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Date	May 2019	Marks available	3	Reference code	19M.2.SL.TZ2.3
Level	Standard level	Paper	Paper 2	Time zone	Time zone 2
Command term	Suggest	Question number	3	Adapted from	N/A

Question

State two causes of the decrease of biomass along food chains in terrestrial ecosystems.

[2]

a.i.

The table shows the global carbon budget over two decades; the years 1990 to 1999 and 2000 to 2009.

[Source: © International Baccalaureate Organization 2019]

Using the table, explain causes of the changes in carbon flux over the two decades.

[3]

a.ii.

Suggest how climate change can influence the natural selection of organisms that live in the Arctic oceans.

[3]

Markscheme

a. «cell» respiration/loss of CO₂/biomass consumed to provide/as a source of energy ✔

b. loss of energy «as heat» between trophic levels means less energy available for building biomass ✔

c. waste products «other than CO₂»/loss of urea/feces/egesta ✔

d. material used/CO₂ released by saprotrophs ✔

e. undigested/uneaten material «teeth, bones, etc»/detritus buried/not consumed
OR
formation of peat/fossils/limestone ✔

a.i.

a. increased CO₂ flux to the atmosphere due to increased burning of fossil fuels by industry/transportation / cement production ✔

b. «land use change leading to» decreased rate of forest burning
OR
better fire suppression leading to decrease in CO₂ release
OR
example of land use changes that uses less fossil fuel
OR
increase in land covered by forests/plants / forests recovering from historical forestry
OR
any other reasonable explanation of land use change that would lead to decreased rate of carbon flow to atmosphere ✔

c. carbon storage in land decreased as less photosynthesis due to fewer forests/more construction
OR
release of methane due to «drying of» wetlands/sealing of land with concrete/buildings/roads ✔

d. carbon storage in ocean increased due to more photosynthesis/algae/greater concentration of CO₂ in the atmosphere
OR
increased diffusion/rate of dissolving of CO₂ into ocean from the atmosphere
OR
limestone/carbonate accumulation «more snails» ✔

a.ii.

a. individuals in a population will show a variation of adaptations to climate change ✔

b. organisms that resist temperature changes
OR
current changes of the ocean/melting ice/more acidity/changes in food chains will survive better ✔

c. reproduce more and pass on their characteristics ✔

d. organisms with less adaptation will disappear with time ✔

e. example «eg polar bears have less ice to be able to catch prey/seals and are starving the ones that manage to find other food sources will survive» OWTTE ✔

f. changes will occur within species
OR
new species may appear «over time» ✔

Accept any valid example of an Arctic ocean organism.

Examiners report

A large proportion of students looked at this question and immediately reeled off an answer in terms of energy (as in many previous years). Few made the connection between biomass and energy, for example in respiration CO₂ is lost from the food chain during respiration, or that urea has mass etc.

a.i.

3a(ii) was the question most discussed on the G2 comments. The expectation was that the students would address each of the four arrows, with an explanation for each. Good candidates did this. Others just combined everything into a general essay on climate change. Weaker candidates just restated the figures without any explanation of them at all. Whilst it is appreciated that none of the candidates was around between 1990 and 1999, references to the Industrial revolution were out by well over a century. This question would have benefitted from a larger answer box.

a.ii.

In 3b weaker candidates seemed to think that they needed to give a detailed description of climate change and filled up most of the box with it. Better candidates were able to state that animals who could cope with warmer surroundings, would survive to breed and pass on their characteristics, while those who could not would perish, and give an example such as the polar bear. There were some G2 comments that Arctic foodwebs were not on the syllabus. It is doubted that any IB student does not know that it is cold in the Arctic, but because of climate change it is warming up slowly.

Syllabus sections

Core » Topic 4: Ecology » 4.4 Climate change

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