Recent Extinction Events
In 2017, Gerardo Ceballos and colleagues published 'Biological Annihilation' in PNAS, calculating that the current extinction rate is 100–1,000 times the geological background rate of 0.1–1 extinctions per million species per year. They found 32% of all vertebrate species are now declining in total numbers. If the trend continues, more than 75% of species could be lost within 240–540 years — the definition of a sixth mass extinction. Australia contributes disproportionately: in 2016, CSIRO declared the Bramble Cay melomys the world's first mammal driven extinct by human-caused climate change, after rising seas destroyed 97% of its habitat on a tiny Torres Strait island.
The dinosaurs went extinct 66 million years ago when an asteroid struck Earth. Today, species are going extinct faster than at almost any time in the fossil record — but there is no asteroid.
Before reading: what do you think is driving the current wave of extinctions, and why might a species be unable to simply adapt and survive when its environment changes?
Know
- The difference between background extinction and mass extinction
- That five mass extinctions are recorded, with a sixth now underway
- The details of a recent extinction event (Bramble Cay melomys)
- The main human-induced selection pressures driving extinction
Understand
- Why the current extinction rate is described as a "sixth mass extinction"
- How rising seas and storm surges caused a specific extinction
- Why extinction occurs when environmental change outpaces adaptation
Can Do
- Explain a recent extinction event with its cause
- Analyse how human activity acts as a selection pressure
- Evaluate the claim that "extinction is natural so it doesn't matter"
Core Content
How fast species disappear — and why now is different
In 2017, Ceballos and colleagues analysed population data for 27,600 vertebrate species and found that the current extinction rate is 100–1,000 times the geological background rate of 0.1–1 per million species per year. Simultaneously, CSIRO confirmed the 2016 extinction of the Bramble Cay melomys — a rodent that lived only on a 4-hectare Torres Strait island, whose habitat was destroyed by rising seas. These two data points connect: the global rate and the local example are both evidence of the same process. Extinction has always occurred at a slow background rate, but what the Ceballos 2017 paper revealed is that the current rate is a spike — potentially the sixth mass extinction in Earth's history.
Extinction is the loss of the last individual of a species. Across geological time, species go extinct at a slow, steady background extinction rate — the normal rate of extinction outside major catastrophes, balanced over time by the formation of new species (speciation).
Punctuating this slow background are mass extinctions: short periods when a large proportion of Earth's species die out. The fossil record shows five mass extinctions — the most famous ending the dinosaurs 66 million years ago (asteroid impact).
Many scientists argue we are now in a sixth mass extinction. Current extinction rates are estimated at tens to hundreds of times the background rate, and — uniquely — the cause is a single species: humans.
The current spike is human-driven and rising far above the background rate — the basis for calling it a sixth mass extinction
The IUCN Red List tracks extinction risk. "Extinct" means no individuals remain anywhere; "Extinct in the Wild" means a species survives only in captivity or cultivation.
Pause — copy the highlighted definitions into your book before the check below.
The normal, slow rate of extinction outside mass-extinction events is called the _____ extinction rate.
The first mammal extinction attributed to climate change
We just saw that today's extinctions are happening unusually fast. That raises a question: what does a modern, human-caused extinction actually look like? This card answers it → a single Australian rodent that vanished as the sea rose around it.
The Bramble Cay melomys lived on one tiny low-lying island; rising seas and storm surges destroyed its habitat, and in 2016 it was declared extinct.
The Bramble Cay melomys (Melomys rubicola) was a small rodent found only on Bramble Cay — a low, vegetated sand island in the Torres Strait, just 3–4 metres above sea level. It was Australia's most isolated mammal.
- The pressure: rising sea levels and increasingly frequent, severe storm surges repeatedly flooded the cay.
- The effect: sea water destroyed the vegetation the melomys depended on for food and shelter — its habitable area shrank dramatically.
- The outcome: none were seen after 2009; surveys in 2014 found nothing; it was officially declared extinct in 2016 (and by the Australian government in 2019).
The Bramble Cay melomys is recognised as the first mammal whose extinction is attributed to human-induced climate change — sea-level rise and storm surges destroyed the habitat of a species confined to a single tiny island.
Add the melomys case (species, cause, year) to your notes before the check below.
What was the primary cause of the Bramble Cay melomys extinction?
Explain a Recent Extinction Event
Pattern — Structured Explanation
In your book, write a structured paragraph (4–6 sentences) explaining the extinction of the Bramble Cay melomys. Your answer must include:
- The species and where it lived (and why its location made it vulnerable).
- The selection pressure that acted on it.
- The link between that pressure and human activity.
- Why the species could not survive or adapt (link to its single, low-lying habitat).
How human activity pushes species past the edge
We just saw one species lost to climate change. That raises a question: is climate change the only way humans drive extinctions? This card answers it → no — there are several major human-induced pressures, often summarised as "HIPPO".
Extinction happens when a selection pressure changes the environment faster than a population can adapt — and human activity supplies several such pressures at once.
A useful way to remember the main human-induced selection pressures is HIPPO:
HIPPO — the major human-induced selection pressures driving species toward extinction
Human-induced selection pressures include habitat destruction, invasive species, pollution, over-harvesting and climate change. Each changes the environment faster than affected species can adapt, so populations shrink and may be driven to extinction.
Critically, these pressures usually act together (cumulatively) and are often faster and more widespread than natural pressures — which is why the modern extinction rate is so high.
Add HIPPO and the cumulative-pressure point to your notes before the check below.
Habitat destruction through land clearing is the single biggest human-induced driver of extinction.
Extinction occurs when environmental change happens faster than a population can adapt.
Human-induced selection pressures only ever act one at a time, never together.
Interpreting Extinction-Rate Data
Pattern — Structured Data Analysis
The table shows estimated extinction rates (extinctions per 10,000 species per 100 years, E/MSY).
| Period / group | Estimated rate (E/MSY) |
|---|---|
| Background (fossil record) | 0.1 – 1 |
| Modern mammals (last 500 yrs) | ~24 |
| Modern birds (last 500 yrs) | ~30 |
| Projected (next century, current trends) | up to ~1000 |
- Roughly how many times higher is the modern mammal rate than the high end of the background rate? Show your calculation.
- Use the data to justify the term "sixth mass extinction".
- The projected future rate is far higher still. Identify two human-induced selection pressures most likely responsible, and explain how each drives extinction.
- Evaluate the limitation of comparing a "projected" rate with measured historical rates.
Extinction rates
- Background rate = normal slow rate; balanced by speciation.
- Five past mass extinctions; a human-driven sixth now underway.
- Current rate ~tens–hundreds × background.
Bramble Cay melomys
- Endemic to one tiny Torres Strait cay (3–4 m above sea level).
- Sea-level rise + storm surges destroyed its vegetation/habitat.
- Declared extinct 2016 — first mammal lost to climate change.
Human-induced pressures (HIPPO)
- Habitat destruction (biggest), Invasive species, Pollution, Population/over-harvesting, climate change.
- Act cumulatively, faster than species can adapt.
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
UnderstandBand 3(3 marks) 1. Distinguish between the background extinction rate and a mass extinction, and state evidence that we may be entering a sixth mass extinction.
1 mark: background rate defined · 1 mark: mass extinction defined · 1 mark: evidence (rate far above background / human-driven)
ApplyBand 4(4 marks) 2. Explain the extinction of the Bramble Cay melomys, identifying the selection pressure responsible and linking it to human activity.
1 mark: species/location · 1 mark: selection pressure (sea-level rise/storm surge) · 1 mark: link to human-induced climate change · 1 mark: why it could not adapt/survive
EvaluateBand 5(5 marks) 3. "Extinction is a natural process, so human-caused extinctions are not a concern." Evaluate this statement, referring to extinction rates and human-induced selection pressures.
Up to 2 marks: acknowledges extinction is natural (background rate) · up to 2 marks: contrasts with elevated, human-driven rate + named pressures · 1 mark: reasoned judgement
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answers
Q1 (3 marks): The background extinction rate is the normal, slow rate at which species go extinct across geological time (outside catastrophes), historically balanced by the formation of new species. A mass extinction is a relatively short period in which a large proportion of Earth's species die out — the fossil record shows five. Evidence that a sixth may be underway is that current extinction rates are estimated at tens to hundreds of times the background rate and are driven by a single species (humans), rather than by a natural catastrophe.
Q2 (4 marks): The Bramble Cay melomys (Melomys rubicola) was a rodent endemic to Bramble Cay, a single low-lying sand island (3–4 m above sea level) in the Torres Strait. The selection pressure was repeated sea-water inundation from rising sea levels and increasingly frequent, severe storm surges, which destroyed the vegetation it relied on for food and shelter and shrank its habitable area. This pressure is human-induced because the sea-level rise and intensified storms are consequences of anthropogenic climate change. The species could not survive or adapt because it was confined to one tiny island with nowhere to migrate, and the environmental change occurred far faster than the population could adapt — so it was declared extinct in 2016, the first mammal extinction attributed to climate change.
Q3 (5 marks): The statement is partly correct: extinction is a natural process — species have always gone extinct at a low background rate, balanced over time by speciation, and five natural mass extinctions are recorded. However, the conclusion that human-caused extinctions are "not a concern" is not supported. Current extinction rates are estimated at tens to hundreds of times the background rate and are driven by human-induced selection pressures — habitat destruction, invasive species, pollution, over-harvesting, and climate change — which often act cumulatively and faster than species can adapt. Because these losses are removing species far faster than ecosystems and evolution can compensate, they threaten biodiversity and the ecosystem services humans depend on. Overall, while extinction itself is natural, the elevated, human-driven rate is a genuine concern, so the statement should be rejected.
Timed questions on extinction events and human-induced selection pressures. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
⚔ Enter the arenaCeballos et al.'s 2017 'Biological Annihilation' study found the extinction rate is now 100–1,000 times the background rate of 0.1–1 per million species per year — and 32% of vertebrate species are declining in total numbers. The Bramble Cay melomys, confirmed extinct by CSIRO in 2016, is a case study in why: its entire population lived on a 4-hectare island. Rising seas — a human-induced climate change effect — destroyed 97% of its habitat before the population could adapt or disperse. Adaptation by natural selection requires heritable variation and time; a population of dozens of individuals on a disappearing island has neither.
The key concept is rate: human-induced selection pressures change environments far faster than populations can adapt, particularly when populations are small, isolated, and have no alternative habitat to retreat to.