DDT in a Food Chain – Answers
1. DDT exists in the water supply and enters a food web through producers such as plankton, which absorb the insecticide and incorporate it into their structure.
2. The organisms in the food web with the most DDT are the dolphins with 5200 parts per billion.
3. The dolphins are at the fourth trophic level.
4. The higher an organism is in the food chain (or the higher its trophic level) the more concentrated the DDT will be in its body.
5. The DDT is 430 000 times greater in fish than in the seawater and 52 million times greater in the dolphins.
6. Student answers will vary: it should be noted that the animals at the top of the food chain are at a much greater risk from poisons in the environment. This is due to the fact that they consume prey that have higher concentrations of the poisons in their tissues from organisms at the lower trophic levels.
Conclude and Apply:
7. Student answers will vary: migration should be included in your response. Many animals migrate at specific times of the year. An organism with DDT in its tissues may migrate a distance from the area it was sprayed; it may be consumed by a carnivore that would store a higher concentration of DDT in its body tissue. Migrating birds can carry DDT in their bodies even though they inhabit and can be consumed in an ecosystem that is distant from the one that is sprayed. The peregrine falcon is a species of bird found to have high concentrations of DDT in its body. The habitat of the falcon is quite broad, ranging from the North American boreal forest to areas of the southern United States. Falcon prey on organisms in areas where use of DDT is still prevalent, then migrate to areas where the use of DDT has been banned.
8. Student answers will vary: it should be noted in the answer that all organisms have a certain amount of body fat. DDT is stored and remains toxic in the body fat, eventually affecting DNA. Affected cells become cancerous, affecting the well-being of the organisms and possibly changing the genetic structure of the offspring. If a pesticide were water-soluble it could be excreted with urine through the kidneys rather than being stored in the body tissue. An effective pesticide would produce toxins that are harmful only to the target pest.
Extend Your Knowledge and Skills:
9. Student answers will vary: your answer should include the possibility that the insects were preyed on by other organisms and experienced a reduction in their population as a result. DDT could have resulted in the death of the insect’s competitors or predators either of which could have allowed the insect population to recover.
10. Did not have to complete
11. Student answers will vary but should include the following:
(a) Seabird eggs taken from the Bay of Fundy appear to have a higher concentration of DDT than those taken from the Atlantic Ocean. Possible reasons for the differences might include the fact that the Atlantic Ocean is more distant from agricultural areas where DDT might be in use, compared with the Bay of Fundy, which is surrounded by agricultural areas. Near the Bay of Fundy, the organisms on which the seabirds feed could be more highly concentrated and easy to catch in the Bay of Fundy than in the Atlantic Ocean.
(b) It appears that the pesticide levels decreased between the late 1960s and the early 1980s. Use of DDT in Canada became restricted in 1969; a decrease after that is expected.
(c) Different species of seabirds have differing amounts of body fat, where DDT is stored. Species with the greatest body fat could also have the highest concentrations of DDT. The storm-petrel and the double-crested cormorant seem to have the highest concentrations of DDT. The double-crested cormorant also feeds on much larger organisms, which have higher concentrations of DDT in their body tissue than the food sources of the other bird species.