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

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

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.

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. 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.

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

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.

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.

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