Question 23M.3.HL.TZ2.18
| Date | May 2023 | Marks available | [Maximum mark: 7] | Reference code | 23M.3.HL.TZ2.18 |
| Level | HL | Paper | 3 | Time zone | TZ2 |
| Command term | Deduce, Outline, State, Suggest | Question number | 18 | Adapted from | N/A |
DDE (dichlorodiphenyldichloroethylene) is formed from the breakdown of DDT (dichlorodiphenyltrichloroethane). In a study done in 1969, eggs of the brown pelican (Pelecanus occidentalis) were collected from colonies in California, South Carolina and Florida, USA. The amount of DDE in each egg was quantified and the percentage of eggshell thinning was measured by comparison with the mean thickness measure before the use of DDT from 1947.
The graph shows the relationship between DDE concentration and eggshell thinning in each of the eggs studied.
[Source: Material from: Blus, L., Gish, C., Belisle, A. and Prouty, R., Logarithmic relationship of DDE residues to eggshell thinning,
published 1972, Nature, reproduced with permission of SNCSC.]
State the relationship between DDE concentration and eggshell thickness.
[1]
the higher the DDE concentration, the lower the thickness/higher the thinning
OR
negative correlation/inverse relationship;
Accept vice versa.
It was generally easy for the vast majority to state the relationship between the DDE concentration and eggshell thickness.
Suggest one reason for a decline in the brown pelican population with the most eggshell thinning.
[1]
more risk of predation/ eggs break more easily/fewer chicks hatch;
OWTTE
It was generally easy for the vast majority to suggest one reason for the population decline.
Deduce, giving reasons, which brown pelican population decreased the most in the years following the study.
[2]
- California, as most eggs have a high level of DDE / most eggshells are thinning;
- (California as) less chicks hatching (results in smaller population size)
OR
(California as) birth rate less than death rate (so decrease in population);
Most deduced that the California population would decline the most, but they limited their answer to only one reason.
Outline the biomagnification of DDE in brown pelicans.
[3]
- DDE accumulates in (fat tissues of) organisms/bioaccumulation;
- (brown) pelicans/birds feed on fish/organisms (that have accumulated DDE);
- (DDE) is not excreted;
- (DDE) concentration increases as trophic level increases;
- (brown) pelicans are in a high trophic level (so have a high accumulation);
Although many candidates provided a complete outline of the biomagnification of DDE, many others didn't seem know the process or had difficulty relating it to the pelican population.