.
In this lesson you learned: