IB ESS SL & HL3.2 Human Impact on BiodiversityPaper 1 & 2~11 min read
Shared Resources and the Tragedy of the Commons
Every individual fishing boat behaves perfectly rationally. Catch more, earn more. The cod still disappear. That gap — between what makes sense for one person and what happens to everyone — is the tragedy of the commons.
📘 What you need to know
The tragedy of the commons describes the overuse and depletion of a shared resource.
It occurs when individuals act in their own self-interest rather than considering the common good.
The result is degradation of the resource, making it unavailable for future use.
Implications: overexploitation leading to resource depletion and possible ecosystem collapse, and impacts on biodiversity through loss of habitats, species and genetic diversity.
These weaken ecosystem resilience.
It applies to contamination and degradation as well as to overharvesting.
Case studies: Grand Banks cod and plastic pollution in ocean gyres.
The mechanism
DefinitionTragedy of the commons — the overuse and depletion of a shared resource, occurring when individuals act in their own self-interest rather than considering the common good, leading to degradation of the resource so it becomes unavailable for future use.
The asymmetry between private benefit and shared cost is the whole mechanism. It is why voluntary restraint alone rarely works.
Implications for sustainability
Overexploitation. Many natural resources are used faster than they can be replenished, causing resource depletion and potentially the collapse of certain ecosystems.
Impact on biodiversity. It causes the loss of habitats and species, and can also reduce genetic diversity. Together these weaken ecosystem resilience, threatening biodiversity.
Case study: overfishing on the Grand Banks
Details
Location
Grand Banks, off the coast of Newfoundland, Canada
Resource affected
Cod fish
Problem
Intensive fishing by local and international fleets, using advanced fishing technologies with no effective regulation
Consequences
Dramatic decline in cod populations; economic collapse of the regional fishing industry; long-term ecological damage
Current situation
Strict regulations and moratoriums on cod fishing. Slow recovery of stocks, but still a long way below historical levels
Note the two ingredients. Advanced technology raised how much each boat could take, and the absence of regulation meant nothing limited how much it would. Either alone is survivable; together they are the tragedy of the commons in its textbook form. Note also that recovery has been slow even after fishing stopped — crossing a threshold is easier than crossing back.
Case study: plastic pollution in ocean gyres
The tragedy of the commons does not only apply where a natural resource is overharvested. It also applies to the contamination and degradation of ecosystems by humans.
Details
Location
Various ocean gyres, including the North Pacific Gyre, home to the Great Pacific Garbage Patch
Resource affected
The marine environment and marine biodiversity
Problem
Accumulation of plastic debris from land-based and marine sources. Plastics are non-biodegradable and persist for hundreds of years
Consequences
Marine animals ingest plastics, leading to injury and death; plastics break down into microplastics that enter the food chain; damage to marine ecosystems and decline in marine biodiversity
Current situation
Efforts to reduce plastic use and increase recycling; international agreements and clean-up initiatives
Recognising the second form is what most students miss. The commons can be a place to take from or a place to dump into.
If you are asked why the tragedy of the commons is hard to solve, the answer is in the mechanism rather than in people’s character. Because the benefit is private and the cost is shared, restraint by one user simply leaves more for the others. That is why the solutions in both case studies are collective: moratoriums, regulations and international agreements. This connects directly to the environmental justice material in 1.3 — those who bear the cost are rarely those who took the benefit.
Worked examples
WE 1
Explaining the concept
Outline what is meant by the tragedy of the commons. (3 marks)
Step 1: the definition
It describes the overuse and depletion of a shared resource.
Step 2: the cause
It occurs when individuals act in their own self-interest rather than considering the common good.
Step 3: the outcome
The resource is degraded, making it unavailable for future use.
Shared resource, individual self-interest, collective lossthe phrase “rather than the common good” is the marking point
WE 2
Applying it to a case study
Explain how the collapse of the Grand Banks cod fishery illustrates the tragedy of the commons. (4 marks)
Step 1: the shared resource
Cod stocks on the Grand Banks off Newfoundland were a shared resource, fished by local and international fleets with no single owner.
Step 2: the self-interest
Each fleet benefited fully from every fish it caught, while the cost of a falling stock was shared by everyone, so there was no incentive for any individual to hold back.
Step 3: the enabling conditions
Advanced fishing technologies raised catch capacity, and there was no effective regulation to limit it.
Step 4: the outcome
Cod populations declined dramatically, the regional fishing industry collapsed economically, and long-term ecological damage followed. Even with moratoriums, recovery has been slow and stocks remain far below historical levels.
The resource everyone depended on was destroyed by everyone acting sensiblythe slow recovery is worth adding — it shows the damage was not easily reversible
WE 3
Beyond overharvesting
Explain how plastic pollution in ocean gyres is an example of the tragedy of the commons. (4 marks)
Step 1: the principle extended
The tragedy of the commons applies not only to overharvesting but also to the contamination and degradation of shared ecosystems.
Step 2: the shared resource
The marine environment is a shared resource that no one owns. Plastic debris from land-based and marine sources accumulates in gyres such as the North Pacific Gyre.
Step 3: the asymmetry
Each producer or user of plastic gains the benefit of cheap disposal, while the cost of pollution is borne by everyone and by marine ecosystems.
Step 4: the consequences
Plastics are non-biodegradable and persist for hundreds of years. Marine animals ingest them, causing injury and death, and plastics break down into microplastics that enter the food chain, damaging ecosystems and reducing marine biodiversity.
A shared sink degraded by many small individually rational decisionsthis second form of the tragedy is often overlooked, so it stands out
💡 Exam tips
Define it using shared resource and self-interest rather than the common good.
Remember it covers contamination as well as overharvesting.
Learn both case studies, including where each one is.
For the Grand Banks, name both causes: advanced technology and no effective regulation.
Link outcomes to ecosystem resilience and genetic diversity, not just species loss.
Note that solutions are collective: regulations, moratoriums, international agreements.
⚠ Common mistakes
Treating it as simple greed. The mechanism works even when everyone behaves reasonably.
Applying it only to harvesting. Pollution of a shared sink counts too.
Forgetting the resource must be shared. A privately owned resource does not fit the model.
Saying the Grand Banks has recovered. Recovery is slow and stocks remain far below historical levels.
Omitting microplastics. Their entry into the food chain is a key consequence.
Proposing only individual solutions. Voluntary restraint does not fix the asymmetry.
That completes the SL content for 3.2. Up next is the HL material, beginning with Pressures on Biodiversity Hotspots — where these threats concentrate, and why conserving them is hardest exactly where it matters most.
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