Lesson 8 of 9 / Organisms and their environment
Biomagnification along a food chain
How does a persistent insecticide become concentrated up a food chain?
In this lesson: Explain biomagnification using persistence, uptake and repeated feeding.
About 5 min
The key ideaSome persistent insecticides accumulate in organisms and become more concentrated at higher trophic levels as consumers eat many contaminated prey.
Concentration is the amount of a substance per unit mass or volume. Compare pollutant concentration in the organisms, not just the total amount in animals of different sizes.
Explore the idea
Compare concentration, not body size
Producer: 0.02 mg/kg. A small starting concentration can enter a food chain.
Original example: each dot represents 0.02 mg in a 1 kg tissue comparison, not one molecule. The tenfold steps are chosen for this example and are not a universal factor.
Explanation
An insecticide is used to kill insects, but it can reach organisms other than the target. Some insecticides persist in the environment and are not readily broken down or excreted by organisms. Repeated exposure can cause the substance to accumulate within an individual.
A consumer eats many prey over time. If the prey contain a persistent substance, the consumer takes in that substance repeatedly. When uptake exceeds removal and the substance is retained, its concentration can become higher in the consumer than in its prey.
This increase in concentration up trophic levels is biomagnification. Top carnivores may receive the highest concentrations in the chain, increasing risks such as impaired reproduction or toxicity even when environmental concentrations were relatively low.
The prediction depends on the substance. Readily broken-down or excreted chemicals do not necessarily show this pattern. Do not explain biomagnification simply by saying that a larger predator contains more total chemical; the important comparison is concentration per unit mass.
Step by step
- 1
Check persistence
The substance must be retained long enough for repeated uptake to matter.
- 2
Follow repeated feeding
A predator takes in pollutant from many contaminated prey.
- 3
Compare concentrations
Use consistent mass-based units and relate the higher top-consumer concentration to biological harm.
Worked example
An original concentration series
A persistent insecticide is measured at 0.02 mg/kg in plants, 0.2 mg/kg in insects and 2 mg/kg in insect-eating birds. Explain the pattern.
One way to explain it
The concentration increases tenfold at each transfer in these data. Consumers repeatedly eat contaminated organisms, and the insecticide is retained because it is not readily broken down or excreted. It therefore becomes more concentrated at higher trophic levels. The exact tenfold factor belongs to this example, not every food chain.
Why this answer works
- Compare the same units at each level.
- Explain repeated uptake and limited removal.
- Avoid treating the numerical factor as universal.
Is this true? "Biomagnification means a large animal contains more pollutant solely because it is larger."
It means higher pollutant concentration at a higher trophic level. A mass-normalised comparison separates concentration from body size.