Our Environment · Lesson 4 of 8
Biological Magnification
“Tiny amounts of persistent chemicals can become a very big problem by the top of a food chain.”
• Explain how pesticides and other persistent chemicals can enter food chains. • Define biological magnification accurately. • Explain why concentrations of non-degradable chemicals increase at higher trophic levels. • Analyse why top consumers can receive the greatest chemical burden. • Connect pesticide use with residues found in common foods.
A food chain transfers more than food energy. Substances present in soil, water or living tissues can also move from one organism to another. This becomes dangerous when a chemical is not readily broken down and is repeatedly transferred upward through a food chain.
Entry Of Harmful Chemicals Into Food Chains
Pesticides and other chemicals used to protect crops can be washed into soil or water bodies. Plants may absorb them from soil along with water and minerals. Aquatic plants and animals can take them up from polluted water. Once the chemical enters living tissue, feeding can carry it to the next trophic level.
Non-degradable Chemicals
Some chemicals are not readily degraded. Instead of being broken down and removed quickly, they can persist in organisms and move from one trophic level to another. Because a consumer may eat many organisms from the level below, the amount accumulated in its body can become greater.
Biological Magnification
Biological Magnification is the progressive increase in the concentration of non-degradable harmful chemicals at successive trophic levels of a food chain.
The key word is concentration. The phenomenon is not simply that the same chemical is present at every trophic level. The concern is that its concentration becomes progressively higher toward the top of the food chain.
Why Higher Trophic Levels Are More Affected
A top consumer obtains food from many organisms below it. If each prey item contains a small amount of a persistent chemical, repeated feeding can deliver a larger total amount to the predator. Because the chemical is not easily degraded, it accumulates.
Humans often occupy high trophic positions in food chains. This helps explain why pesticide residues can be found in foods such as wheat, rice, vegetables, fruits and meat, and why exposure can continue through many food sources.
A Reasoning Strategy For Magnification Problems
Identify the pollutant and confirm that it is persistent or non-degradable. Write the food chain in order. Track the pollutant from the environmental source into producers or aquatic organisms and upward through feeding. The highest trophic level is expected to have the greatest concentration.
Problem
A pesticide enters soil and moves through grass → goat → human. Which level is expected to have the greatest concentration?
- 1.The pesticide enters the producer, grass.
- 2.Goat feeds on grass and receives pesticide through feeding.
- 3.Human occupies the highest trophic level in this simplified chain.
- 4.Because the pesticide is persistent, its concentration becomes greatest at the highest level.
- 5.Therefore the human is expected to have the greatest concentration.
Problem
A persistent chemical enters a pond, then aquatic plants, small fish and large fish. Explain the trend.
- 1.The chemical first enters organisms at a low trophic level.
- 2.Small fish obtain it through feeding.
- 3.Large fish consume many contaminated prey organisms.
- 4.The chemical is not readily degraded, so it accumulates.
- 5.Concentration therefore becomes greater at higher trophic levels.
Problem
Why can pesticide exposure not always be eliminated by washing food?
- 1.Some residues may be on surfaces and may be reduced by washing.
- 2.But chemicals can also be absorbed into plant or animal tissues.
- 3.A chemical inside tissues is not simply removed by rinsing the outside.
- 4.Therefore washing cannot always eliminate residues that have entered the food chain.
Pesticides And Food
Pesticide residues may reach prepared foods through ingredients that already contain traces. The same chemicals can reach humans through grains, vegetables, fruits or animal products. Reducing exposure therefore requires thinking about the whole pathway from chemical use to food production.
Food energy becomes smaller at higher trophic levels, but concentrations of persistent chemicals can become larger. These are opposite trends because energy is used and lost, whereas a non-degradable chemical can accumulate.
Quiz
What is biological magnification?
How can a pesticide first enter a terrestrial food chain?
Why can top consumers contain the greatest concentration of a persistent chemical?
Which statement correctly compares energy and a persistent pollutant along a food chain?
Why may washing food fail to remove all pesticide residues?
Practice Problems
- Trace how a pesticide applied to a crop field could reach a human through a three-step food chain.
- Explain why a non-degradable chemical behaves differently from ordinary food energy as it moves through trophic levels.
- An aquatic pollutant enters algae, then small fish, then large fish, then humans. Rank the trophic levels from expected lowest to highest pollutant concentration and explain.
- A student says, 'There is less energy at higher trophic levels, so there must also be less pesticide.' Explain why this is incorrect.
- Suggest two broad ways pesticide exposure might be reduced, using the pathway of entry into food chains as the basis for your reasoning.
Key Takeaways
• Persistent pesticides and other chemicals can enter food chains through soil and water. • Non-degradable chemicals can remain in organisms and pass from one trophic level to the next. • Biological magnification is the progressive increase in concentration of such chemicals at higher trophic levels. • Top consumers are especially vulnerable because they feed on many organisms from lower trophic levels. • Pesticide residues can occur in grains, vegetables, fruits and meat and cannot always be removed simply by washing.