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First teaching 2024

First exams 2026

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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)

Pyramid of numbers
Pyramid of numbers
  • 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)

Inverted pyramid of numbers
Pyramids of numbers are not always pyramid-shaped (they can be inverted, like the one shown above)

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

Worked Example

The table below shows:

  • A food chain with four trophic levels

  • The total mass of organisms at each trophic level

 

Clover →

Snail   

Thrush    →

Sparrowhawk

 Biomass (kg)

    80

  30

   10

       2

Draw a pyramid of biomass for the food chain in Table 1.

Constructing a pyramid of biomass
  • 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 biomass
Pyramids of biomass

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

Pyramid of energy
The energy stored in the biomass of organisms can be represented by a pyramid of productivity
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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies Content Creator

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Bridgette Barrett

Reviewer: Bridgette Barrett

Expertise: Geography Lead

After graduating with a degree in Geography, Bridgette completed a PGCE over 25 years ago. She later gained an MA Learning, Technology and Education from the University of Nottingham focussing on online learning. At a time when the study of geography has never been more important, Bridgette is passionate about creating content which supports students in achieving their potential in geography and builds their confidence.

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