Question 21N.2.SL.TZ0.1b.ii
| Date | November 2021 | Marks available | [Maximum mark: 1] | Reference code | 21N.2.SL.TZ0.1b.ii |
| Level | SL | Paper | 2 | Time zone | TZ0 |
| Command term | Identify | Question number | b.ii | Adapted from | N/A |
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 %.
[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.]
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]
50 % / 25 % / 49 %
OR
25–49 % (coral cover);
Most were able to able to gain the mark in 1b (ii) for saying that the threshold level was 25 – 49 %.
