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Date November 2020 Marks available 1 Reference code 20N.2.HL.TZ0.1
Level Higher level Paper Paper 2 Time zone TZ0 / no time zone
Command term Outline Question number 1 Adapted from N/A

Question

White clover (Trifolium repens) is native to Eurasia but is now a common plant found worldwide in lawns, next to roads, in pastures and similar habitats.

[Source: © International Baccalaureate Organization 2020.]

Some T. repens plants are able to produce the toxin hydrogen cyanide (HCN) by cyanogenesis. A study at 128 sites (2509 plants) in Toronto (Canada) looked at the proportion of T. repens plants producing HCN. The sites were at regular intervals from the city centre towards rural areas.

[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]

To determine whether similar patterns in cyanogenesis were seen in other cities, studies were also carried out around New York City and Boston (USA) and Montreal (Canada).

[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]

The researchers considered two possible ecological causes for the observed gradients in cyanogenesis. First, the researchers assessed the extent of herbivory of T. repens plants in Toronto by measuring the percentage of leaf area that was eaten. The graph shows the results for both cyanogenic and non-cyanogenic plants.

[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]

The researchers then investigated a possible correlation between cyanogenesis and exposure to freezing conditions. It had been proposed that when a cyanogenic plant freezes, its cells burst, releasing HCN which is toxic to the plant. Snow can insulate the ground and plants from freezing temperatures. However, snow is more likely to melt in cities, which then exposes plants to freezing temperatures.

All four of the cities studied receive below freezing temperatures and winter snowfall. Researchers looked at the number of days below freezing (0 °C) that did not have snow cover in these cities.

[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]

State the distance from the city centre at which the highest proportion of plants sampled contained HCN.

[1]
a.i.

Outline the relationship shown in the graph.

[1]
a.ii.

Deduce whether the pattern of cyanogenesis was the same in all of the areas around all four cities.

[2]
b.

Discuss whether the data supports the hypothesis that the gradient in cyanogenesis is due to its benefits against herbivory in rural areas.

[2]
c.

Identify with a reason the city where the plants were more insulated from freezing temperatures.

[1]
d.i.

Using all of the data so far, suggest whether exposure to freezing temperatures in the four cities is supported as a reason for the differences in HCN production in T. repens.

[2]
d.ii.

Markscheme

32 (km);

Accept answers in the range of 31 to 33 (km).

a.i.

positive correlation / (proportion with) HCN increases as distance increases;

a.ii.

a. Toronto, NYC and Boston show same pattern/all show positive correlation/relationship/WTTE;

b. Montreal shows negative correlation/negative relationship/WTTE so is different;

b.

a. (hypothesis) not supported;

b. large overlap/little difference between cyanogenic and non-cyanogenic (in herbivory);

c. smaller difference between cyanogenic and non-cyanogenic in rural areas;

d. both show negative correlation between herbivory and distance from city center/same trend;

e. some support/hypothesis partly supported by lower herbivory in cyanogenic (at all distances).

Do not accept ‘No’ unqualified as an answer.

Do not award mpe if the answer states that the hypothesis is supported without doubt/fully.

c.

Montreal because it has the lowest number of days below 0 °C without snow cover;

Do not award the mark for Montreal if the reason is not given.

d.i.

a. cities with more days without snow cover have positive correlation between distance from city center and HCN / vice versa for Montreal;

b. fewer plants with HCN within cities that have more days without snow cover/have more exposure to freezing temperatures / converse for Montreal which has fewer days without snow cover;

c. HCN is 0.2 (or less) HCN in cities that have more days without snow cover proportion whereas city with fewer days/Montreal it is 0.5/more than 0.4;

d. in Toronto cyanogenic and non-cyanogenic plants show little difference in herbivory;

e. support for hypothesis/exposure to freezing temperatures as reason.

Do not accept ‘Yes’ unqualified as an answer, but accept it if supported by reasoning.

d.ii.

Examiners report

This was intended to be an easy first mark but only 70 %  answered it correctly, with others mostly giving the maximum distance rather than the distance with the highest proportion of plants containing cyanide.

a.i.

This was answered more successfully with 90 % of candidates gaining the mark. The answer ‘positive relationship’ was not enough and either ‘positive correlation’ or description of it were required.

a.ii.

Here candidates were expected to state that the four cities did not all show the same pattern, as three have a positive correlation, but the fourth city (Montreal) has a weak negative correlation. A few failed to include Toronto among the cities with a positive correlation and instead singled it out for showing a stronger correlation than the others.

b.

This was possibly the hardest part of the data-based question. Candidates were spread approximately equally between 0, 1 and 2 marks. Many candidates did not realise that the differences between herbivory in cyanogenic and non-cyanogenic plants were unlikely to be significant, especially in the rural areas that were furthest from the city. The hypothesis stated in the question was therefore not supported. Candidates should be encouraged to consider both the closeness of means and also the spread, when analysing data.

c.

90 % of candidates identified Montreal as the city where plants were more insulated because it was where there were fewer freezing days without snow cover.

d.i.

Many candidates found this to be another difficult question. There was a lot of data to consider and as in (c) it was necessary to take into account whether differences were likely to have been significant. In the histogram showing the amount of cold experienced, the large error bars show that only differences between Montreal and the other three cities are great enough to be considered reliable. When combined with data for the proportion of plants with HCN in each city, it was clear that there was support for exposure to cold as a reason.

d.ii.

Syllabus sections

Core » Topic 4: Ecology » 4.1 Species, communities and ecosystems
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