Pollution occurs when harmful substances are added to an ecosystem faster than they can be degraded or transformed into harmless substances. Therefore, waste does not automatically cause pollution; pollution begins when its input exceeds the ecosystem’s capacity to process it.
The Mechanism
This is a core SL and HL understanding in Subtopic 7.3: Solid waste. Ecosystems can absorb and process some waste through dilution, decomposition, nutrient uptake and chemical transformation. This processing ability is sometimes described as the ecosystem’s assimilative capacity.
Using a systems approach, waste entering an ecosystem is an input. If the rate of input is greater than the rates of decomposition, transformation or removal, waste accumulates in a storage such as soil, water, sediment or biomass. Harmful concentrations may then disrupt organisms, food webs and ecosystem services.
| Factor | Effect on pollution risk |
|---|---|
| High rate or volume of waste input | Waste accumulates faster than it can be processed |
| Persistent or non-biodegradable waste | Breakdown is extremely slow, allowing long-term accumulation |
| Toxicity | Even a relatively small input may harm organisms |
| Low ecosystem resilience | The system is less able to absorb disturbance and maintain stability |
| Bioaccumulation and biomagnification | Pollutant concentrations increase within organisms and along food chains |
A named example is Minamata Bay, Japan, where industrial wastewater containing mercury entered the aquatic ecosystem. Mercury was converted into methylmercury, accumulated in organisms and biomagnified through the food chain, severely affecting fishing communities. This also demonstrates environmental justice, because local residents experienced health and livelihood costs created by industrial activity.
Exam Technique
For an IB ESS explain question, state the rate relationship explicitly: pollution occurs when waste input exceeds degradation or harmless transformation. Then develop the systems mechanism using inputs, storages and outputs, and support it with a named example. Avoid the misconception that all waste is automatically pollution.
