Ecological Pyramids (DP IB Environmental Systems & Societies (ESS)): Revision Note

Ecological Pyramids

• Ecological pyramids include:

  • Pyramids of numbers

  • Pyramids of biomass

  • Pyramids of energy (also known as pyramids of productivity)

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

Pyramids of numbers

• 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 organisms at each trophic level would have without including all the water that is in the organisms:

  • This is known as their ‘dry mass’

• As per the second law of thermodynamics, the quantities of biomass generally decrease along food chains, so the pyramids become narrower towards the top

  • 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

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

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

  • However, they can occasionally be inverted and show higher quantities at higher trophic levels

  • These inverted pyramids sometimes occur due to marked seasonal variations

    • For example, in some marine ecosystems, the standing crop of phytoplankton, the major producers, is lower than the mass of the primary consumers, such as zooplankton

    • This is because the phytoplankton reproduce very quickly and are constantly being consumed by the primary consumers, which leads to a lower standing crop but higher productivity

    • This can occur because phytoplankton can vary greatly in productivity (and therefore biomass) depending on sunlight intensity

Pyramids of energy

• Pyramids of energy (also referred to as pyramids of productivity) show the flow of energy through trophic levels, indicating the rate at which that energy is being generated

• Pyramids of productivity illustrate the amount of energy or biomass of organisms at each trophic level per unit area per unit time

  • Productivity is measured in units of flow

  • The units are mass or energy per metre squared per year (g/kg m^-2 yr^-1 or J/kJ m^-2 yr^-1)

• The length of each box, or bar, represents the quantity of energy present

• These pyramids are always widest at the base and decrease in size as they go up

  • This is because pyramids of productivity for entire ecosystems over a year always show a decrease along the food chain, following the second law of thermodynamics

• The base is wide 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 10 % of the energy is passed on)

• Energy is lost at each trophic level due to:

  • Incomplete consumption

  • Incomplete digestion

  • Loss of heat energy to the environment during respiration

  • Excretion of the waste products of metabolism e.g. carbon dioxide, water, and urea