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Date May 2021 Marks available 4 Reference code 21M.2.SL.TZ0.6
Level Standard Level Paper Paper 2 Time zone Time zone 0
Command term Outline Question number 6 Adapted from N/A

Question

With reference to four different properties of a soil, outline how each can contribute to high primary productivity.

[4]
a.

Explain how the level of primary productivity of different biomes influences their resilience.

[7]
b.

Discuss the role of feedback mechanisms in maintaining the stability and promoting the restoration of plant communities threatened by human impacts.

[9]
c.

Markscheme

particle size affects ability of soil to store/retain water necessary for productivity;
high mineral content provides nutrients for healthy growth/productivity;
high organic content / deep humus provides long term storage of nutrients (released through decomposition);
air spaces provide more O2 to roots for growth/respiration / allow deeper penetration of roots;
appropriate porosity allows soil to hold enough water for plant growth;
better drainage prevents water-logging that inhibits growth/productivity;
abundant biota help to aerate/break up the soil allowing for better root growth/recycle nutrients;
microorganisms contribute to mineral-cycling promoting growth/productivity;
neutral to slightly acidic pH is the optimal for most plants (6.0–7.5);
low or no slope prevents water erosion / loss of soil;

No credit for ground cover reducing wind erosion / soil conservation/management (not soil properties).

a.

resilience is the ability to withstand disturbances / tendency to maintain stability/avoid tipping points;
generally, biomes with higher primary productivity (e.g. rainforests/estuaries/wetlands) are more resilient than those with lower productivity (e.g. tundra/deserts);
more productive biomes can support more species/diversity;
diversity increases resilience because loss of one species is more easily replaced by others;
more productive biomes support more branching food chains / greater complexity of interrelationships;
…that allows for more negative feedback mechanisms/shifting feeding habits maintaining stability/providing more resilience;
more productive biomes produce larger biotic storages;
larger storages are less likely to be eliminated/reduced beyond a tipping point so contribute to greater resilience;
larger storages provide higher maximum sustainable yields so are less prone to overharvesting;
higher productivity entails faster plant growth, thus more effective regeneration after a disturbance;
oceanic biomes have low productivity per unit area but their large size increases their resilience;
coral reefs have high productivity but narrow niche requirements give them low resilience;

Allow credit for valid counterexamples as in last two MPs.

b.

Refer to paper 2 markbands, available under “your tests”  tab > supplemental materials.

The following guide for using the markbands suggests certain features that may be offered in responses. The five headings coincide with the criteria given in each of the markbands (although “ESS terminology” has been conflated with “Understanding concepts”). This guide simply provides some possible inclusions and should not be seen as requisite or comprehensive. It outlines the kind of elements to look for when deciding on the appropriate markband and the specific mark within that band.

Answers may include:

c.

Examiners report

Candidates generally demonstrated considerable appropriate knowledge in response to this question but often struggled to identify four discreet points.

a.

Most were able to link high productivity with high resilience but were often limited in the extent of their explanation.

b.

Candidates struggled with this question, mainly due to a limited grasp of the nature of feedback, in particular, how positive feedback mechanisms can be advantageous in the context of regrowth and re-colonisation through succession. Few candidates could clearly apply concepts of negative or positive feedback to the context of plant communities.

c.

Syllabus sections

Topic 5: Soil systems and terrestrial food production systems and societies » 5.1 Introduction to soil systems
Topic 5: Soil systems and terrestrial food production systems and societies

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