Question 21N.2.HL.TZ0.b
| Date | November 2021 | Marks available | [Maximum mark: 2] | Reference code | 21N.2.HL.TZ0.b |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Compare and contrast | Question number | b | Adapted from | N/A |
A community living in the water of an estuary was used to investigate how climate change may affect ecological systems. The food web in this community included phytoplankton (producers), zooplankton (consumers) and saprotrophic bacteria. Small plastic mesocosms were set up with water from the estuary containing only these three groups of organisms. The mesocosms were subjected to four different temperatures and two nutrient levels (control and nutrients added) to replicate local variations of the conditions in the estuary during springtime warming.
The graph shows the biomass of the community for each of the eight mesocosms at the end of the experimental period. Biomass was measured in terms of the amount of carbon present. The horizontal line indicates the initial biomass.
[Source: adapted from O’Connor, M.I., Piehler, M.F., Leech, D.M., Anton, A. and Bruno, J.F., 2009.
PLOS Biology, [e-journal] 7(9). https://doi.org/10.1371/journal.pbio.1000178.]
The graph shows the mean biomass of autotrophs and heterotrophs in the eight mesocosms. The horizontal lines indicate the initial biomasses.
[Source: adapted from O’Connor, M.I., Piehler, M.F., Leech, D.M., Anton, A. and Bruno, J.F., 2009.
PLOS Biology, [e-journal] 7(9). https://doi.org/10.1371/journal.pbio.1000178.]
Compare and contrast the effects of temperature on the biomass of autotrophs and heterotrophs with added nutrients.
[2]
a. autotroph biomass decreases and heterotroph biomass increases with higher temperatures;
b. decrease in autotrophs is greater/larger/more than increase in heterotrophs
OR
little difference in biomass (between auto and heterotrophs) at highest temperature/27 °C;
c. autotrophs show smaller and smaller gains in biomass from initial as temperature rises/WTTE;
d. heterotrophs no gain in biomass at 21 °C then larger and larger gains as temperature rises;
Most candidates stated that autotroph biomass decreased and heterotroph biomass increased as temperature increased, but this contrast was only given one mark. Far fewer candidates gave a second worthwhile comparison or contrast. Very few candidates realised that at all temperatures the autotrophs had gained biomass from the initial level, but the gains were less as temperature rose. Similarly, few candidates stated that the heterotrophs had not gained mass at 21°C but gained increasing amounts of biomass as the temperature rose.
