Define net secondary productivity.

Identify the data required to calculate the value of net secondary productivity for a named population.

Explain how the first and second laws of thermodynamics are demonstrated as energy from the sun flows through the primary producers in a food chain.

Including reference to their relative efficiency, discuss whether terrestrial or aquatic food production systems show the greatest potential for feeding a growing human population. Support your conclusion with valid reasons or evidence.

Definition:
NSP = the gain by consumers in energy/biomass after deducting/allowing for respiratory losses;

N.B. Do not credit the simple formula NSP = GSP − R, unless GSP is also defined in words ie food/biomass/energy absorbed/assimilated by consumer. Definition must include reference to “consumers” to gain credit.

[1 max]

Data required:
(rate of) food eaten − faecal loss;
(rate of) food absorbtion / gross secondary production;
(rate of) respiration;
per unit area, per time;
biomass/weight at time t+1 minus biomass/weight at time t;

No mark to be given for named consumer, but award [2 max] if population is unnamed or not an appropriate consumer.
Accept a generic name e.g. mouse, but not eg ‘mammal’ or ‘herbivore’.

[3 max]

first law is law of conservation of energy / in transformations energy is conserved / energy is not created or destroyed;
… demonstrated in that all chemical energy comes from light energy / is converted by photosynthesis;
but no new energy is “created”/chemical energy is converted to heat energy but not “destroyed”;
energy entering producers is equal to energy stored + energy dissipated as heat;

second law states that in an isolated system entropy tends to increase spontaneously / in any transformation there is a net increase in entropy/dissipation of energy;
… demonstrated in that while some energy is stored as chemical energy in producer;
… there is a net dissipation of energy / lost as heat through respiration/to environment;
(as for all transformations) efficiency of photosynthesis/conversion of solar to chemical energy is less than 100 % / (often) only 10 % is passed on / 90 % is lost before next trophic level;
producers maintain order/low entropy through this dissipation of high entropy/heat energy/the continuous input of solar energy;

Award [4 max] for responses that only address one law.

[6 max]

Arguments for terrestrial systems having greater potential:
terrestrial systems tend to harvest food at lower trophic levels than aquatic systems;
… so that there is less energy loss along food chain;
… so larger areas/volumes are necessary to produce same total harvest;
aquatic systems tend to be less efficient at trapping/fixing solar energy;
… due to greater reflection/absorption of light by water;
harvesting may be more efficient/easier in terrestrial systems than marine fisheries (e.g. distance travelled by fishing vehicles);
(inland) populations may find efficiency of terrestrial food supply more efficient in terms of convenience/transportation involved;
inland aquacultural systems may have even greater demand on water supply than equivalent irrigated farmland;
there is a far greater variety of producers suitable for farming/harvesting in terrestrial systems;

Arguments for aquatic systems having greater potential:
aquatic systems tend to have more efficient energy conversions along food chains;
ie lower respiratory losses than terrestrial systems;
terrestrial systems require considerable inputs of water/irrigation;
… which may deplete local resources/require energy subsidy;
arable land is becoming limited due to increasing urbanisation/human settlement;
global productivity of marine systems is greater than global productivity of arable land offering greater potential harvest overall;
soils can become degraded through intensive terrestrial farming;
… whereas marine systems tend to be more resilient/readily replenished;
aquatic animal species tend to have higher reproductive rates than terrestrial animals;

Alternative points of equivalent validity, significance and relevance to those given, should be credited.

Award [1 max] for an explicit and valid conclusion.

Note to examiners: An isolated statement e.g. “terrestrial systems show the greatest potential for food production” or an unjustified opinion e.g. “I think aquatic systems are best for food production” should not be considered as a valid conclusion. The conclusion must be supported/justified by points raised that must have at least addressed both sides of argument. A valid conclusion may, however, be stated within the body of the response rather than at the end, and may involve some balanced decision:

e.g. while terrestrial systems provide the greatest potential for food production in terms of quantity, aquatic systems will make a valuable contribution to providing full dietary/feeding needs/quality; [1 max]

Award [5 max] for responses that only explore one side of argument.
Otherwise, award [7 max] for marking points above, and [1 max] for a clear conclusion that is justified by points raised.

[8 max]

Expression of ideas: [2 max]

Generally poor. Candidates were often unable to give a sufficiently precise definition.

Most were able to identify a couple of factors, but rarely gained the full credit available.

Very good accounts of first law of thermodynamics, but much weaker, vaguer application of second law.

Generally OK. Many candidates were able to address significance of trophic levels used and available insolation in terrestrial v aquatic food production, but few went beyond this. Discussions often lacked valid conclusions derived from balanced argument.