Ecological Pyramids Drawn to Scale – Interdependence Among Organisms in an Ecosystem (Senior 3 Biology) | Ganzaa

Interdependence Among Organisms in an Ecosystem

Lessons

Senior 3 · Biology

Interdependence Among Organisms in an Ecosystem

5 lessons

Lesson 2 of 5

Ecological Pyramids Drawn to Scale

25 min

One tree, thousands of caterpillars

Walk under a fig tree in the rainy season and you may find its leaves crowded with caterpillars, and small birds hopping through the branches to eat them. Count the living things and the tree is outnumbered two thousand to one. Weigh them and the tree outweighs all the caterpillars more than a hundred times over. Which picture of this community is "right"? Both are, but they answer different questions.

In Senior 2 you met the three ecological pyramids and the idea that about 10% of the energy passes from one trophic level to the next. In this lesson you draw pyramids to scale from data, calculate the efficiency of each energy transfer, and explain what each shape tells you.

Definition

Pyramid of numbers: bars showing the number of individual organisms at each trophic level, producers at the bottom.

Pyramid of biomass: bars showing the total dry mass of the organisms at each trophic level, usually in $\mathrm{g\,m^{-2}}$ .

Pyramid of energy: bars showing the energy passing through each trophic level in a given area over a given time, for example in $\mathrm{kJ\,m^{-2}\,yr^{-1}}$ or $\mathrm{kcal\,m^{-2}\,yr^{-1}}$ .

Drawn to scale: the width of every bar is proportional to its value, using one stated scale, for example "1 cm represents 100 organisms".

Ecological efficiency: the percentage of the energy at one trophic level that reaches the next level.

How to draw a pyramid to scale

List the trophic levels in order, with the producers at the bottom.
Choose a scale that makes the largest bar fit your page. A bar of 10 cm fits an exercise book.
Work out each width: $\text{width of bar} = \frac{\text{value}}{\text{value represented by 1 cm}}$
Draw bars of equal height, one on top of the other, each centred on the same vertical line.
Label each bar with the trophic level (or organism) and its value, and write the scale and the units.

Only the width of a bar carries information. If you change the heights, the drawing is wrong.

Worked example 1: a pyramid of numbers from a grassland sample

Students counted the organisms in one food chain in a 10 m by 10 m plot of grassland: 1,000 grass plants, 200 grasshoppers, 40 wolf spiders and 5 agama lizards. (Illustrative figures.) Draw the pyramid of numbers to a scale of 1 cm to 100 organisms.

Grass plants: $\frac{1000}{100} = 10\ \text{cm}$ .
Grasshoppers: $\frac{200}{100} = 2\ \text{cm}$ .
Wolf spiders: $\frac{40}{100} = 0.4\ \text{cm} = 4\ \text{mm}$ .
Agama lizards: $\frac{5}{100} = 0.05\ \text{cm} = 0.5\ \text{mm}$ : barely more than a pencil line.

Answer: Figure 1. Each bar is narrower than the one below, so the pyramid is upright. Notice how thin the top bar is: drawn to scale, a top level can almost disappear.

Figure 1: The grassland pyramid of numbers drawn to scale (illustrative data).

Warning

Common mistakes. (1) Drawing the bars as a triangle with sloping sides: an ecological pyramid is made of rectangular bars. (2) Making the top bars wider "so they can be seen": that breaks the scale. If a bar is too narrow to draw, draw a line and write its value beside it. (3) Using wet (fresh) mass for biomass: water content varies, so biomass is measured as dry mass.

Worked example 2: the same community, counted and weighed

A fig tree supports 2,000 caterpillars, which are eaten by 20 birds; the birds carry 600 feather lice, which are parasites. The mean dry masses are: tree 50,000 g, caterpillar 0.2 g, bird 10 g, louse 0.0001 g. (Illustrative figures.) (a) Calculate the biomass at each level. (b) Compare the shapes of the pyramids of numbers and biomass, and explain the difference.

Use $\text{biomass} = \text{number} \times \text{mean dry mass}$ .
Tree: $1 \times 50\,000 = 50\,000\ \text{g}$ . Caterpillars: $2000 \times 0.2 = 400\ \text{g}$ . Birds: $20 \times 10 = 200\ \text{g}$ . Lice: $600 \times 0.0001 = 0.06\ \text{g}$ .
Numbers (Figure 2): inverted at the base (1 tree, 2,000 caterpillars) and wider again at the top (600 lice on 20 birds). A pyramid of numbers ignores size: one huge producer feeds many small herbivores, and many tiny parasites live on each host.
Biomass (Figure 3): upright, because the one tree has far more mass than everything that feeds on it.

Figure 2: Pyramid of numbers for the fig tree community, drawn to scale. The bar for the single tree is a thin line.

Figure 3: Pyramid of biomass for the same community, drawn to scale. Every consumer level is a thin line, because the tree holds almost all the biomass.

A real energy pyramid: Silver Springs

Energy pyramids need long, careful measurements, so few complete ones exist. A famous one comes from Silver Springs, a clear spring-fed river in Florida, USA, studied by the ecologist Howard T. Odum in the 1950s. He measured the energy flowing through each trophic level in $\mathrm{kcal\,m^{-2}\,yr^{-1}}$ ( $1\ \text{kcal} = 4.184\ \text{kJ}$ ).

Trophic level	Energy (kcal m⁻² yr⁻¹)
Producers (water plants and algae)	20,810
Primary consumers	3,368
Secondary consumers	383
Tertiary consumers	21

Figure 4: The Silver Springs pyramid of energy drawn to scale. The tertiary bar would be 0.1 mm wide, so it is shown as a line.

Worked example 3: efficiency at each step

Use the Silver Springs table. Calculate the ecological efficiency of each transfer, and the percentage of the producers' energy that reaches the tertiary consumers.

Formula: $\text{efficiency} = \frac{\text{energy at the higher level}}{\text{energy at the lower level}} \times 100\%$
Producers to primary: $\frac{3368}{20\,810} \times 100\% = 16.2\%$ .
Primary to secondary: $\frac{383}{3368} \times 100\% = 11.4\%$ .
Secondary to tertiary: $\frac{21}{383} \times 100\% = 5.5\%$ .
Producers to tertiary: $\frac{21}{20\,810} \times 100\% = 0.10\%$ .

Interpretation: the efficiencies are close to the "10% rule" but not equal to it; real ecosystems vary. Only about one thousandth of the producers' energy reaches the top level, which is why there is room for so few top consumers, and why a pyramid of energy can never be inverted.

Ecological Pyramids (FuseSchool). Watch for the three kinds of pyramid and why a pyramid of numbers can have an odd shape while a pyramid of energy cannot.

Tip

Memory hook: "count, weigh, burn". Numbers = count them; biomass = dry them and weigh them; energy = measure the energy they take in over a year. Only the last can never be upside down. In an exam, when a pyramid of numbers is inverted, look for a big producer (a tree) or small parasites.

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Practice: drawing and reading pyramids

State what the width of a bar shows in a pyramid of biomass, and the usual unit.
A sample contains 600 bean plants, 120 aphids, 30 ladybirds and 3 spiders. Using 1 cm to 60 organisms, calculate the width of each bar.
Why must the bars of a pyramid all have the same height?
Explain why biomass is measured as dry mass, not fresh mass.
In a pond, producers hold 12,000 kJ m⁻² yr⁻¹ and primary consumers 1,500 kJ m⁻² yr⁻¹. Calculate the ecological efficiency.
One cow carries 150 ticks. Sketch the pyramid of numbers for grass (many plants) → 1 cow → 150 ticks, and describe its shape.
Using the Silver Springs data, explain why a fifth trophic level would be very unlikely there.
A student says: "Pyramids of numbers are useless because they can be upside down." Evaluate this statement.

Show answersHide answers

The total dry mass of the organisms at that trophic level; grams per square metre (g m⁻²).
Bean plants $\frac{600}{60} = 10$ cm; aphids $\frac{120}{60} = 2$ cm; ladybirds $\frac{30}{60} = 0.5$ cm; spiders $\frac{3}{60} = 0.05$ cm.
Only the width carries the value. Equal heights mean the eye compares the widths fairly; unequal heights would change the areas and mislead.
Fresh mass includes water, and the amount of water varies from organism to organism and from day to day. Dry mass measures only the living material.
$\frac{1500}{12\,000} \times 100\% = 12.5\%$ .
A wide bar for the grass, a very narrow bar (almost a line) for the one cow, then a wider bar for the 150 ticks: narrow in the middle, because one large host carries many small parasites.
The tertiary consumers receive only 21 kcal m⁻² yr⁻¹. At about 5–10% efficiency, a fifth level would receive only about 1–2 kcal m⁻² yr⁻¹, too little to support a population of animals.
Partly wrong. A pyramid of numbers is quick and easy to make, and useful for comparing similar communities, but it ignores the size of organisms. It is misleading where producers are large or parasites small; pyramids of biomass or energy give a truer picture of the food supply.

Key points

To draw to scale, keep one scale for all bars, equal bar heights, and bars centred one above the other.
Pyramids of numbers can be inverted (large producers, small parasites); pyramids of biomass occasionally; pyramids of energy never.
$\text{biomass} = \text{number} \times \text{mean dry mass}$ ; $\text{efficiency} = \frac{\text{higher}}{\text{lower}} \times 100\%$ .
At Silver Springs only about 0.1% of the producers' energy reached the tertiary consumers.

Sources and credits

20.1: Energy Flow through Ecosystems, from Concepts of Biology by Samantha Fowler, Rebecca Roush and James Wise (OpenStax), via LibreTexts Biology, licensed under CC BY 4.0. Adapted by Ganzaa.
Silver Springs data: H. T. Odum, "Trophic structure and productivity of Silver Springs, Florida", Ecological Monographs 27 (1957), as reproduced in OpenStax Biology 2e, section 46.2.
Video: Ecological Pyramids, FuseSchool - Global Education, embedded from YouTube.

Written by Ganzaa: the drawing method, Figures 1 to 4, the grassland and fig tree data (illustrative), the worked examples and the practice questions.

1One tree, thousands of caterpillars

2How to draw a pyramid to scale

3A real energy pyramid: Silver Springs

One tree, thousands of caterpillars

How to draw a pyramid to scale

A real energy pyramid: Silver Springs