Distinguish between two named biomes and the factors that cause their distribution.

Evaluate one method for measuring primary productivity in a named ecosystem.

Discuss how human activities impact the flows and stores in the nitrogen cycle.

e.g. tundra vs tropical rainforest:

Distinguishing features [3 max]:
tundra has lower insolation / TRF has the highest insolation of all biomes;
TRF has constant insolation throughout the year / tundra has long, dark winters;
tundra has lower mean annual temperature / TRF has warmer temperatures;
tundra has 6–10 months of freezing temperatures / TRF has constant warm temperatures;
TRF has the largest annual precipitation compared to any biome / tundra precipitation is as low as in deserts;
TRF has almost constant precipitation throughout the year;
tundra precipitation mostly in form of snow / tundra has a characteristic layer of frozen ground below the surface/permafrost;
TRFs have the highest biodiversity of all biomes;

Explanation of distribution: [2 max]
tundra found at the poles, while TRFs at the tropics (above and below the equator);
Hadley cell rises at the Equator causing huge precipitation (low pressure zone) / as warm air cools and its moisture condenses;
tundra is found in the low pressure area of polar cell / at a region where there is net loss of solar energy (causing freezing temperatures) / alpine tundra found on mountaintops, where temperatures are below 0 most time of the year due to high altitude;

Notes: Award [3] max for correctly indentified distinguishing features i.e. insolation / temperature / precipitation / permafrost / high biodiversity etc.
Award [2] max for explanation of distribution.
Award [2] max overall if no biomes are named.

Alternative 1:
Method:
light and dark bottle for an aquatic ecosystem;
measure dissolved oxygen at start and end of experiment;
compare measurements in a transparent (with light) and opaque (without light) bottle containing sample of water from ecosystem;
net productivity is equivalent to change in dissolved oxygen in light bottle;
gross productivity is equivalent to change in dissolved oxygen in light bottle plus the loss of dissolved oxygen in the dark bottle (due to respiration);
measurements taken for a set period of time, eg one week;

Evaluation:
simple, easy to conduct method;
ethical method – samples can be returned to ecosystem;
difficult to isolate primary producers from consumers in ecosystem sample;
only collecting productivity for submerged subset of ecosystem;
measurements dependent upon temperature;
quality of measurements depends on precision of instruments;

Alternative 2:
Method:
three comparison plots, one covered in opaque plastic for terrestrial ecosystem;
measure dry biomass at start and end of experiment;
compare measurements from an open (with light) and covered (without light) plot in ecosystem;
net productivity is equivalent to change in biomass in open plot;
gross productivity is equivalent to change in biomass plus the loss of biomass in the covered plot (due to respiration);
measurements taken for a set period of time, eg one week;

Evaluation:
easy to isolate primary producers in ecosystem sample;
difficult to collect all biomass;
ethical problems as samples need to be killed to measure dry biomass;
measurements dependent upon temperature;
productivity easier to measure in simpler systems;
difficult to measure with larger producers/trees;

Notes: Award [3] max for description of method. Evaluation should address inherent strengths and weaknesses of method. Do not credit examples of poor execution eg inaccurate measurement/samples too small etc.
Award [6] max if ecosystem is not named.
No credit can be given to responses that mistakenly address eg biodiversity rather than productivity.

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 systems approach; flows and (biotic and/or abiotic) stores in nitrogen cycle; atmospheric content; farming practices (aquatic and terrestrial); soil; eutrophication; urbanization, deforestation; transportation; forest fires; use of fossil fuels;
• breadth in addressing and linking climate change; photochemical smog; secondary pollutant; acid deposition; scrubbers/catalytic converters; renewable vs. non-renewable energy sources; population growth; EVSs; sustainable development;
• examples of farming practices (aquatic and terrestrial) which affect nitrogen flows; eutrophication/pollution management strategies; specific human activities causing atmospheric pollution;
• balanced analysis discussing activities which increase nitrogen flows and stores, as well as decreasing or managing these flows and stores;
• a conclusion that is consistent with, and supported by, analysis and examples given eg probably the greatest human disturbance to steady state equilibrium in the nitrogen cycle is the increase of inorganic stores such as nitrogen oxides in the atmosphere and nitrates in aquatic systems.

Q4 was by far and away the least popular question and, when attempted, was certainly found to be the most challenging. This seemed largely because candidates did not have a clear idea of how to measure primary productivity in a system or to identify human impacts on the nitrogen cycle. Q(4) (a) Most candidates could give sufficient distinguishing features of two named biomes.

Very few candidates showed awareness of suitable methods for measuring primary productivity of a system and, of those that did, they tended to just describe such methods and not "evaluate" them.

It was clear from most responses that candidates were struggling to link in many issues of atmospheric pollution, eutrophication etc to an account of the nitrogen cycle. They knew aspects of the natural cycle but seemed unfamiliar with human impacts on that cycle.