A pollutant can be present at three parts per million in the water and still wipe out a population of eagles. This page is about how a concentration that low turns into a concentration that lethal – and it happens because of something you already know about food chains.
📚 What you need to know
Biomagnification is the increase in concentration of persistent pollutants with ascending trophic level.
Bioaccumulation is the build-up of pollutants within an organism, or within a single trophic level.
Concentration rises up a chain because of the decrease in total biomass at higher trophic levels.
DDT and mercury are the two named pollutants; DDT caused eggshell thinning in birds of prey.
Plastics are non-biodegradable; macroplastics are over 5 mm, microplastics under 5 mm.
NOS: clear communication of research findings can change public behaviour.
Two words that are not the same
These get mixed up constantly, and the distinction is a genuinely easy mark to pick up.
If the question mentions moving up a food chain, it is biomagnification. If it stays with one organism or one level, it is bioaccumulation.
DefinitionsBiomagnification is the increase in concentration of persistent or non-biodegradable pollutants with ascending trophic level through a food chain. Bioaccumulation is the build-up of pollutants within an organism, or within a single trophic level.
Why the concentration goes up
As pollutants are passed up a food chain from one trophic level to the next, they become more concentrated, and the reason is the decrease in total biomass at higher trophic levels – the same 10 % rule you met in the energy unit, seen from the other side.
Put in terms of individuals: the small organisms at the bottom of the chain each consume a small volume of pollutant. An organism at the top consumes many of those smaller organisms, and so receives a much larger dose. The pollutant does not break down and is not excreted, so each meal adds to the total.
DDT
DDT – dichlorodiphenyltrichloroethane – is a persistent pollutant that enters food chains. It was a widely used insecticide in the mid-20th century, and it turned out to have harmful effects on top predators such as birds of prey.
The route it took is worth learning as a chain:
DDT sprayed on crops leached into waterways and eventually entered freshwater and marine ecosystems.
It entered food chains via plankton and accumulated in the bodies of fish.
Those fish were eaten by birds, which accumulated higher concentrations still.
Drawn on an ordinary scale, the water bar would be invisible and the top five bars would all look identical. That is what a logarithmic axis is for.
Because DDT is persistent and does not break down easily, it continues to accumulate in the bodies of animals at higher trophic levels, leading to harmful effects such as thinning of eggshells and reduced reproductive success. The mechanism there is worth spelling out: thin eggshells could not withstand the weight of the parent bird during incubation, so eggs broke, fewer young hatched, and over time bird populations fell.
DDT has now been banned worldwide, with one exception – its use in areas where it is essential for dealing with mosquitos that transmit malaria.
That exception is worth remembering, because it is a good example of a genuine trade-off rather than a simple right answer. A ban that saves eagles but costs lives to malaria is not obviously the better outcome, which is exactly why the exception exists.
Mercury
Mercury is the second named example, and it follows the same pattern with a chemical twist. It is released into the environment through activities such as coal-fired power plants and gold mining.
Once in the environment, mercury can be converted by microorganisms into a highly toxic form called methyl mercury, which accumulates in the bodies of fish. As larger fish eat smaller fish, the concentration of methyl mercury in their tissues increases – which means potential harm for humans who eat large predatory fish such as tuna or swordfish.
Notice where humans sit. In the mercury example, we are the top of the food chain, so biomagnification delivers its highest dose to us. It is one of the few places in the syllabus where the ecology has a direct dietary consequence for the reader.
Plastic pollution
Plastics have a large negative impact on both land and water habitats because they are non-biodegradable. Two size categories are named:
Type
Size
Examples and origin
Macroplastics
More than 5 mm in length
Plastic bags, bottles, food packaging and fishing nets
Microplastics
Less than 5 mm in length
Come from macroplastics broken into smaller pieces by, for example, wave action or UV rays
In marine habitats the damage takes several forms:
Animals try to eat plastic. Turtles may attempt to eat a plastic bag that resembles a jellyfish. Albatrosses may accidentally consume plastic when they fish and then give it to their chicks when they regurgitate food – the chicks may later starve because their stomachs are full of plastic.
Animals become caught in plastic such as fishing lines, leading to injury and death.
As plastic breaks down it can release toxins, which can then undergo biomagnification in the food chain.
Once broken down into very small particles, plastic is commonly ingested and enters the food chain directly.
NOS: communication changes behaviour
The syllabus makes a specific point here: scientists can influence the actions of citizens if they provide clear information about their research findings.
Plastic pollution is the example, and it is a good one because the change happened quickly and visibly. The impacts have become very well understood by the public in recent years because of effective communication of scientists’ findings – popular wildlife documentaries showing footage of seabirds feeding plastic to their chicks, for instance. That kind of clear science communication can change public behaviour:
People have petitioned food companies and supermarkets to reduce plastic packaging.
People may choose to shop at stores that use less plastic.
People may get better at household recycling and at taking their rubbish home.
People may opt for non-plastic items such as bamboo toothbrushes or paper straws.
Worked examples
WE 1
Distinguish the two terms
Distinguish between bioaccumulation and biomagnification. (2 marks)
Point 1: bioaccumulation
The build-up of a pollutant within an organism, or within a single trophic level.
Point 2: biomagnification
The increase in concentration of a pollutant with ascending trophic level through a food chain.
One organism versus up the chain“distinguish” means give both and make the contrast explicit – do not define only one
WE 2
Explain the mechanism
Explain why the concentration of DDT was highest in birds of prey. (4 marks)
Point 1: persistence
DDT is persistent, so it does not break down easily and is not excreted.
Point 2: entry to the chain
It leached into waterways and entered food chains via plankton, accumulating in the bodies of fish.
Point 3: biomassTotal biomass decreases at higher trophic levels, so the same quantity of pollutant is contained in less tissue.
Point 4: consumption
Each bird eats many fish, so it receives the combined dose of all of them, giving the highest concentration at the top of the chain.
Does not break down, and the tissue it sits in keeps shrinkingthe biomass point is the one that separates a full-mark answer from a three-mark one
WE 3
Explain a population decline
Explain how DDT led to a reduction in the populations of birds of prey. (3 marks)
Point 1: the accumulation
DDT accumulated to high concentrations in the tissues of birds at the top of the food chain.
Point 2: the effect
This caused thinning of eggshells, so the shells could not withstand the weight of the parent bird during incubation.
Point 3: the consequence
Eggs broke, fewer young birds hatched, so reproductive success and therefore population size fell over time.
The DDT did not kill the adults – it broke the eggsgive the mechanical reason the eggs broke; “the shells were weak” alone is vague
💡 Exam tips
Use the phrase ascending trophic level in the biomagnification definition.
Explain the concentration rise using the decrease in total biomass.
For DDT, name eggshell thinning and explain why that reduces hatching.
For mercury, name the toxic form: methyl mercury.
Quote the 5 mm boundary between macro- and microplastics.
In NOS questions, give a concrete example of a behaviour change, not just “people care more”.
⚠ Common mistakes
Using bioaccumulation and biomagnification interchangeably. They describe different things.
Saying the pollutant is produced by the organisms. It is ingested and retained.
Explaining biomagnification without mentioning biomass. That is the actual reason.
Reading a logarithmic graph as if it were linear. Check the axis before comparing bars.
Saying microplastics are manufactured small. They come from larger plastics breaking down.
Claiming DDT is banned everywhere. It is still used against malarial mosquitos where essential.
Up next: Restoring Ecosystems – having spent three pages on ways ecosystems get damaged, we look at what can be done to put one back together.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
