Ecological Pyramids (Cambridge (CIE) AS Environmental Management)

Ecological Pyramids

Energy Transfer & Energy Loss in Food Chains

• The transfer of energy in a food chain is not 100 % efficient:

  • Energy is lost to the environment at each trophic level

• When a consumer ingests another organism, not all the chemical energy in the consumer's food is transferred to the consumer's biomass

  • Only around 10% of the energy is available to the consumer to store in their tissues

  • This is because around 90% of the energy is lost to the environment

• So much energy is lost to the environment because:

  • Not every part of the food organism is eaten, e.g. the roots and woody parts of plants or the bones of animals, meaning that the stored energy in these uneaten tissues is lost to the environment

  • Consumers are not able to digest all of the food they ingest, e.g. cellulose in plants or the fur of animals, so some is egested as faeces; the chemical energy in this undigested food is then lost to the environment

  • Energy is lost to the environment in the form of heat when consumers respire

  • Energy is lost to the environment when organisms excrete the waste products of metabolism e.g. urea in urine

• The energy that is left after these losses is available to the consumer to fuel their life functions, including being stored in biomass during growth

Pyramids of Numbers

• Ecological pyramids include:

  • Pyramids of numbers

  • Pyramids of biomass

  • Pyramids of energy

• They are quantitative models usually measured for a given time and given area

• A pyramid of numbers shows how many organisms we are talking about at each level of a food chain

• The width of the box indicates the number of organisms at that trophic level

• For example, consider the following food chain:

grass → vole → owl

• A pyramid of numbers for this food chain would look like the one shown below

  • Often, the number of organisms decreases along food chains, as there is a decrease in available energy since some energy is lost to the surrounding environment at each trophic level

  • Therefore pyramids of numbers usually become narrower towards the apex (the top)

• Despite the name, a pyramid of numbers doesn’t always have to be pyramid-shaped

• For example, consider the following food chain:

oak tree → insects → woodpecker

• The pyramids of numbers for this food chain will display a different pattern to the first food chain

• When individuals at lower trophic levels are relatively large, like the oak tree, the pyramid becomes inverted:

  • Only a single oak tree is needed to support large numbers of insects (which can then support large numbers of woodpeckers)

Pyramids of Biomass

• A pyramid of biomass shows how much mass the creatures at each level would have without including all the water that is in the organisms:

  • This is known as their ‘dry mass’

• Pyramids of biomass are usually pyramid-shaped, regardless of what the pyramid of numbers for that food chain looks like

• This is because the mass of organisms decreases as you go up a food chain:

  • If we take our first food chain as an example, it would be impossible to have 10kg of grass feeding 50kg of voles feeding 100kg of barn owls

• Pyramids of biomass provide a much better idea of the quantity of the plant or animal material at each level of a food chain and therefore are a better way of representing interdependence within the food chain

• Being able to construct accurate pyramids of biomass from appropriate data is an important skill

Pyramids of Energy

• Pyramids of energy illustrate the amount of energy contained within the biomass of individuals within different trophic levels

• These pyramids always have a wide base (due to the large amount of energy contained within the biomass of producers)

• As you move up the pyramid to higher trophic levels the quantity of energy decreases as not all energy is transferred to the biomass of the next trophic level (roughly only 10% of the energy is passed on)