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Date November 2020 Marks available 2 Reference code 20N.3.HL.TZ0.15
Level Higher level Paper Paper 3 Time zone TZ0 / no time zone
Command term Outline Question number 15 Adapted from N/A

Question

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. 
Reprinted with permission from AAAS. Readers may view, browse, and/or download material for temporary copying purposes
only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further
reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or in part, without prior
written permission from the publisher.]

Outline a method that could have been used in this study to measure the numbers of sea urchins per 0.25 m2.

[2]
a.

Sea otters are considered keystone species in this environment. Suggest how the presence of sea otters could affect the algae population.

[3]
b.

Sea urchins are destroying the giant kelp alga (Macrocystis pyrifera) marine forests of South East Australia. Suggest a reason that sea urchins might be an invasive species in this environment.

[1]
c.

Markscheme

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;

a.

a. sea otters (feeding on sea urchins) limit sea urchin population;

b. the largest sea urchins are eaten;

c. shown by low biomass/small size (of sea urchins);

d. fewer/smaller sea urchins allow for increase in algae population;

e. sea otters have a top down effect;

Allow converse reasoning.

b.

the sea urchins’ limiting factors in their original habitat are missing
OR
lack of (natural) predators for sea urchins

c.

Examiners report

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.

a.

The majority of candidates could suggest that the sea algae population could be affected by sea otters feeding on sea urchins; most nevertheless did not use the graphs or mention that a top down effect was present.

b.

Many suggested that sea urchins' predators were not present, but too many irrelevant or blank answers were seen.

c.

Syllabus sections

Option C: Ecology and conservation » Option C: Ecology and conservation (Core topics) » C.2 Communities and ecosystems
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Option C: Ecology and conservation » Option C: Ecology and conservation (Core topics)
Option C: Ecology and conservation

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