| Date | May 2011 | Marks available | 2 | Reference code | 11M.3.SL.TZ1.19 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Suggest | Question number | 19 | 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.
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 (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;
Examiners report
Most answered correctly within the range of 162-168 mg m-2.
This part of the question required some reference to the months of the year given in the graph, and many answers simply mentioned seasons, ie spring, summer etc, and so did not gain any marks.
The majority answered correctly.
It seems that many students did not fully understand the question, and so were unable to suggest any explanation for the movement of aquatic invertebrates. Perhaps this is due to a lack of knowledge of invertebrate life cycles,which would help to interpret the data.
