Question 21N.2.HL.TZ0.1g
| Date | November 2021 | Marks available | [Maximum mark: 2] | Reference code | 21N.2.HL.TZ0.1g |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Discuss | Question number | g | 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.]
Chlorophyll concentration was used as an estimate of the photosynthetic capacity of the community. The rate of photosynthesis and mass of chlorophyll per unit volume were measured in a mesocosm at three different temperatures.
[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.]
In a larger study, California grassland was exposed to elevated temperature and nitrate concentration for five years. The graph shows the total biomass production in individual and in combined treatments. Error bars denote one standard error.
[Source: adapted from Dukes, J.S., Chiariello, N.R., Cleland, E.E., Moore, L.A., Shaw, M.R., Thayer,S., Tobeck, T.,
Mooney, H.A. and Field, C.B., 2005. PLOS Biology, 3(10), e319. https://doi.org/10.1371/journal.pbio.0030319.]
The first study used mesocosms and the second study was carried out in natural grassland. Discuss the use of mesocosms as opposed to a study in a natural environment.
[2]
advantages of mesocosms/converse problems with studies in natural environments
a. easier to manipulate/control variables/conditions / less susceptible to outside influences
OR
easier to replicate
OR
take up less space;
disadvantages of mesocosms/converse opportunities with studies in natural environments
b. some trophic levels missing/incomplete food chains in mesocosms
OR
large animals cannot be included / ethical concerns about enclosing animals in mesocosms
OR
some variables lacking in mesocosms / doesn’t show what happens in natural ecosystems;
Allow only one mark for an advantage and one mark for a disadvantage as this is a discuss question.
Because this was a ‘discuss’ question, one mark was awarded for arguments in favour or mesocosms and one mark for counter arguments. The best answers weighed up the relative advantage of these two approaches, rather than just singing the praises of one of them.
