| Date | May 2011 | Marks available | 2 | Reference code | 11M.3.HL.TZ1.10 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Describe | Question number | 10 | Adapted from | N/A |
Question
The graph below shows the monthly mean values of terrestrial invertebrates from May 1997 to June 1998 in the northern hemisphere. The light line shows the biomass of invertebrates which are prey to forest birds (terrestrial invertebrate biomass). The darker line shows the invertebrates which lived in the stream and have moved to the forest (aquatic invertebrate flux or movement). The black bars on the horizontal line at the bottom show periods when trees have leaves and the white bars show periods of defoliation.
State the mean terrestrial invertebrate biomass measured in August.
Describe the trend in the aquatic invertebrate flux.
Suggest the relationship between defoliation and the amount of terrestrial invertebrates in the forest.
Suggest a possible explanation for the pattern in aquatic invertebrate flux to the forest seen between the months of June and December.
Allochthonous organisms are those which have migrated from one place to another, such as the aquatic invertebrates in this study. Suggest one effect of allochthonous invertebrates in this environment.
Markscheme
166 mg m–2 (Allow answers in the range of 162–168 mg m–2)
rapid rise and fall between April and August;
peak in May/June;
fluctuates between August/September and December;
low December/January until February/March;
cyclical;
negative relationship/during period of defoliation, biomass/amount (of terrestrial invertebrates) is at its lowest;
less leaves means less food/habitats / easier for predators to see invertebrates;
defoliation occurs in winter/autumn and the cold may kill invertebrates;
(aquatic invertebrate flux) decreases because movement to the forest has occurred (by adult forms) / fewer aquatic invertebrates left in the stream so fewer are moving;
fluctuation due to movement of different species/different life cycles/second generation;
decreases because invertebrates left at the beginning of winter/cold season;
(adult forms) move to utilize (changes in) food supply in forest;
increased/alternative food source for (forest) organisms/forest birds;
decreased food supply for aquatic organisms;
can cause changes in the food webs/pyramids of energy;
Examiners report
Almost all candidates correctly read the graph to find the mean terrestrial invertebrate biomass.
Most candidates were able to correctly describe the trend in the aquatic invertebrate flux for two marks.
The majority of candidates were able to get one mark for seeing the negative relationship between defoliation and biomass and some were able to get a second mark for suggesting why this might be.
Candidates struggled to suggest explanations for the pattern of movement between the aquatic environment and forest. Some were able to get a mark for suggesting adult forms moved to the forest to utilize food supplies. Almost none were able to suggest that fewer were moving into the forest as the adult forms of the invertebrates had already migrated and therefore few were left in the stream or that the fluctuations were due to different life cycles or species. In retrospect, this question did seem too difficult for candidates considering the information they were given.
Candidates also struggled with this section of the question. Although the word allochonthous was briefly given in the question stem, it seemed that candidates were still not able to understand enough to answer the question. A few were able to get the mark for indicating that the aquatic invertebrates that migrated to the forest would provide an alternative food source for the forest birds and others indicated that this would cause changes to the food web.
