Outline the processes involved in the formation of fertile soils from bare rock.

Explain how negative and positive feedback mechanisms may influence the growth of decomposer populations in the soil.

To what extent are natural limiting factors more likely than population policies to limit global human population growth in the future?

(mechanical/chemical) weathering/breaking up of parent rock producing small particles;
deposition of sediment/eroded material (through wind/water) increasing soil depth;
dissolving of particle material to release soluble minerals;
colonization of parental rock/sediments by plants/animals/decomposers/pioneer species;
biological/atmospheric nitrogen fixation adds available nitrogen (nitrates, ammonium) to soil;
activity of mycorrhizal fungi/decomposition of dead organisms/leaf litter to release mineral/nutrients/organic material/humus;
growth of mosses form mats that stabilize soils in sterile/barren ecosystems;
earthworms/burrowing insects spread soil particles/open soil pores;
precipitation adding water to soil;

negative feedback occurs when the output of a process inhibits or reverses the same process / in such a way as to reduce change/counteract deviation/maintain equilibrium;

positive feedback occurs when the output of a process accelerates that same process (will tend to amplify changes) / deviate away from equilibrium / or drive the system toward a tipping point (adopting a new equilibrium);

Negative feedback (occurring at carrying capacity):
growth of decomposer populations will reduce available organic material;
…thus leading to reduced growth in population;
growth of decomposer population may lead to increase in predators feeding on decomposers;
…thus leading to reduction in population growth;

Positive feedback (occurring during phase of exponential growth):
increase in number of decomposers will increase potential for reproduction;
…thus leading to increased rate of population growth;
increase in decomposers may favourably modify environment (e.g. increase temperature/improve soil);
…thus leading to increased rate of population growth;

Note: Candidates may answer with the help of a diagram (e.g. feedback-links diagram) for which credit should be given.
Award [2] max for each valid feedback loop correctly identified.
Only give credit if a complete loop is identified i.e. an increase of factor X leading eventually to a decrease or further increase of factor X (or the converse).
Do not credit simple changes in decomposer populations.

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 limits to growth: water supply, food supply, pollution, environmental degradation; disease; carrying capacity; ecological footprints; demographic transition; population/development policies; crude birth & death rates; natural increase rates; population projections; Malthus/Boserup models, etc.;
• breadth in addressing and linking human population growth with range of anti-natal policies, limiting resources, environmental degradation over time and in different locations/nations, etc.;
• examples of potentially limiting natural resources, population policies and populations, etc.;
• balanced analysis evaluating the extent to which natural limiting factors or population policies are most likely to limit global human population growth along with limitations and counterarguments, etc.;
• a conclusion that is consistent with, and supported by, analysis and examples given e.g. “although technological developments are currently going a long way to keep pace with resource availability in line with Boserup’s approach, there has to be an upper limit and ultimately populations will need to reduce their own rate of increase through management policies such as is already happening in some countries”;

Most were able to identify key processes in the formation of fertile soil apparently from their understanding of early succession which was quite appropriate.

Although candidates appeared to grasp the principles of negative and positive feedback they generally couldn't fine the imagination and fresh application to identify examples in the context of decomposers. 

This question was often answered very well including references to many potentially limiting factors, Malthusian principles, Boserup's counterarguments, population policies and their limitations.