Food chains, webs and the arrow that trips students

A food chain shows the transfer of energy from one organism to the next, beginning with a producer. The single most-marked error is the arrow direction: arrows point from the organism being eaten to the organism that eats it, because they show the direction of energy flow, not 'who eats whom'.

Learn the terms precisely:

  • Producer: an organism that makes its own organic nutrients, usually by photosynthesis (e.g. a plant).
  • Consumer: gets energy by feeding on other organisms; primary consumers (herbivores) eat producers, secondary consumers eat primary consumers, and so on.
  • Decomposer: gets energy by breaking down dead organisms and waste (e.g. bacteria and fungi).

A food web shows interconnected food chains. The trophic level is the position of an organism in a food chain. Examiners reward correct trophic-level naming (producer, primary consumer, etc.) and correct arrows. Get both and these are free marks.

Energy flow: explaining the losses

The high-value ecology mark is explaining why energy decreases along a food chain. Energy enters food chains as light energy, captured by producers in photosynthesis. At each transfer, much energy is lost, so less is available to the next level. The reasons examiners want:

  • Energy is lost as heat from respiration.
  • Energy is lost in waste/faeces and urine (excretion/egestion).
  • Not all of an organism is eaten or digested.

Because energy is lost at each stage, food chains are usually short (rarely more than four or five links). There is not enough energy to support more levels. This also explains why there is usually a greater biomass/number of producers than top consumers. (Extended only) This energy loss is why it is more energy-efficient for humans to eat plants (crops) directly than to feed crops to animals and eat the animals. A frequent food-supply application. State the reasons explicitly; 'energy is just lost' without naming respiration/heat caps your marks.

Pyramids of numbers, biomass and energy

Three pyramids appear, and examiners test which is most reliable:

  • Pyramid of numbers: counts organisms at each level. It can be irregular (e.g. one large tree supporting many insects gives a narrow base), so it is not always pyramid-shaped.
  • Pyramid of biomass: shows the mass of living material at each level; almost always a true pyramid shape because biomass decreases up the levels.
  • Pyramid of energy (Extended). Shows energy at each level; always a perfect pyramid because energy is always lost between levels.

The exam skill is explaining anomalies: a pyramid of numbers can be inverted when a single large producer supports many small consumers, but a pyramid of biomass corrects for size. When asked which is most reliable, the answer is the pyramid of energy, because energy is always lost up the chain. Always read the axis label to know which pyramid you are dealing with.

The carbon cycle

The carbon cycle is examined as a set of processes that add or remove carbon dioxide from the air. You should be able to state each:

  • Photosynthesis removes CO₂ from the atmosphere (carbon fixed into glucose by producers).
  • Respiration (by plants, animals and decomposers) releases CO₂ back into the air.
  • Decomposition by bacteria and fungi releases CO₂ as they respire while breaking down dead matter.
  • Combustion (burning) of fossil fuels and wood releases CO₂.
  • Feeding transfers carbon (as compounds) along the food chain.

Examiners often give a diagram with blanks and ask you to name the process for each arrow. The two most-confused are photosynthesis (removes CO₂) versus respiration/combustion (add CO₂). Make sure the direction matches. (Extended only) Increased combustion of fossil fuels raises atmospheric CO₂, contributing to the enhanced greenhouse effect and climate change. Photosynthesis links to plant nutrition and respiration to respiration.

The nitrogen cycle and populations (Extended)

(Extended only) The nitrogen cycle has named stages and named bacteria. A common exam blank-filling exercise:

  • Nitrogen-fixing bacteria convert nitrogen gas into nitrogen compounds (some in root nodules of legumes).
  • Decomposers break down proteins in dead organisms, releasing ammonium compounds.
  • Nitrifying bacteria convert ammonium compounds to nitrites then nitrates.
  • Denitrifying bacteria convert nitrates back to nitrogen gas (reducing soil fertility).
  • Plants absorb nitrate ions to make amino acids and proteins.

The exam trap is muddling nitrifying with denitrifying. Remember denitrifying removes nitrates from soil. On populations, define a population as all the organisms of one species in an area, and a community as all the populations together. Population growth shows a sigmoid (S-shaped) curve: a lag phase, an exponential/log phase, a stationary phase (limited by food, space, predators, disease) and sometimes a death phase. To practise these cycle diagrams with a specialist, take a free trial class.