IB ESS SL3.2 Human Impact on BiodiversityPaper 1 & 2Case studies~14 min read
Case Studies in Decline and Recovery
You need three species at your fingertips: one that is gone, one that is nearly gone, and one that was nearly gone and came back. Not the names alone — what pushed it down, what the loss cost, what was tried, and whether it worked.
📚 What you need to know
You must know one extinct, one critically endangered and one improving species in detail.
For each, learn the causes of decline, the impacts of that decline, the strategies used, and whether they succeeded.
The passenger pigeon went from billions of individuals to extinct in 1914 through overhunting and habitat destruction.
The thylacine was lost to hunting, habitat loss and disease from introduced species; the last died in captivity in 1936.
The Sumatran orangutan and black rhinoceros are critically endangered, from habitat loss and poaching respectively.
The southern white rhinoceros and bald eagle both recovered from near extinction through deliberate intervention.
You do not have to use these examples, but you do need the same four pieces of information for whichever you choose.
What a case study answer needs
🧩 Four things to learn for every species
Causes of decline. Name them and sort them into direct and indirect threats.
Impacts of the decline. On the ecosystem, and on people — both are asked about.
Strategies used. Protection, restoration, law enforcement, captive breeding, bans.
Did it work? A number or a clear trend. This is where most answers stop too early.
Note the green line does not reach the red line’s starting height. Recovery usually means “no longer dying out”, not “back to how it was”.
Extinct species
Passenger pigeon (Ectopistes migratorius)
Passenger pigeon
What it was
Once one of the most abundant bird species in North America, numbering in the billions of individuals
Causes of decline
Overhunting for meat on an industrial scale, plus destruction of the forest habitat it depended on
Timeline
In serious decline by the late 1800s; despite some conservation attempts, it went extinct in 1914
Impacts
Loss of a species that shaped its forests through seed dispersal and disturbance; a warning that abundance is no protection
Did anything work?
No. Action came far too late, and by then the flocks were too small to breed successfully
This is the one to remember if you take away nothing else. A species numbering in the billions was wiped out inside a human lifetime. “There are still lots of them” has never been a safety guarantee, and examiners like students who understand that.
Thylacine or Tasmanian tiger (Thylacinus cynocephalus)
A carnivorous marsupial that once lived on the Australian island of Tasmania.
Declined through hunting (it was blamed for killing livestock), habitat loss, and disease transmitted by introduced species.
The last known individual died in captivity in 1936, marking the extinction of the species.
Its loss removed Tasmania’s largest marsupial predator, with knock-on effects on the species it had controlled.
Note the pattern: three threats acting together, exactly the cumulative effect from the first page of this topic.
Critically endangered species
Sumatran orangutan (Pongo abelii)
Sumatran orangutan
Where
One of three orangutan species, found only on the Indonesian island of Sumatra — so it is endemic
Main cause
Habitat destruction and fragmentation from logging, conversion of forest to agriculture, and infrastructure development
Second cause
Illegal hunting and capture for the pet trade
Status
Critically endangered, with around 14 000 individuals remaining in the wild
Why it is hard to fix
Slow breeding, dependence on continuous forest canopy, and economic pressure to clear more land
Black rhinoceros (Diceros bicornis)
A large mammal native to Africa, now critically endangered.
Causes: poaching for its horns, habitat loss, and civil unrest across parts of its range, which makes protection almost impossible.
The population declined by over 90% since the 1960s, leaving roughly 3 000 mature individuals in the wild.
Strategies in use: anti-poaching patrols, habitat restoration and captive breeding programmes.
Poaching is driven by demand elsewhere, so protection at the reserve alone cannot solve it — demand reduction matters too.
Improving species
Southern white rhinoceros (Ceratotherium simum)
Was on the brink of extinction from poaching for horn, with only a handful of individuals surviving in the wild in South Africa in the early 20th century.
Strategies: increased law enforcement, habitat protection and captive breeding programmes.
The population has recovered to over 18 000 individuals today.
It is still threatened by poaching and habitat loss, but its conservation status has greatly improved through human intervention.
Bald eagle (Haliaeetus leucocephalus)
Bald eagle
Causes of decline
Habitat destruction, hunting, and pesticide use — DDT caused eggshell thinning and reproductive failure
Low point
Fewer than 500 pairs in the 1960s
Strategies
Banning DDT, habitat protection, and captive breeding programmes
Result
Recovered to over 10 000 pairs today
Why it worked
The main cause was identified precisely and then removed, rather than just managed around
Why the eagle is the best recovery example. DDT accumulated up the food chain, so a top predator received the highest dose — the trophic level factor from the last page in action. Once the chemical was banned, the pressure genuinely stopped. Compare that with rhino poaching, where the pressure is still there and only being held back by constant effort. One is a cure; the other is life support.
Patterns worth spotting
Pattern
What it tells you
Habitat loss is nearly always involved
It appears in almost every case, usually alongside a direct threat
Threats combine
The thylacine faced hunting, habitat loss and disease; single-cause extinctions are rare
Slow breeders recover slowly
Rhinos and orangutans cannot rebound quickly even when protected
Recoveries need the cause removed
Banning DDT worked; managing around poaching only holds the line
Recovery takes decades
Both improving species needed most of a century of sustained effort
Late action fails
Conservation attempts for the passenger pigeon came when the population was already too small
Worked examples
WE 1
Explain an extinction
With reference to a named extinct species, explain the factors that led to its extinction. (4 marks)
The species
The passenger pigeon (Ectopistes migratorius), once one of the most abundant birds in North America with billions of individuals.
Direct threat
Overhunting for meat removed enormous numbers of birds, a direct threat acting on the population itself.
Indirect threat
Destruction of its forest habitat removed the nesting and feeding sites it depended on.
The outcome
Numbers fell steeply through the late 1800s and, despite late conservation attempts, the species went extinct in 1914.
Abundance did not protect it once both threats acted togethername the species properly and give the date; vague case studies lose easy marks
WE 2
Explain a successful recovery
With reference to a named species, explain how conservation efforts have improved its status. (4 marks)
The species and its problem
The bald eagle declined to fewer than 500 pairs in the 1960s, from habitat destruction, hunting, and DDT causing eggshell thinning and reproductive failure.
Strategy 1
DDT was banned, removing the cause of the reproductive failure rather than managing around it.
Strategy 2
Habitat was protected and captive breeding programmes were run.
The result
The population recovered to over 10 000 pairs, so its conservation status has greatly improved.
Identify the real cause, remove it, then give it decadesquote the before and after figures; they are what turn a description into evidence
WE 3
Compare two case studies
Suggest why conservation has been more successful for the southern white rhinoceros than for the black rhinoceros. (3 marks)
Point 1: the shared threat
Both were driven down mainly by poaching for horn, so the underlying pressure is the same.
Point 2: what differed
Southern white rhino recovery followed sustained law enforcement, habitat protection and captive breeding, allowing numbers to reach over 18 000.
Point 3: why the black rhino lags
Black rhino range includes areas of civil unrest, which makes patrolling and enforcement extremely difficult, and its population had already fallen by over 90% since the 1960s.
Same threat, very different ability to enforce protectionpolitical stability is a legitimate ESS answer — conservation is a social system as much as an ecological one
💡 Exam tips
Learn one species from each group properly rather than six species vaguely.
Always give the scientific name if you know it, and a date or number.
Sort the causes into direct and indirect as you write — it structures the answer.
Include the impacts of the decline, on the ecosystem and on people.
Say whether the strategies succeeded, and give the evidence.
Note that recovery rarely returns a species to its original numbers.
⚠ Common mistakes
Naming a species with no detail. The name alone is worth nothing.
Giving only one cause. Nearly every case involves several threats together.
Leaving out the outcome. “Conservation efforts were made” does not answer whether they worked.
Calling an improving species safe. Both examples here are still threatened.
Confusing the two rhinos. Black is critically endangered; southern white has recovered.
Forgetting people. Declines affect livelihoods, culture and economies, not just ecosystems.
Up next: Shared Resources and the Tragedy of the Commons — a lot of these declines share one underlying shape, and it has a name.
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