Ecosystems and interdependence · Process
Toxic build-up in a food chain
A concentration far too low to harm anything in the water can still kill the bird at the top of the chain. Nothing is added along the way. The arithmetic does it.
Start here
Safe in the water. Lethal in the osprey.
A pesticide is sprayed on farmland and washes into a lake at 0.003 parts per million — three thousandths of a milligram in a litre, far below anything that could harm a fish. Twenty years later the fish-eating birds are gone, and the pesticide in their bodies measures 300 parts per million.
How did the concentration rise by a hundred thousand times?
Nobody added anything. Two facts do all the work: the chemical is not broken down or excreted, so it stays in the body for life; and each organism eats many of the organisms below it. A fish that eats a thousand contaminated water fleas keeps a thousand doses in one body. Do that five times up a chain and the concentration climbs by five factors of ten.
At the bench · one lake, six levels
Follow the concentration up
level 1 of 6
Persistence — whether the body can break the chemical down — is the dial that decides everything.
The chemical
Cannot be broken down by the body and dissolves in fat, so it is stored rather than excreted. Every dose an organism ever takes in stays with it.
Lake water
the source
0.0030 ppm
no measurable effect
Algae
absorbs from the water
0.030 ppm
no measurable effect
Water fleas
eat thousands of algae
0.300 ppm
no measurable effect
Minnows
eat hundreds of water fleas
3.0 ppm
no measurable effect
Perch
eat dozens of minnows
30 ppm
no measurable effect
Ospreys
eat hundreds of perch a year
300 ppm
above the level that causes harm
The ospreys are carrying 300 ppm — roughly 100,000 times the concentration in the water, and enough to stop them breeding. Not one molecule was added after the spraying. Every one of them came out of the lake.
Two conditions, and it needs both
Remove either one and nothing accumulates.
Condition one
It persists
The chemical is not broken down by enzymes and not excreted — usually because it dissolves in fat rather than water, so it is stored in the body instead of being filtered out.
Condition two
Predators eat many
Each organism eats many of the level below over its life. All the toxin from all those bodies ends up in one, so the concentration multiplies.
Why the top suffers
Long chains, small numbers
The animals at the top are few, slow-breeding and long-lived — so they accumulate for longest, and a population that loses its adults takes decades to recover.
Key fact
A toxic substance that cannot be broken down or excreted accumulates in each organism and becomes more concentrated at every step up a food chain, because each predator eats many of the organisms below it. The animals at the top are harmed first, at concentrations that are harmless lower down.
Think again
“The poison gets stronger as it goes up the chain.”
The molecule is identical at the top of the chain and at the bottom — it has not been strengthened, activated or changed in any way. What rises is the concentration: how much of it there is in each kilogram of animal. Nothing is added; things are collected. A water flea takes in a tiny amount and keeps it. A minnow eats a hundred water fleas and now holds a hundred tiny amounts in one small body. A perch eats fifty minnows, and an osprey eats a hundred perch across a season. At every step the toxin from many bodies is packed into one, while most of the food's mass is respired away as carbon dioxide and water — which is the arithmetic of Food chains and food webs again, running in the other direction. The energy shrinks by ten times a level; the toxin does not shrink at all, so its concentration climbs by roughly the same factor.
“If the level in the water is safe, the ecosystem is safe.”
This was the actual reasoning behind several decades of pesticide approvals, and it is why the effect took so long to notice. A safety limit set by testing the water, or by testing a single dose on a single animal, misses two things: that the chemical persists rather than passing through, and that a food chain concentrates whatever persists. The osprey is not exposed to lake water — it is exposed to fifty years of lake water, collected by other organisms and delivered in a fish. Modern approval of a pesticide therefore asks a different question: not only how toxic is it but how long does it last and does it dissolve in fat. Those two properties, not the raw toxicity, decide whether a chemical becomes a problem at the top of a chain.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Which chemicals build up
Which property makes a pollutant likely to accumulate up a food chain?
Rung 2 · The one that catches people
Why is the concentration higher in a perch than in the minnows it eats?
Rung 3 · Explain the build-up
A pesticide measured at 0.003 ppm in a lake is found at 300 ppm in the ospreys nesting there. Explain how the concentration rose by roughly a hundred thousand times, given that nobody added any more pesticide.
Rung 4 · Take it somewhere new
Health advice tells anyone who is pregnant or might be to limit how much tuna and swordfish they eat, because of mercury, while sardines and prawns carry no such warning. Explain the difference using what you know about food chains.
Key note
A toxic substance builds up in a food chain when it cannot be broken down or excreted. Each organism keeps what it takes in, and each predator eats many of the organisms below it, so the concentration multiplies at every level. Animals at the top of long chains are affected first, at concentrations that are harmless in the water and harmless in the organisms lower down.
Going further
DDT was the miracle insecticide of the 1940s: cheap, effective, and credited with saving millions of lives from malaria and typhus. Its inventor received a Nobel Prize. Two decades later, ornithologists noticed that peregrines, ospreys and bald eagles were laying eggs with shells so thin they broke under the weight of the incubating parent, and the populations were collapsing. The chemical was persistent and fat-soluble — the two properties on the bench above — and it had concentrated up every chain it entered. Rachel Carson's Silent Spring put the evidence in front of the public in 1962; DDT was banned for agricultural use in most countries during the 1970s, and the birds recovered. The awkward part of the story is still live: DDT is still used in some countries for malaria control indoors, because the disease kills people now and the ecological cost is judged smaller than the alternative. Both facts are true at once, which is what makes it a real decision rather than a moral.
Before this lesson
Connects to
At GCSE this becomes
- Bioaccumulation and biomagnification, persistent organic pollutants, and the regulation of pesticides.
Where to next
Ask Mr Badmus AI
Want to work out which chemicals would and would not build up?
The concentrations on the bench are round numbers chosen to show the pattern; they are of the same order as figures reported for DDT in lake food chains but are not taken from a particular study. Real accumulation depends on the chemical, the species, the length of the chain and how long the exposure has lasted.
Lesson content © MrBadmusAI.