IB Biology SLProtecting BiodiversityPaper 1 & 2Case studies~10 min read
Causes of Extinction
Extinction is not new. Species have been disappearing for as long as there have been species. What is new is the cause: for the first time, one species is driving the losses. That is what “anthropogenic” means, and it is the word the whole page hangs on.
📚 What you need to know
Anthropogenic extinction means extinction caused by human activity.
There have been five mass extinction events in Earth’s history. Many scientists think we are causing a sixth.
You must be able to discuss three case studies: the North Island giant moa, the Caribbean monk seal, and one of your own choice.
The North Island giant moa was a large flightless bird in New Zealand, hunted out by roughly 1300, within about a century of humans arriving.
The Caribbean monk seal was hunted for its oil and meat and declared extinct in 2008, though it had probably gone long before.
Your third case study should be local or familiar, and the species must have been lost through human activity.
You do not need Latin binomial names in the exam.
Background extinction and mass extinction
Species go extinct all the time, at a low steady rate, as environments shift and competitors appear. That is background extinction and it is completely normal.
A mass extinction is different: a large proportion of all species on Earth disappearing in a geologically short window. The fossil record contains five of them.
The dates are approximate and the causes varied — asteroid impact, huge volcanic episodes, rapid climate shifts. What links them is scale: a big share of all species lost in a short time.
Careful with the wording: we say we may be in a sixth mass extinction. It is a well-supported scientific view, not a settled fact, and IB rewards that kind of precision.
The pattern behind most anthropogenic extinctions
Read the two case studies below and you will notice they are the same story twice. That pattern is worth learning, because it lets you write a sensible answer about almost any species.
Step 3 is the one students skip. A species that lays one egg a year cannot absorb the same losses as one that produces hundreds of young.
Case study 1: the North Island giant moa
🧩 What to say about it
What it was. A large, flightless, plant-eating bird found only in New Zealand.
Where it lived. New Zealand only, so it had nowhere else to retreat to.
What changed. Humans arrived roughly between 1200 and 1300.
What happened. The moas were hunted for meat. Being large, flightless and unafraid of people made them easy targets.
The outcome. They are thought to have been hunted to extinction by about 1300 — within roughly a century of first contact.
Notice the timescale. Around one hundred years is nothing in evolutionary terms — nowhere near long enough for a large slow-breeding bird to evolve any defence against a brand new predator.
Case study 2: the Caribbean monk seal
🧩 What to say about it
What it was. A seal living in the waters of the Caribbean and the Gulf of Mexico.
What made it vulnerable. It was docile and habitually hauled out onto rocks to rest, so it was easy to approach and kill.
What humans wanted. European colonists hunted it for its oil and meat.
The outcome. Numbers fell until none were left. It was formally declared extinct in 2008, although it is thought to have actually disappeared well before that date.
Why the two dates differ: a species is only declared extinct after a long search finds nothing. So the declaration date is always later than the real disappearance — a useful point if a question asks why extinction dates are uncertain.
Case study 3: choosing your own
You are expected to research a third example yourself. Three rules for picking a good one:
It should be from a local or familiar region, so you can say something specific rather than generic.
The extinction must be due to human activity, not a natural event.
You should be able to give the same level of detail as the two case studies above: what it was, where it lived, what humans did, and the outcome.
Widely used examples include the baiji river dolphin, the passenger pigeon, the paradise parrot, the golden toad and the St Helena darter dragonfly. Pick one, look up the specifics, and write yourself four bullet points.
✓ A GOOD CHOICE
Clearly caused by humans
You can name the activity: hunting, draining, clearing, pollution
You know roughly when and where
Local enough that you can add detail
✗ A WEAK CHOICE
Dinosaurs — not anthropogenic
A species that is endangered but not extinct
One you cannot name a cause for
Anything you only half remember
Worked examples
WORKED EXAMPLE
Outline the causes of the extinction of the North Island giant moa. [3]
Point 1: name the human activityHumans arriving in New Zealand around 1200 to 1300 hunted moas for meat.Point 2: say why they were easy to killThey were large and flightless, and had no evolved response to a new predator, so they could not escape.Point 3: link to the populationHunting removed individuals faster than the population could replace them, and they were gone by about 1300“Anthropogenic extinction” is worth writing explicitly — it shows you know the term.
WORKED EXAMPLE
Suggest two features that made both the giant moa and the Caribbean monk seal especially vulnerable to hunting. [2]
Feature 1: easy to approachNeither had evolved alongside human hunters, so neither fled. The moa could not fly and the seal rested openly on rocks.Feature 2: limited rangeBoth were restricted to one region, so there was no untouched population elsewhere to replace lossesSlow reproduction would also score here.
WORKED EXAMPLE
Explain why some scientists describe the present day as a sixth mass extinction. [3]
Point 1: define the comparisonA mass extinction is the loss of a large proportion of Earth’s species in a short period. Five are recorded in the fossil record.Point 2: the current rateExtinctions are now happening far faster than the background rate, across many groups at once.Point 3: the causeThe losses are largely anthropogenic — hunting, habitat destruction, pollution and climate changeSay “many scientists believe”, not “it is a fact”.
💡 Exam tip
Use the word anthropogenic and define it once. It is a term the syllabus names explicitly.
For any case study, give four things: what the species was, where it lived, the human activity, and the outcome with a date.
Always explain why the species could not cope, not just that it was hunted. That is where the higher marks sit.
Have your third case study written out and memorised before the exam. It will not come to you on the day.
Latin names are not required. Do not waste time or risk a misspelling.
Keep “five mass extinctions, possibly a sixth” as a ready fact.
⚠ Common mix-up
Confusing extinct with endangered. Extinct means no individuals survive anywhere.
Using a natural extinction as an anthropogenic case study. The dinosaurs do not count.
Saying “humans killed them” and stopping. Explain the biology: slow breeding, small range, no fear response.
Mixing up the date declared extinct with the date it actually disappeared. The monk seal shows why they differ.
Claiming the sixth mass extinction as established fact. Write “many scientists believe” or “evidence suggests”.
Thinking hunting is the only anthropogenic cause. Habitat loss, pollution, invasive species and disease all count.
Up next: Ecosystem Loss — losing a species is bad, but losing an entire habitat takes everything living in it at the same time.
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