Biodiversity — Measurement, Importance and Ecosystem Stability
The IUCN Red List 2022 update recorded that since European settlement, Australia has lost 34 mammal species — the highest mammal extinction rate of any country on Earth. The quoll has lost over 90% of its pre-European range. The bettong and bilby have similarly retreated to isolated refugia. These species were not ecologically neutral: they were soil engineers, fungal spore dispersers, and prey base for surviving predators. Their extinction has measurably weakened the ecosystems they left behind, demonstrating why biodiversity — genetic, species, and ecosystem — matters beyond aesthetics.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits 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.
Core Content
Biodiversity operates at three nested scales — from alleles in a population to habitat types across a landscape
The 2022 IUCN Red List documented 34 mammal extinctions in Australia since European settlement — the highest rate of any country on Earth. The quoll, bettong, and bilby have each lost over 90% of their pre-European range. But what does that mean for the ecosystem? Bettongs were soil engineers — their digging aerated soil and dispersed fungal spores. Their loss reduces soil water infiltration, weakens mycorrhizal networks, and removes a prey base for quolls and wedge-tailed eagles. Each extinction removes a functional role. Biodiversity is not a single number: it operates at three nested scales, and losing any one level degrades all the others.
Definition: 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. When the environment changes — new disease, climate shift, habitat loss — genetically diverse populations are more likely to contain individuals with traits that allow survival.
Australian example — Tasmanian devils: 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: Species richness (total number of species) + species evenness (how evenly individuals are distributed). Both contribute to a high Simpson's Diversity Index value.
Example: A forest with 10 species, each representing 10% of individuals, has higher evenness — and higher diversity — than one where a single species makes up 90%.
Definition: The variety of habitat types, communities, and ecological processes within a region.
Australian example — 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 operates at three nested scales: Genetic (allele variation within a species), Species (richness + evenness in a community), and Ecosystem (variety of habitat types and ecological processes across a landscape).
Pause — copy the highlighted three levels into your book before continuing.
Which level of biodiversity refers to variation in allele frequencies within a single species?
We defined three levels of biodiversity. That sets up the key question: why does it actually matter if biodiversity declines? This card builds the three-pillar argument — ecosystem stability, ecosystem services, and intrinsic value.
Ecosystem stability, services, and intrinsic value are the three pillars of biodiversity's importance
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.
Biodiversity matters for three reasons: (1) Ecosystem stability — higher diversity increases resilience through functional redundancy; (2) Ecosystem services — provisioning (food/water), regulating (carbon, pollination), cultural (recreation/heritage); (3) Intrinsic value — species have a right to exist independent of human utility.
Pause — copy the highlighted three pillars into your book.
A wetland filters agricultural runoff, removing excess nutrients before they reach a river. This is an example of which category of ecosystem service?
We've built the case for why biodiversity matters. Now the hard evidence: Australia's extinction record shows exactly what happens when biodiversity is lost — and the ecosystem consequences continue to ripple outward decades later.
Australia has the highest mammal extinction rate of any country — and ecosystem consequences continue to ripple outward
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.
- 34 mammal species extinct since European settlement — highest mammal extinction rate of any country
- Over 100 plant species extinct or possibly extinct
- More than 1,800 species currently listed as threatened under the EPBC Act
- Key drivers: Habitat destruction (land clearing), introduced predators (cats, foxes), altered fire regimes, invasive species competition, and climate change
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.
- Reduced redundancy: Fewer species performing key functions means no backup when one is lost.
- Reduced resilience: Simplified ecosystems recover more slowly from disturbance.
- Trophic cascade risk: Loss of apex predators or keystone species triggers cascading collapses.
- Reduced genetic diversity: Small, isolated populations suffer inbreeding depression.
- Loss of ecosystem services: Declining pollinators reduce crop yields; loss of wetland vegetation reduces water purification.
Australia has lost 34 mammal species since European settlement — the highest rate globally. Key drivers: introduced predators (cats, foxes), habitat destruction, altered fire regimes. Effects include reduced functional redundancy, resilience, and ecosystem services. IUCN Red List: LC → NT → VU → EN → CR → EW → EX.
Pause — copy the highlighted Australia extinction summary into your book.
Community A has 10 species with equal numbers of individuals. Community B has 10 species where one species makes up 90% of individuals. Which community has higher species diversity?
Two sites were surveyed for bird species. Use the data below to answer the questions.
| 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 |
- Calculate the species richness for each site. (1 mark)
- Which site has higher species evenness? Explain your reasoning without calculating. (2 marks)
- Predict which site would be more resilient to an outbreak of avian disease. Justify using the concept of redundancy. (3 marks)
- A developer proposes draining Site A for housing. Evaluate this proposal using functional redundancy and one other ecological principle. (2 marks)
Australia has the highest mammal extinction rate in the world. Since European colonisation, at least 34 mammal species have gone extinct, including the Tasmanian tiger, pig-footed bandicoot, and lesser bilby.
- Identify the three main drivers of mammal extinction in Australia and rank them by impact. (3 marks)
- Explain how the loss of a single digging mammal species reduces ecosystem function through its role in soil engineering. (2 marks)
- A politician claims that extinction is a natural process and we should not intervene. Analyse this claim by comparing background extinction rates with the current rate in Australia. (3 marks)
Tasmanian devils suffered catastrophic population declines from facial tumour disease primarily because:
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
ApplyBand 4(4 marks) 1. Distinguish between species richness and species evenness. In your answer, define each term and explain how a community could have high richness but low evenness.
AnalyseBand 4(4 marks) 2. Explain two reasons why high biodiversity increases ecosystem resilience. For each reason, describe what would happen to an ecosystem with low biodiversity when faced with the same disturbance.
EvaluateBand 5–6(6 marks) 3. Australia has the highest mammal extinction rate of any country since European settlement, losing species such as the lesser bilby, toolache wallaby, and desert rat-kangaroo. (a) Explain the ecological consequences of losing digging mammals such as the lesser bilby. Refer to soil structure and ecosystem services. (b) Connect Australia's mammal extinction record to two concepts from this lesson (genetic diversity, functional redundancy, or ecosystem services).
Show all answers
Activity 1 — Diversity Indices
1. Site A richness = 2 species; Site B richness = 8 species.
2. Site B has higher evenness — all 8 species have exactly 5 individuals (12.5% each). Site A has very low evenness — one species (black swans) makes up 75% of all individuals.
3. Site B would be more resilient. With 8 species, functional redundancy is higher — if one bird species is killed by disease, 7 others remain to continue ecological functions (seed dispersal, nutrient cycling). Site A with only 2 species has minimal redundancy; disease affecting the dominant black swan (75%) would devastate ecosystem function.
4. Draining Site B would eliminate 8 species and all associated ecosystem services including water filtration, habitat for amphibians, and carbon storage. The functional redundancy of 8 species would be lost — a much greater ecological cost than draining a 2-species system. Ethically, each of the 8 species also has intrinsic value independent of human utility.
Activity 2 — Australia's Extinction Record
1. 1st: Introduced predators (cats, foxes) — directly killed billions of small mammals with no evolutionary defence. 2nd: Habitat destruction (land clearing) — eliminated breeding grounds. 3rd: Altered fire regimes — disrupted mosaic habitats that many species required.
2. Digging mammals like the lesser bilby created burrows that turned over and aerated soil, increasing water infiltration and bringing nutrient-rich subsoil to the surface. Their extinction has caused soil compaction, reducing water penetration and increasing runoff — degrading vegetation and reducing carrying capacity for remaining herbivores.
3. The politician is incorrect. The background extinction rate is approximately 1 species per million species per year. The current extinction rate in Australia is estimated at 1,000× the background rate. This acceleration is driven by human activity — habitat destruction, introduced species — not natural evolutionary processes. Natural extinction is gradual; the current Australian rate is catastrophic and unprecedented outside of mass extinction events.
Short Answer Model Answers
Q1 (4 marks): Species richness = total number of different species present in a community (1 mark). Species evenness = how evenly individuals are distributed across species (1 mark). A community can have high richness but low evenness when many species are present but one or a few dominate numerically (1 mark). For example, a forest with 20 tree species where 95% of individuals belong to one pine species has high richness (20 species) but very low evenness (1 mark).
Q2 (4 marks): Reason 1 — Functional redundancy: multiple species perform the same function; if one pollinator is lost, another fills the gap (1 mark). In a low-biodiversity system, loss of the only pollinator causes plant reproductive failure and cascading collapse (1 mark). Reason 2 — Diverse disturbance response: high-biodiversity communities contain species with varied tolerances; some survive drought, some flood, some fire. At least some species maintain function after disturbance (1 mark). Low-biodiversity systems share similar vulnerabilities and may collapse entirely when their shared tolerance is exceeded (1 mark).
Q3 (6 marks): (a) Digging mammals were ecosystem engineers — their burrowing aerated soil, increased water infiltration, and brought nutrients to the surface (1 mark). Without them, soil has compacted, reducing water penetration and increasing runoff. This has degraded vegetation and the regulating service of water cycling has been impaired (1 mark). No native species has fully replaced this soil-engineering function (1 mark). (b) Functional redundancy: the collective loss of 34 mammal species eliminated redundancy in functions like digging, seed dispersal, and fungal spore transport. With no backup species, the ecosystem is fragile under further stress (1.5 marks). Ecosystem services: extinct mammals provided regulating services (soil aeration, nutrient cycling) and supporting services (mycorrhizal spore dispersal). Their loss has degraded pasture quality and plant reproduction across arid Australia (1.5 marks).
Five timed questions integrating biodiversity levels, ecosystem services, and Australia's extinction record. Beat the boss to bank a tier.
Enter the arenaThe 2022 IUCN Red List confirmed Australia's 34 mammal extinctions since European settlement — a record no other country matches. The quoll, bettong, and bilby have each lost over 90% of their pre-European range. Each loss removed a functional role: burrowing mammals aerated soil and dispersed fungal spores; small predators suppressed rodent populations; prey species supported predator food webs. These are measurable biodiversity losses at three levels: genetic (reduced allele diversity in remaining populations), species (34 species gone), and ecosystem (functional roles vacant).
Return to your Think First response. Write one sentence explaining why a high Simpson's Diversity Index value indicates a more resilient ecosystem than a low value — and apply it to the rainforest vs plantation comparison.