DP Biology (last assessment 2024)

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Question 23M.3.HL.TZ2.17

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Date May 2023 Marks available [Maximum mark: 5] Reference code 23M.3.HL.TZ2.17
Level HL Paper 3 Time zone TZ2
Command term Describe, Outline Question number 17 Adapted from N/A
17.
[Maximum mark: 5]
23M.3.HL.TZ2.17

The diagram represents primary succession that occurs in an Arctic ecosystem, on a river floodplain in Alaska, USA. Permafrost is permanently frozen subsoil found in Arctic regions.

[Source: Bonanza Creek LTER.]

(a)

Outline primary succession.

[1]

Markscheme

colonization of areas that were barren/not previously inhabited/not occupied by organisms;

Examiners report

Not universally well answered as several candidates were unable to outline primary succession in terms of an area that was previously not inhabited. 

(b)

Describe two limiting factors on this ecosystem.

[2]

Markscheme
  1. light limits photosynthesis
    OR
    daylength limits growth season;
  2. (low) temperature limits growth/slows metabolism
    OR
    (low) temperature slows activity of decomposers;
  3. (lack of) water due to frozen water/permafrost
    OR
    (lack of) water/rain limits photosynthesis/productivity;
  4. nutrients (in permafrost)/quality/depth of soil limit production of organic molecules / OWTTE
    OR
    insufficient soil / permafrost for tree growth;

Award marks only for factors accompanied by a description.

Examiners report

It was difficult for many to describe instead of listing limiting factors; many referred to factors not relevant to the given arctic ecosystem and failed to see the link with permafrost and water being frozen. 

(c)

Outline processes that must occur over time to produce deeper soil.

[2]

Markscheme
  1. weathering of rock accumulates matter (allowing pioneer species to grow);
  2. decay/decomposition of organisms increase organic matter/detritus/litter;
  3. increased soil allows larger/more plants to grow / roots preventing erosion;
  4. as more plants are decomposed, soil depth/amount of soil available increases;
Examiners report

Outlining the formation of soil was also difficult for many who could only mention decomposition.

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