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Date	November 2021	Marks available	1	Reference code	21N.2.SL.TZ0.1
Level	Standard level	Paper	Paper 2	Time zone	TZ0 / no time zone
Command term	Deduce	Question number	1	Adapted from	N/A

Question

The incidence of white syndrome, an infectious disease of coral, was investigated in a six-year study on Australia’s Great Barrier Reef. The map shows disease conditions on coral reefs at six study sites.

[Source: adapted from Bruno, J.F., Selig, E.R., Casey, K.S., Page, C.A., Willis, B.L., Harvell, C.D., Sweatman,
H. and Melendy, A.M., 2007. PLOS Biology, [e-journal] 5(6), e124. https://doi.org/10.1371/journal.pbio.0050124.]

Satellites were used to record sea surface temperatures. The temperatures each week above a reef were compared with mean temperatures for that week between 1985 and 2004. If the sea surface temperature was 1 °C or more above the mean, this was recorded as a weekly sea surface temperature anomaly (WSSTA). The number of WSSTAs was calculated for the twelve months preceding the date on which a reef was surveyed for white syndrome.

On each reef, the number of cases of white syndrome in a 1500 m² sample area was surveyed once per year. The table shows these cases in relation to numbers of WSSTAs and coral cover on the reef. Low coral cover was 0–24 % and high coral cover was 50–75 %.

The graphs show the relationship between the weekly sea surface temperature anomalies (WSSTA) and coral cover during two twelve-month periods (1998–99 and 2002–03), which were the warmest in the six-year study. Each dot represents one studied reef.

Identify the site with the highest incidence of diseased colonies.

[1]

a.i.

Deduce whether there is a trend in the incidence of white syndrome over the north-south range of latitude.

[1]

a.ii.

Describe the evidence that is provided by the data in the table for the harmful effects of rising sea temperatures on corals.

[2]

b.i.

The researchers concluded that there was a threshold coral cover percentage, below which infection rates tended to remain fairly low. Using the data in the table, identify this threshold level.

[1]

b.ii.

Suggest a reason for a larger percentage of corals being infected with white syndrome on reefs with a higher cover of corals.

[1]

b.iii.

Compare and contrast the data for 1998–1999 and 2002–2003.

[2]

c.i.

Suggest a reason for the correlation between coral cover and WSSTA in 1998–1999.

[1]

c.ii.

Some scientists predict that, if humans continue to produce carbon dioxide at the current rate, the pH of the oceans will become more acidic. Suggest possible effects on the coral reefs.

[3]

This study was carried out over six years on the Australian Great Barrier Reef. State one advantage of field investigations compared with mesocosm experiments to study ecological processes.

[1]

Markscheme

Capricorn and Bunkers group;

a.i.

there is no clear trend;

a.ii.

a. effects (generally) increase with temperature anomalies/WSSTA;
b. effects increase for all different coral covers
OR
in the 25-49 cover there is an anomaly/is not an increasing trend;
c. harmful effect is higher in coral covers between1 50-75 % / vice versa;

b.i.

50 % / 25 % / 49 %
OR
25–49 % (coral cover);

b.ii.

a. the closer the corals are (to each other) the easier the transmission;
b. other organisms in the community may act as vectors/carriers / OWTTE;

b.iii.

Similarities
a. similar range of cover in both periods
OR
weak correlation/no correlation in either;

Differences
b. higher WSSTA range/more WSSTA in 1998–1999 than 2002–2003
OR
positive (2002–2003) versus negative correlation (98–99)
OR
more coral reefs with very low % of coral cover (0–20 %) in 2002–2003;

Must respond with one similarity and one difference for full marks.

c.i.

coral cover lower on reefs with higher temperature;

c.ii.

a. corals would not be able to maintain their skeletons;
b. (if coral reefs are lost) habitat will be lost;
c. some organisms/coral can decline if the water becomes too acidified;
d. coral bleaching could occur/become worse
OR
coral could expel their mutualistic alga/zooxanthellae;
e. enzymes could be denatured;

real environmental conditions / larger scale investigations / more variables studied;

Examiners report

In general, the data analysis question was well attempted by the majority of students. In 1a almost all were able to correctly name the Capricorn and Bunkers Group.

a.i.

[N/A]

a.ii.

In 1b many just described the incidence at various points, instead of stating that there was no clear trend. There were a few G2 comments that the map did not include compass points, which, perhaps, has some justification. However, by convention maps are usually shown with North at the top.

The expression ‘coral cover’ did confuse some of the weaker students who imagined that the corals somehow has a sort of cover over them. Most students gained at least 1 mark in 1b (i), despite the criticism from the G2 forms that it was very complex.

b.i.

Most were able to able to gain the mark in 1b (ii) for saying that the threshold level was 25 – 49 %.

b.ii.

In 1b (iii) the better students realised that it was to do with higher density of corals, not just the more vague ‘higher cover’ given in the stem.

b.iii.

In 1c contrasts were common, but few gained the comparison mark as well.

c.i.

[N/A]

c.ii.

In 1d weaker students incorrectly wrote about the sea becoming acidic as the pH was increasing (sic). Indeed, it is becoming more acidic, as the pH is decreasing slightly, leading to more bleaching and the decrease in population of some creatures.

In 1e most were able to give a reason why the study was better than a mesocosm.

Syllabus sections

Core » Topic 4: Ecology » 4.4 Climate change

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