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 reliable 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 trial class.

Key terms to connect with this topic

Use these definitions inside explanations and questions rather than memorising them as isolated sentences.

  • Biodiversity: The variety of living organisms in an area, including the diversity of species, genetic diversity within species, and the diversity of ecosystems. High biodiversity indicates a healthy, stable ecosystem, while low biodiversity may indicate environmental stress.
  • Carbon cycle: The biogeochemical cycle in which carbon is recycled between the atmosphere, living organisms, the ocean and the Earth's crust. Carbon dioxide is removed from the atmosphere by photosynthesis and returned by respiration, combustion and decomposition.
  • Carnivore: An animal that feeds on other animals. Carnivores can be secondary consumers (eating herbivores), tertiary consumers (eating other carnivores) or top predators. Examples include foxes, hawks and lions.
  • Community: All the populations of different species living and interacting in the same area at the same time. A community includes all the plants, animals, fungi, bacteria and other organisms in a defined habitat.
  • Consumer: An organism that obtains energy by feeding on other organisms. Primary consumers eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers. Consumers are heterotrophs.
  • Decomposer: An organism that gets its energy from dead or waste organic material, breaking it down and returning nutrients to the environment.
  • Denitrification: The conversion of nitrates in the soil back into nitrogen gas (N2), which is released into the atmosphere. This process is carried out by denitrifying bacteria, which are anaerobic and thrive in waterlogged, oxygen-poor soils.
  • Ecosystem: A unit containing the community of organisms and their environment, interacting together. For example, a decomposing log, a pond, or a rainforest.
  • Eutrophication: The over-enrichment of water with nutrients (from fertiliser run-off or sewage), causing an algal bloom that blocks light, the death and decomposition of plants and algae, oxygen depletion by decomposer respiration, and the death of aquatic animals.
  • Food chain: A diagram showing the transfer of energy from one organism to the next, beginning with a producer. For example: grass → grasshopper → frog → snake.
  • Food web: A diagram showing the interconnected food chains within an ecosystem. A food web is more realistic than a single food chain because most organisms feed on more than one species and are eaten by more than one predator.
  • Habitat: The specific place or environment in which an organism lives, providing the conditions and resources it needs to survive. Examples include a pond, a woodland floor or a coral reef.
  • Herbivore: An animal that feeds only on plants or other producers. Herbivores are primary consumers and occupy the second trophic level in a food chain. Examples include rabbits, caterpillars and cows.
  • Niche: The role of an organism within its ecosystem, including what it feeds on, what feeds on it, and how it interacts with both the living and non-living parts of its environment. No two species can occupy exactly the same niche in the same habitat.
  • Nitrification: The conversion of ammonium compounds in the soil into nitrites and then into nitrates by nitrifying bacteria. This is an aerobic process and is important because plants can absorb nitrogen in the form of nitrate ions through their roots.
  • Nitrogen cycle: The biogeochemical cycle in which nitrogen is converted between various chemical forms as it circulates through the atmosphere, soil and organisms. Key processes include nitrogen fixation, nitrification, denitrification and decomposition.
  • Nitrogen fixation: The conversion of atmospheric nitrogen gas (N2) into ammonium compounds or nitrates that can be used by plants. This is carried out by nitrogen-fixing bacteria, some of which live freely in the soil and others in root nodules of leguminous plants. Lightning can also fix nitrogen.
  • Omnivore: An animal that feeds on both plants and animals. Omnivores can occupy more than one trophic level in a food web. Examples include humans, bears and pigs.
  • Population: A group of organisms of the same species living in the same area at the same time and able to interbreed. Population size is affected by birth rate, death rate, immigration and emigration.
  • Producer: An organism that makes its own organic nutrients from simple inorganic molecules, usually by photosynthesis. Producers are autotrophs and form the first trophic level of a food chain. Green plants and algae are the main producers in most ecosystems.
  • Pyramid of biomass: A diagram showing the total dry mass of organisms at each trophic level in a food chain at a given time. Pyramids of biomass are almost always pyramid-shaped, with the greatest biomass at the producer level and decreasing biomass at each successive level.
  • Pyramid of energy: A diagram showing the total amount of energy flowing through each trophic level in a food chain over a set period of time, measured in kilojoules per square metre per year. Pyramids of energy are always true pyramids because energy is always lost between trophic levels.
  • Quadrat: A square frame (usually 0.25 m squared or 1 m squared) placed on the ground to sample the organisms within it. Quadrats are used with random sampling or along a transect to estimate population size and distribution of organisms in a habitat.
  • Transect: A line placed across a habitat along which organisms are sampled at regular intervals, usually with quadrats. A transect is used to study how the distribution of organisms changes across an environmental gradient, such as from a shoreline to inland or from light to shade.
  • Trophic level: The position of an organism in a food chain, food web or pyramid. Producer, primary consumer, secondary consumer, and so on.

Browse the full IGCSE Biology glossary when a related term needs checking.

Apply the topic in practical work

These practical guides show how the Biology appears in methods, variables, measurements, graphs and evaluation.

  • Sampling with Quadrats: Quadrats are square frames placed on the ground to sample organisms in a defined area. Random sampling uses randomly generated coordinates to place quadrats. Systematic sampling places quadrats at regular intervals along a transect line to study how distribution changes across a habitat.
  • Writing Conclusions and Hypotheses: A hypothesis is a testable prediction that states the expected relationship between the independent and dependent variables. A conclusion summarises what the results show, referencing specific data, and states whether the hypothesis is supported.