IB ESS SL & HL3.2 Human Impact on BiodiversityPaper 1 & 2~13 min read
Case Studies in Decline and Recovery
The passenger pigeon numbered in the billions and was gone by 1914. The bald eagle fell below 500 pairs and now exceeds 10 000. Same century, same country, opposite outcomes — and the difference is what these case studies are for.
📘 What you need to know
You need at least one detailed example each of an extinct, a critically endangered and an improving species.
For each: the factors causing decline, the impacts on ecosystems and societies, and the conservation strategies used and whether they worked.
Critically endangered: Sumatran orangutan (~14 000 left), black rhinoceros (~3 000 mature individuals, over 90% decline since the 1960s).
Improving: southern white rhinoceros (now over 18 000), bald eagle (under 500 pairs in the 1960s to over 10 000 pairs).
You do not have to use these particular examples.
Extinct species
Passenger pigeon (Ectopistes migratorius)
Tasmanian tiger (Thylacinus cynocephalus)
What it was
Once one of the most abundant bird species in North America, numbering in the billions
A carnivorous marsupial, also called the thylacine or Tasmanian wolf, native to the Australian island of Tasmania
Causes of decline
Overhunting for meat, and destruction of forest habitat
Hunting, habitat loss, and disease transmission by introduced species
Outcome
In serious decline by the late 1800s. Despite some conservation attempts, it went extinct in 1914
The last known individual died in captivity in 1936
The passenger pigeon is the point of this page. A species numbering in the billions was driven to extinction inside a single human lifetime. Abundance is not safety — a species can be extremely numerous and still collapse if the rate of removal exceeds its rate of reproduction, which is the overharvesting mechanism from the previous note.
Critically endangered species
Sumatran orangutan (Pongo abelii)
Black rhinoceros (Diceros bicornis)
Where
One of three orangutan species, found only on the Indonesian island of Sumatra
A large mammal native to Africa
Causes of decline
Habitat destruction and fragmentation from logging, conversion of forest to agriculture and infrastructure development. Also illegal hunting and capture for the pet trade
Poaching for their horns, habitat loss, and civil unrest in the countries of their range
Current status
Critically endangered, with only around 14 000 individuals remaining in the wild
Population has declined by over 90% since the 1960s; only around 3 000 mature individuals remain
Conservation
—
Anti-poaching patrols, habitat restoration and captive breeding programmes
Improving species
The 1960s bar is barely visible at this scale, which is the honest way to show a twentyfold recovery.
Southern white rhinoceros (Ceratotherium simum)
Bald eagle (Haliaeetus leucocephalus)
The low point
Once on the brink of extinction from poaching for their horns, with only a handful of individuals surviving in the wild in South Africa in the early 20th century
On the brink of extinction from habitat destruction, hunting and pesticide use, which caused eggshell thinning and reproductive failure. Fewer than 500 pairs in the 1960s
What was done
Increased law enforcement, habitat protection and captive breeding programmes
Habitat protection, captive breeding programmes, and the banning of harmful pesticides like DDT
Today
Recovered to over 18 000 individuals. Still threatened by poaching and habitat loss, but status greatly improved
Recovered to over 10 000 pairs. Status greatly improved thanks to human intervention
Reading the pattern
The bald eagle recovered because DDT was banned. Protecting nests alone would not have fixed eggshell thinning.
Learn one species from each of the three groups properly rather than all six vaguely. For each, you need the factors that caused decline, the impacts on ecosystems and societies, and the conservation strategies used and whether they succeeded. The bald eagle is the strongest single case study because the causal chain is so clean: DDT thinned eggshells, DDT was banned, the population recovered twentyfold.
Worked examples
WE 1
An extinct species
Using a named example, explain the factors that led to a species becoming extinct. (4 marks)
The species
The passenger pigeon was once one of the most abundant bird species in North America, numbering in the billions.
Factor 1: direct
It was hunted intensively for meat, a direct threat that removed individuals faster than the population could reproduce.
Factor 2: indirect
Its forest habitat was destroyed, removing the resources and support systems it needed.
The outcome
By the late 1800s it was in serious decline, and despite some attempts at conservation it went extinct in 1914.
Even a species numbering in the billions can be lost within a lifetimenaming both a direct and an indirect factor shows you can apply the classification
WE 2
A critically endangered species
Describe the threats facing the black rhinoceros and the conservation measures being used. (4 marks)
Threat 1: poaching
The black rhinoceros is poached for its horns, a direct threat which has driven the sharpest losses.
Threat 2: habitat and instability
It also faces habitat loss and civil unrest in the countries of its range, which makes protection difficult to enforce.
The scale of decline
The population has fallen by over 90% since the 1960s, leaving only around 3 000 mature individuals in the wild.
Conservation measures
Anti-poaching patrols, habitat restoration and captive breeding programmes are all under way.
Poaching, habitat loss and instability, met with patrols, restoration and breedingboth figures — the 90% and the 3 000 — are worth quoting
WE 3
Why some recoveries succeed
Using a named example, evaluate the success of a conservation programme. (4 marks)
The starting position
The bald eagle was on the brink of extinction from habitat destruction, hunting and pesticide use, which caused eggshell thinning and reproductive failure. Fewer than 500 pairs remained in the 1960s.
The measures used
Habitat protection, captive breeding programmes, and the banning of harmful pesticides such as DDT.
The outcome
The population recovered to over 10 000 pairs, a twentyfold increase, and its conservation status greatly improved.
Why it worked
Crucially, the underlying cause was removed rather than merely managed: banning DDT addressed the reproductive failure directly. Compare the southern white rhinoceros, which recovered to over 18 000 but remains threatened by poaching because that driver still operates.
Recovery succeeds when the cause is removed, not just the symptoms treatedthe comparison in the last step is what lifts this into evaluation
💡 Exam tips
Prepare one detailed example from each group: extinct, critically endangered, improving.
For each, cover causes of decline, impacts, and conservation strategies and their success.
Learn the numbers: 1914, 1936, 14 000, 3 000, 18 000, 500 to 10 000 pairs.
Use scientific names if you can recall them, but never at the expense of the detail.
You may use your own examples instead of these.
For evaluation, ask whether the underlying driver was removed.
⚠ Common mistakes
Assuming abundance means safety. The passenger pigeon numbered in the billions.
Giving a species name with no detail. The marks are in the causes and strategies.
Confusing the black and southern white rhinoceros. One is critically endangered, the other improving.
Describing improving species as saved. Both remain threatened by poaching and habitat loss.
Omitting DDT. It is the specific mechanism behind the bald eagle’s decline and recovery.
Ignoring societal impacts. The syllabus asks for effects on ecosystems and societies.
Up next: Shared Resources and the Tragedy of the Commons. Several of these declines happened in places nobody owned. The next page explains why that is not a coincidence.
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