In the early 1800s, the passenger pigeon may have been the single most abundant bird on the planet, flocks so vast that a single migrating column could take hours to pass overhead and was recorded blocking out the sun. By September 1, 1914, when a bird named Martha died in the Cincinnati Zoo, the species was extinct.
A species numbering in the billions went to zero in about five decades. What single factor best explains a collapse that fast?
Both, plus something less obvious. Deforestation removed the pigeon's oak and beech mast forests, and the telegraph and railroad turned hunting into an industrial supply chain: hunters could learn a flock's location by wire and ship barrels of pigeons to city markets by rail within days. But the final collapse had a third cause. Passenger pigeons had evolved to breed only in enormous, densely packed colonies, a strategy that likely helped swamp predators. Once flocks fell below a certain size, breeding success collapsed even where birds still survived, a species that could not reproduce well in small numbers. Statement 3.2.6 asks for three named species with three different fates; the passenger pigeon is the one that ran out of time.
- Population estimates for the early-to-mid 1800s range from 3 to 5 billion birds, plausibly a quarter or more of all birds in North America.
- Commercial hunters shot and netted pigeons for meat markets and live-pigeon shooting; by the late 1800s, nearly half of the United States' native forest had been cleared for agriculture and timber.
- The last confirmed wild bird was shot in Pike County, Ohio, in March 1900. The last known individual of the species, a captive female named Martha, died at the Cincinnati Zoo on September 1, 1914.
Sources: Smithsonian National Museum of Natural History; Scientific American, "3 Billion to Zero: What Happened to the Passenger Pigeon?"; Natural History Museum, London.
Direct and indirect threats to biodiversity
Biological diversity is being adversely affected by both direct and indirect influences. Direct threats act on a species or population immediately and physically: overharvesting, poaching, and the illegal pet trade. Indirect threats work through a change to the wider system the species depends on: habitat loss, climate change, pollution, and invasive alien species.
Practice: classify each pressure as a direct or an indirect threat. Ask whether it removes individuals from a population immediately, or whether it changes the conditions the population depends on.
Rhinos are killed for their horns by armed poachers operating inside a protected reserve.
A tract of rainforest is cleared for soy farmland, removing the tree cover an understory bird species nests in.
Wild-caught parrot chicks are taken from nests and sold into the exotic pet trade.
Rising ocean temperatures cause repeated coral bleaching events across a reef system.
Most ecosystems face multiple impacts at once
Real ecosystems are rarely exposed to just one threat in isolation. Human impacts are increasing in number, and their effects are amplified when combined: a threat that a resilient system could absorb on its own becomes far more damaging once resilience has already been reduced by something else. Climate change is the clearest example. Once warming has weakened a system's ability to recover, a second pressure, such as an invasive species that a healthier system might have resisted, can do far more damage than either pressure would alone.
This is the same idea as resilience covered in 1.2 Systems: a system's capacity to absorb disturbance without changing state. Reduced resilience does not cause damage by itself, it removes the buffer that would otherwise have absorbed the next disturbance.
Invasive alien species: the cane toad in Australia
An invasive alien species is a non-native species that, once introduced to a new ecosystem, spreads and causes ecological or economic harm. Invasive species reduce local biodiversity through competition for limited resources, predation, and the introduction of diseases or parasites that native species have no defense against.
- In 1935, 2,400 cane toads (native to Central and South America) were deliberately released near Gordonvale, Queensland, intended to control cane beetle species, pests of sugar cane. No environmental impact assessment was carried out first, and the toads were never shown to actually control the target beetles.
- Cane toads are toxic at every life stage. Native predators that evolved no tolerance for the toxin, quolls, goannas, snakes, and freshwater crocodiles among them, die after attempting to eat them, and several of these predator populations have collapsed in newly invaded areas.
- The toad population is now estimated in the hundreds of millions, spreading at roughly 30-60 km per year across Queensland, the Northern Territory, Western Australia, and New South Wales.
- Management today focuses on control rather than elimination: community "toad-busting" removal events, research into pheromone traps that lure toad tadpoles, and breeding native predators that show early signs of learned or genetic toxin tolerance.
Sources: National Museum of Australia; Australian Government Department of Climate Change, Energy, the Environment and Water, cane toad fact sheet.
Photo: Wilfredor, via Wikimedia Commons (public domain).
Quick check. Quoll and goanna populations have collapsed in areas cane toads have invaded. Which mechanism from the definition above explains this specific decline?
The IUCN Red List
The global conservation status of a species is assessed by the International Union for Conservation of Nature (IUCN) and published as the IUCN Red List of Threatened Species. Status is assigned using precise criteria: number of individuals remaining, the rate at which the population is increasing or decreasing, breeding potential, geographic range, and known threats.
Vulnerable, endangered and critically endangered are collectively described as "threatened." Least concern and near threatened are not. Data deficient (not enough evidence to assess) sits outside this ranked sequence entirely.
Why assigning status matters
Assigning a global conservation status does more than record a fact: it publicizes a species' vulnerability and gives governments, non-governmental organizations, and individual citizens a shared basis for selecting conservation priorities and management strategies. Different groups tend to weigh that information differently. A national government may prioritize status alongside economic development pressure and its own legal obligations; a conservation NGO may prioritize status alongside where its funding and expertise can have the most impact; an individual citizen may prioritize status alongside which species or causes feel most immediate or visible to them.
If asked to compare perspectives on conservation status, be specific about whose interests are at stake and why they diverge, not just that "everyone has different opinions." Governments answer to national economic and legal pressures, NGOs answer to funders and mission focus, and individuals respond to visibility and personal connection.
Three species, three fates
Statement 3.2.6 asks for three different named species: one driven extinct by human activity, one that is critically endangered today, and one whose conservation status has been improved by intervention. Explore each to see what contributed to its decline, and, for the two still living, what conservation strategy is or is not working.
Photo: Naturalis Biodiversity Center, via Wikimedia Commons (CC BY-SA 3.0).
Quick check. The panda's status improved while the vaquita's has not. What actually explains that difference?
A species being critically endangered does not mean its outlook is hopeless, and a species being downlisted does not mean it is safe. The giant panda remains vulnerable, not a low-risk species; the vaquita's population has shown small recent upticks in survey counts without changing its critically endangered status. Status describes current risk level against fixed criteria, not a verdict on the future.
- Passenger pigeon population and extinction: Smithsonian National Museum of Natural History; Scientific American; Natural History Museum, London.
- Cane toads, Australia: National Museum of Australia; Australian Government Department of Climate Change, Energy, the Environment and Water.
- IUCN Red List categories and criteria: IUCN Red List of Threatened Species.
- Vaquita population and gillnet/totoaba connection: SeafoodSource, 2025; Marine Mammal Commission; Scientific American.
- Giant panda IUCN status: IUCN Red List; San Diego Zoo Wildlife Alliance; WWF.
- Passenger pigeon specimen photo: Naturalis Biodiversity Center, via Wikimedia Commons (CC BY-SA 3.0).
- Cane toad photo: Wilfredor, via Wikimedia Commons (public domain).
- Vaquita photo: Paula Olson / NOAA, via Wikimedia Commons (public domain).
- Giant panda photo: Colegota, via Wikimedia Commons (CC BY-SA 2.5 Spain).
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 3, Subtopic 3.2, statements 3.2.1-3.2.6.
Biodiversity hotspots are under threat
Because biodiversity is spread so unevenly across the planet, threats are unevenly distributed too. Habitat destruction inside a biodiversity hotspot, an area holding an exceptionally large share of the world's species, threatens a disproportionate amount of global biological diversity for the area of land or sea involved. Many hotspots sit in tropical biomes, which frequently overlap with developing countries, adding economic development pressure to the challenge of protecting them.
3.1 Biodiversity and Evolution covers what makes a hotspot in the first place, using Madagascar and the Coral Triangle to show how isolation and marine species-richness each produce extreme endemism. This section picks up from there: what happens when that concentrated biodiversity comes under threat.
Key biodiversity areas
Key biodiversity areas (KBAs) are sites identified, using globally standardized IUCN criteria, as contributing significantly to the global persistence of biodiversity. A site can qualify by holding species at risk of extinction, an ecosystem at risk of collapse, or an exceptionally large concentration of a species' entire population. Unlike a biodiversity hotspot, which is a broad biogeographic region, a KBA is a specific, mappable site that conservation planners and governments can act on directly.
Cape Floristic Region, South Africa
- The smallest of the world's six recognized floral kingdoms, covering less than 0.5% of Africa's land area
- Home to over 9,000 vascular plant species, around 69% found nowhere else on Earth
- Recognized as a UNESCO World Heritage property (Cape Floral Region Protected Areas) specifically for this concentration of endemic plant diversity
Phobjikha Valley, Bhutan
- A high-altitude wetland valley in the Eastern Himalaya biodiversity hotspot
- Winter home to around 500 black-necked cranes (Grus nigricollis), migrating from the Tibetan plateau each year, a species IUCN classified as near threatened in 2020
- Crane tourism directly benefits local communities, and the species holds deep cultural significance in Bhutan, an example of a KBA where conservation and local livelihoods reinforce each other
Image: NASA Earth Observatory (Wanmei Liang), public domain.
Sources: CEPF, Cape Floristic Region hotspot profile; UNESCO World Heritage listing 1007; IB Environmental Systems and Societies specimen paper materials, Figure 8, Eastern Himalaya biodiversity hotspot and the black-necked crane.
Conflict in key biodiversity areas: palm oil in Malaysia and Indonesia
Inside many KBAs, conflict exists between exploitation, sustainable development, and conservation. Palm oil expansion in Malaysia and Indonesia is one of the most extensively documented examples.
- Malaysia and Indonesia together supply roughly 85% of the world's palm oil, the most widely used vegetable oil on Earth.
- Palm oil plantations accounted for at least 39% of all forest loss on the island of Borneo between 2000 and 2018.
- Deforestation and hunting killed an estimated 150,000 Bornean orangutans between 1999 and 2015; fewer than 80,000 are believed to survive today, and suitable habitat has fallen by more than 80% in 20 years.
- Sumatra's elephant population has crashed to fewer than 3,000 individuals as herds lose habitat and come into escalating conflict with plantation communities.
Sources: WWF, "Endangered Species Threatened by Unsustainable Palm Oil"; Save the Orangutan; Mongabay, 2023.
Palm oil is also Malaysia and Indonesia's most valuable agricultural export and supports millions of livelihoods, and oil palm produces far more oil per hectare than alternative crops such as soy or rapeseed, meaning that switching to a different vegetable oil could require clearing even more land elsewhere. Certification schemes such as the Roundtable on Sustainable Palm Oil (RSPO) attempt to address the conflict without simply halting the industry, with mixed and contested success.
Traditional indigenous land management under threat
Traditional indigenous approaches to land management are often more sustainable than the systems that replace them, built on generations of accumulated ecological knowledge. But they face mounting pressure from population growth, economic development, climate change, and, frequently, a lack of governmental support or legal protection.
- The Kayapo people of the Brazilian Amazon practice diversified agroforestry, planting native fruit and forest species rather than clearing land for single-crop agriculture. In one documented project, Kayapo women restored 13 hectares of degraded land this way.
- Fair-trade production of non-timber forest products, including honey, has generated income for nearly 70 indigenous families without further deforestation.
- The Kayapo fought for and won official legal recognition of their territory in the 1980s and 1990s, among the largest indigenous land grants in the Amazon.
- That recognition is now under sustained pressure: illegal logging, gold mining, land grabs for soy and cattle, and oil drilling all continue to encroach on Kayapo land, and indigenous and environmental defenders across the Amazon region have faced violence, including hundreds of killings between 2012 and 2022.
Sources: National Geographic, "Rain Forest Warriors: How Indigenous Tribes Protect the Amazon"; Global Citizen; Amazon Watch.
The threat to indigenous land management is not only external. Legal land title stops outside actors from moving in, but it does not stop the slower pressure of a community's own younger generation moving toward wage labor, urban migration, or commercial farming as national economic development draws people away from traditional practices.
Environmental justice in conservation
Environmental justice must be considered when conservation efforts address biodiversity loss. Areas expected to experience the most severe consequences of biodiversity and ecosystem loss are disproportionately home to indigenous and marginalized communities, often low-income and without ready access to legal support. Conservation itself can become the source of harm when a protected area is created on land a community depends on for its livelihood.
- In June 2022, the Tanzanian government moved to demarcate 1,500 km² of Maasai ancestral land in Loliondo division, Ngorongoro district, as a game reserve, land the Maasai had used for grazing for generations.
- The plan threatened the eviction of an estimated 70,000 Maasai people and over 200,000 livestock, framed officially as necessary for wildlife conservation and safari tourism.
- Human rights organizations documented violent clashes between security forces and Maasai residents during the demarcation process.
Sources: Human Rights Watch, 2023; Amnesty International, 2023; International Work Group for Indigenous Affairs.
1.3 Sustainability builds this case out in full, including the specific stakeholder perspectives and a self-marking practice question on the justice trade-offs involved. Environmental justice is not limited to indigenous displacement from protected land, either: the Skills tab on this page works through a real Portland, Oregon dataset connecting urban tree canopy coverage to household income, a very different flavor of the same underlying concept.
The "loss of biosphere integrity" planetary boundary
The planetary boundaries framework treats "loss of biosphere integrity" as one of nine boundaries defining a safe operating space for humanity. It is assessed using two control variables, and both have been pushed well past their proposed safe limits.
Genetic diversity (extinction rate)
Measured in extinctions per million species-years (E/MSY). The proposed safe boundary is 10 E/MSY; the current rate is estimated above 100 E/MSY, more than ten times the limit.
Functional integrity (HANPP)
Measured as the human appropriation of net primary production (HANPP), the share of the planet's plant growth diverted to human use. The proposed safe boundary is 10%; the current level is estimated near 30%, inside the framework's high-risk zone.
1.3 Sustainability covers the full nine-boundary planetary boundaries framework and its current status, seven of nine boundaries transgressed as of the Stockholm Resilience Centre and Potsdam Institute's Planetary Health Check, September 2025. Biosphere integrity was among the first boundaries confirmed crossed, alongside the nitrogen cycle covered in 2.3.
Apply it: a new mining concession
A government grants a mining company a concession to extract minerals inside a key biodiversity area that is also traditional land for an indigenous community with no formal legal title. The KBA holds a critically endangered amphibian found nowhere else, and the community has managed the land sustainably for generations without government support.
Think it through, then check your reasoning against the model answer below. Identify one direct and one indirect threat this concession creates for the amphibian, and explain why environmental justice is relevant to how this conflict should be resolved.
- Direct threat: mining activity itself, such as excavation, could destroy or fragment the amphibian's habitat and directly kill or displace individuals.
- Indirect threat: runoff, sedimentation, or chemical contamination of water sources downstream of the mine could degrade breeding habitat without removing individuals directly.
- Environmental justice is relevant because the community with the strongest historical claim and the most sustainable track record on this land has no formal legal title, meaning it has the least power to contest the concession, while bearing the most direct cost if the land is degraded.
- A strong answer explicitly connects the community's lack of legal title to their lack of power in the decision, rather than treating the human and ecological harms as two separate, unrelated issues.
- Biodiversity hotspots cross-reference: covered in full on 3.1 Biodiversity and Evolution.
- Cape Floristic Region: CEPF hotspot profile; UNESCO World Heritage listing 1007.
- Cape Floristic Region satellite image: NASA Earth Observatory (Wanmei Liang), public domain.
- Phobjikha Valley and the black-necked crane: IB Environmental Systems and Societies specimen paper materials; IUCN Red List.
- Palm oil expansion, Malaysia and Indonesia: WWF; Save the Orangutan; Mongabay, 2023.
- Kayapo agroforestry: National Geographic; Global Citizen; Amazon Watch.
- 2022 Loliondo evictions and planetary boundaries figure: covered in full on 1.3 Sustainability.
- Loss of biosphere integrity boundary status: Stockholm Resilience Centre, Planetary Health Check, September 2025.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 3, Subtopic 3.2, statements 3.2.8-3.2.13.
Order the IUCN Red List by extinction risk
Statement 3.2.4 introduces the IUCN Red List categories. Click the categories below, in order, from lowest extinction risk to highest.
Reading real data: tree canopy and income in Portland, Oregon
Environmental justice claims should be backed by real data, not just intuition. Researchers ranked every neighborhood in Portland, Oregon by household income and split the city into five equal-sized groups, called quintiles, from Quintile 1 (the lowest-income fifth of the city) to Quintile 5 (the highest-income fifth). They then measured how much of each group's neighborhoods was actually covered by trees.
Source: Voelkel, J. (2017), Canopy Analysis: CBG, Portland State University Sustaining Urban Places Research Lab, measured across 477 neighborhoods citywide, as reported in Portland Parks & Recreation's 2018 tree planting strategy.
(a) Describe the relationship shown between household income group and tree canopy cover. [1]
(b) Suggest two reasons for this relationship. [2]
(c) Discuss how addressing this imbalance could also help address climate change. [2]
Tick each point you're confident your answer actually included:
Self-marked score: 0 / 5
- Tree canopy cover by household-income quintile, Portland, Oregon: Voelkel, J. (2017), Canopy Analysis: CBG, Portland State University Sustaining Urban Places Research Lab; reported in Portland Parks & Recreation (2018), Growing a More Equitable Urban Forest: Portland's Citywide Tree Planting Strategy.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 3, Subtopic 3.2.
Glossary
- Direct threat
- A pressure that removes individuals from a population immediately and physically, such as overharvesting, poaching, or the illegal pet trade.
- Indirect threat
- A pressure that acts through a change to the wider system a species depends on, such as habitat loss, climate change, pollution, or invasive alien species.
- Invasive alien species
- A non-native species that, once introduced to a new ecosystem, spreads and reduces local biodiversity through competition, predation, or the introduction of disease.
- IUCN Red List
- A global assessment of species' extinction risk, published by the International Union for Conservation of Nature, ranking species from least concern to extinct using standardized criteria.
- Conservation status
- The assigned risk category of a species based on population size, rate of change, breeding potential, geographic range, and known threats.
- Key biodiversity area (KBA)
- A specific, mappable site identified using standardized IUCN criteria as contributing significantly to the global persistence of biodiversity.
- Biodiversity hotspot
- A broad biogeographic region holding an exceptionally large proportion of the world's species, especially rare and endemic ones, frequently located in tropical biomes.
- Environmental justice
- The principle that the costs and benefits of environmental decisions, including conservation efforts, should not fall disproportionately on marginalized or low-income communities.
- Planetary boundary
- One of nine proposed limits within which humanity can operate safely without triggering abrupt or irreversible environmental change.
- Loss of biosphere integrity
- The planetary boundary assessing genetic diversity (extinction rate) and functional integrity (human appropriation of net primary production); both control variables are currently assessed as transgressed.
- Extinction rate (E/MSY)
- Extinctions per million species-years, the metric used to track the genetic diversity control variable of the biosphere integrity boundary. The proposed safe limit is 10 E/MSY.
- Human appropriation of net primary production (HANPP)
- The share of the planet's total plant growth (net primary production) diverted to human use, used as the functional integrity control variable of the biosphere integrity boundary.
- Traditional indigenous land management
- Land management practices developed by indigenous communities over generations, often more sustainable than the systems that replace them, but increasingly threatened by outside and internal economic pressure.
- Tragedy of the commons HL
- The pattern of negative outcomes that results when a shared, unrestricted resource is overexploited because individuals act in their own self-interest rather than the shared long-term interest.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 3, Subtopic 3.2.
Test Yourself is coming soon for this subtopic
Practice questions and markschemes for this page are still being written. Check back once they have been added.
The tragedy of the commons
The tragedy of the commons describes the negative outcomes that result when a shared, unrestricted resource, a "commons", is overexploited because individuals act in their own self-interest rather than the group's shared long-term interest. No single user is entirely to blame: each acts rationally from their own position, taking a small amount for themselves while the cost of that small amount is spread across everyone who shares the resource. The combined effect of many individually rational choices is a resource degraded or depleted for everyone, including the people who overused it.
The tragedy of the commons is not simply "overharvesting happened." The concept specifically requires a shared resource with no clear ownership or restriction on access, so that no individual user has either the responsibility or the incentive to limit their own use. A privately owned, well-managed fishery that still collapses from bad management is not automatically a tragedy of the commons in the strict sense; an unregulated, open-access resource is.
Case: the Great Pacific Garbage Patch
Plastic pollution in ocean gyres is a widely used real-world example of the tragedy of the commons. The open ocean has no clear owner and no single country or authority responsible for it, so the cost of any one country's or company's plastic waste is spread across the entire shared resource.
- A zone of concentrated marine debris in the North Pacific Ocean spanning roughly 1.6 million km², more than three times the area of France.
- Estimated to hold at least 79,000 tonnes of ocean plastic, made up of around 1.8 trillion plastic fragments, most smaller than a grain of rice.
- Fishing nets and gear account for roughly 46% of the total mass, evidence that commercial fishing fleets, not just consumer litter, are a major contributor.
- No individual country, company, or fishing fleet created the Garbage Patch. It is the accumulated, combined effect of many separate, individually small contributions to a shared ocean commons over decades.
Sources: The Ocean Cleanup; Scientific Reports, "Evidence that the Great Pacific Garbage Patch is rapidly accumulating plastic," 2018.
Graphic: Plastic Atlas, Appenzeller/Hecher/Sack, via Wikimedia Commons (CC BY 4.0).
Quick check. What makes the Great Pacific Garbage Patch a clear example of the tragedy of the commons, rather than just a large pollution problem?
Case: the Grand Banks cod collapse
A second clear example is a resource overharvested outright: fish stocks on the Grand Banks. The Newfoundland northern cod fishery is already covered in full depth on 1.3 Sustainability, including the timeline of the collapse, the 32-year moratorium, and its controversial 2024 reopening, so this page builds only the tragedy-of-the-commons framing on top of that existing case rather than repeating it.
See 1.3 Sustainability for the full case. In tragedy-of-the-commons terms: the Grand Banks were, for centuries, treated as an open-access resource. Individual fishing vessels and nations had every short-term incentive to catch as much as possible before a competitor did, and no vessel bore the full future cost of the stock's decline alone. By the time Canada declared a moratorium in July 1992, the incentive structure had already driven the stock to collapse.
HL practice
Running total: 0 / 8
Explain how the accumulation of microplastic in the Great Pacific Garbage Patch demonstrates the concept of the tragedy of the commons.
Maximum 2 marks if the Great Pacific Garbage Patch itself is not explicitly referenced in the answer.
Self-marked score: 0 / 4
Compare the Great Pacific Garbage Patch and the Grand Banks cod collapse as examples of the tragedy of the commons.
Self-marked score: 0 / 4
- Great Pacific Garbage Patch: The Ocean Cleanup; Scientific Reports, 2018.
- Great Pacific Garbage Patch size-comparison graphic: Plastic Atlas, Appenzeller/Hecher/Sack, via Wikimedia Commons (CC BY 4.0).
- Grand Banks cod collapse: covered in full on 1.3 Sustainability.
- Tragedy of the commons framing: informed by IB Environmental Systems and Societies specimen paper materials.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 3, Subtopic 3.2, statement 3.2.7.