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

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

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

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).

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.

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:

• understanding concepts and terminology of negative and positive feedback, steady state and dynamic equilibria, tipping points, resilience, sustainability, colonization, pioneer communities, succession, biodiversity, variety of nutrient and energy pathways, human threats e.g. climate change, eutrophication, deforestation, land degradation, marine pollution, toxic pollution of lakes;
• breadth in addressing and linking negative feedback with stability, steady state, resilience in natural systems mitigating adverse impacts; and positive feedback with dynamic equilibria, growth, succession in restoration as well as amplifying/exacerbating human disturbance and destabilization of systems, etc.;
• examples of organisms, feeding and non-feeding relationships, abiotic & biotic interactions generating negative feedback loops in natural systems and positive feedback promoting population growth, succession (i.e. earlier successional stages modifying environment to allow more and more colonization of later stages), and human impacts leading to positive feedback through e.g. agriculture, unsustainable exploitation, overharvesting, eutrophication, global warming, etc.;
• balanced analysis of the extent to which feedback mechanisms maintain stability and promote restoration in face of human impact with counter examples of positive feedback leading to greater destabilization, or to a new equilibrium (past a tipping point), etc.;
• a conclusion that is consistent with, and supported by, analysis and examples given e.g. “generally, negative feedback is significant in maintaining stability, while positive feedback promotes restoration of plant communities. However, human impacts frequently drive systems beyond their tipping point, and then positive feedback drives the system even further from its naturally stable equilibrium”;

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

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

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.