Question 20N.3.HL.TZ0.a
| Date | November 2020 | Marks available | [Maximum mark: 2] | Reference code | 20N.3.HL.TZ0.a |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Outline | Question number | a | Adapted from | N/A |
Sea urchins (Strongylocentrotus) are echinoderms that consume large quantities of algae. Sea otters (Enhydra lutris) feed on sea urchins.
The graphs show the biomass of sea urchins in an area of 0.25 m2 plotted against their size. Data were collected in two Aleutian Islands, Amchitka and Shemya. In Amchitka the sea otter population is high while at Shemya there are no sea otters.
[Source: From Estes, J.A. and Palmisano, J.F., 1974. Sea Otters: Their Role in Structuring Nearshore Communities. Science,
185(4156), pp.1058–1060.16. DOI: 10.1126/science.185.4156.1058, https://science.sciencemag.org/content/185/4156/1058.
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Outline a method that could have been used in this study to measure the numbers of sea urchins per 0.25 m2.
[2]
a. with quadrats (of 0.5 m side / 0.25m2);
b. (quadrats) position determined at fixed distance by transects
OR
(quadrats) position determined at random;
c. random sampling / capture-recapture;
d. average number calculated;
Most candidates did well on this question, especially strong ones, although there was evidence that many candidates are not familiar with population evaluation skills. Candidates should be aware that writing "quadrants" (with a "n") is questionable, and that although it may not be assumed that they knew what echinoderms or sea urchins were, the capture-mark-release-recapture method was not really appropriate to evaluate their numbers, given the very limited areas of 0.25 m2.
