Agriculture

A hectare of land can produce grain containing 30 million kJ of food energy. If the grain is instead fed to cattle, explain roughly how much food energy people obtain and why.

1. 01Direct consumption

   Eaten directly, the grain provides people with about 30 million kJ of food energy from the hectare.

2. 02Via cattle

   Feeding the grain to cattle adds a trophic level; only about 10% of the energy is transferred to the animals' edible tissue, so people obtain roughly 3 million kJ.

3. 03Explain

   Energy is lost as heat and in waste at each transfer, so eating plants directly captures far more food energy per hectare than eating animals fed on those plants.

Result: Direct grain gives about 30 million kJ, but via cattle only about 3 million kJ, because energy is lost at the extra trophic level.

A grower's crop is attacked by aphids. Compare using a chemical pesticide with introducing ladybirds, and recommend an approach.

1. 01Chemical pesticide

   Kills aphids quickly and reliably, but may harm pollinators and the ladybirds themselves, can persist and biomagnify, and selects for resistant aphids.

2. 02Biological control

   Introducing ladybirds, a natural predator, avoids chemicals and is self-sustaining, but acts more slowly and less predictably and cannot eliminate the pest completely.

3. 03Recommend

   Use integrated pest management: rely on the ladybirds and monitoring, applying a targeted pesticide only if the aphids exceed a damaging threshold, to control the pest with least harm.

Result: Integrated pest management, favouring biological control and using pesticide only when needed, controls the aphids with least environmental harm.

A river running through intensively farmed land suffers eutrophication and a decline in insect-eating birds. Explain how farming practices could cause both.

1. 01Eutrophication

   Nitrogen and phosphate fertiliser runs off the fields into the river, enriching it and causing an algal bloom whose decomposition removes oxygen (eutrophication).

2. 02Decline in birds

   Pesticides reduce the insect populations the birds feed on, and persistent pesticides can biomagnify and poison the birds directly.

3. 03Link

   Both impacts arise from inputs used to raise yield, showing that intensification concentrates pollution and biodiversity loss.

Result: Fertiliser runoff causes eutrophication and pesticides reduce insects and biomagnify, so both impacts trace to yield-raising inputs.

A region must increase food production while protecting a river from eutrophication. Compare intensifying with adopting sustainable methods and recommend an approach.

1. 01Intensifying

   More fertiliser and pesticide would raise yield per hectare but worsen the runoff that causes eutrophication, so it conflicts with protecting the river.

2. 02Sustainable methods

   Crop rotation, precise fertiliser use, cover crops and buffer strips reduce runoff and protect the river, but may give a somewhat lower yield per hectare.

3. 03Recommend

   Adopt precision and sustainable methods that maintain most of the yield while cutting runoff, combined with reducing food waste, rather than intensifying in a way that would harm the river.

Result: Sustainable and precision methods best reconcile higher production with protecting the river, whereas simple intensification would worsen eutrophication.