Different Types of Ecological Pyramids: Pyramid of Number, Pyramid of Biomass and Pyramid of Energy

Do you know about the Great Pyramids of Giza? Pyramids are symbols of the ancient Egyptian culture and architecture which were built by the Egyptian pharaohs as tombs to treasure their bodies after death. In fact the Pyramid of Khufu is the largest pyramid of Egypt and is considered to be one of the seven wonders of the world that still stands strong even after 2000 years of it being built.

Have you noticed the shape of a pyramid? It is a three dimensional structure which has a polygonal base and flat triangular faces that meet at an apex at the top. Connecting each edge of the bases of the triangular faces to the apex helps to form the pyramid.

Did you know that pyramids can be used for graphical representation of information regarding an ecosystem? Different ecological parameters such as number of individuals, energy levels and the amount of biomass at different trophic levels of an ecosystem can be represented in the form of pyramids known as ecological pyramids.

But what is the underlying principle behind this representation? What are the different types of ecological pyramids? Do all ecosystems have similar ecological pyramids for all parameters? To get answers to all these questions, let us dive into the topic of Ecological Pyramids.

Table of Content

• What are Ecological Pyramids?
• Pyramid of numbers
• Pyramid of Biomass
• Pyramid of Energy
• Limitations of ecological pyramids
• Practice Problems
• FAQs

What are Ecological Pyramids?

An ecological pyramid is a graphical representation of ecological pyramids such as the number of individuals, energy levels and the amount of biomass at different trophic levels of an ecosystem can be represented in the form of pyramids.

You must have noticed that the Egyptian pyramids have steps and the pyramids being broader at the base and becoming increasingly narrower towards the apex, the length of the steps also keep reducing as we move from the base towards the apex. Yes! you guessed it right, just like the steps in a real life pyramid, the trophic levels form the different levels in these ecological pyramids. The lowest trophic level, that is producers are kept at the base of the pyramid and the top carnivores which form the highest trophic levels are placed at the apex. The intermediate trophic levels lie in between. The quantity at each trophic level is indicated by the length of the step or bar of the pyramid in a graph.

Based on the parameters that are represented using the ecological pyramids, these can be of three types -

• Pyramid of Numbers
• Pyramid of Biomass
• Pyramid of Energy

Fig: Ecological pyramids

The shape of an ecological pyramid can be -

• Upright with a broad base and gradually tapering towards the apex.
• Inverted with a pointed or narrow base and gradually broadening towards the apex.
• Spindle-shaped with a narrow base and apex and a broad centre.

All the calculations with respect to biomass, numbers and energy content of a particular trophic level must include all the organisms in that trophic level as calculations based on generalisations made after taking only a few organisms of that trophic level into consideration, will not be true.

Pyramid of Numbers

It is the relationship between different trophic levels of a food chain with respect to the number of individuals they comprise. Let us take some examples -

Upright Pyramid of Numbers

In a pond ecosystem, the base of the pyramid, that is the lowest trophic level, is occupied by producers which are numerous. The second trophic level is occupied by the primary consumers or zooplanktons which are relatively less abundant. The third trophic level is occupied by a comparatively small number of secondary consumers or primary carnivores which include small-sized fishes and the fourth trophic level includes an even smaller number of secondary carnivores, i.e, medium sized fishes. At the apex lie the large-sized top carnivores such as whales and sharks which are much less in number.

Fig: Upright pyramid of numbers in pond ecosystem

This pyramid shows why the number of top carnivores is limited. This is because a huge prey population will be needed to feed a large number of such carnivores but the number of individuals generally decreases in successive trophic levels of such ecosystems.

A forest ecosystem and a grassland ecosystem also exhibit an upright pyramid of numbers.

Fig: Upright pyramid of numbers in grassland ecosystem

Inverted Pyramid of Numbers

In a parasitic food chain, the pyramid of numbers is inverted as the number of parasites are more than their hosts. For example, a single tree supports a large number of fruit eating birds which in turn harbour a larger number of parasites such as bugs and lice.

Fig: Inverted pyramid of numbers in parasitic food chain

Spindle-shaped Pyramid of Numbers

A big tree supports a larger number of herbivorous animals such as birds, elephants, deer, etc. These herbivores are however eaten by a smaller population of top carnivores such as tigers, lions, etc. Thus the number of primary consumers is not only higher than the number of the producer, which is the single big tree, but also higher than the number of top carnivores which feed on them. This gives a spindle shape to the pyramid of numbers.

Fig: Spindle-shaped pyramid of numbers in a big tree

Pyramid of Biomass

Biomass represents the total mass of living organisms of a population present at a given time at a given area. The pyramid of biomass represents the biomass of successive trophic levels of an ecosystem, arranged in the form of a pyramid with the lowest trophic level at the base and going higher up.

Generally, the biomass of producers is the highest in an ecosystem. The biomass of primary consumers or herbivores is comparatively less and that of secondary consumers (primary carnivores) is even lesser, and so on. Thus, there is a gradual decrease in biomass at each trophic level from the base of the pyramid to the apex.

Fig: Pyramid of biomass in terrestrial ecosystem

Pyramid of Energy

The total energy at successive trophic levels of a food chain is represented by the pyramid energy. It starts with producers at the base and successive trophic levels are arranged one above the other. Top carnivores are present at the apex of the pyramid.

As we know, the transfer of energy from one trophic level to the next higher trophic level of a food chain, follows the 10% law of energy transfer. This indicates that only 10% of the energy retained by an organism at a particular trophic level is transferred to the organism of the next higher trophic level when it is eaten by the latter in a food chain. The rest is used up in respiration and also lost as heat generated during respiration. Thus, the energy content keeps reducing at successive trophic levels as we go from the base of the pyramid to its apex.

Fig: Pyramid of energy

Practice Problems

1. If a big oak tree is considered as an ecosystem in itself, then the pyramid of numbers, representing the number of organisms at various trophic levels supported by it, would look like

1. An upright pyramid
2. An inverted pyramid
3. A spindle-shaped structure
4. A cylindrical structure

Solution: A big tree supports a larger number of herbivorous animals such as small fruit-eating birds, elephants, deer, etc. A smaller population of top carnivores such as large birds of prey, tigers, lions, etc. feeds on these herbivores. Thus the number of primary consumers is not only higher than the number of the producer, which is the single big tree, but also higher than the number of top carnivores which feed on them. Thus, the pyramid of numbers looks like a spindle shaped structure.

Hence, the correct option is c.

2. Producers use only 1% of the photosynthetically active radiation (PAR) coming from the sun. So, if the total amount of PAR available to the producers is 1,00,000 J, what is the energy available at the level of secondary consumers?

1. 1000 J
2. 100 J
3. 10 J
4. 0.1 J

Solution: Total amount of available solar energy for photosynthesis, i.e, PAR = 1,00,000 J

Energy produced at the level of producers = 1% of PAR = (1/100) X 1,00,000 J = 1,000 J

According to the 10 % of law of energy transfer,

The energy available at the level of herbivores or primary consumers = 10% of 1,000 J = (10/100) X 1000 J = 100 J

The energy available at the level of primary carnivores or secondary consumers = (10/100) X 100 J = 10 J

Thus, the correct option is c.

3. Why are pyramids of energy always upright?

Answer: A pyramid of energy starts with producers at the base and successive trophic levels are arranged one above the other with top carnivores present at the apex of the pyramid.

Only 10% of the energy retained by an organism at a particular trophic level is transferred to the organism of the next higher trophic level when it is eaten by the latter in a food chain. The rest is used up in respiration and also lost as heat generated during respiration. Thus, the energy content of an organism at a lower trophic level is always more compared to the organism at the next higher trophic level. This is why the pyramid of energy is always upright.

4. The pyramid of numbers will be inverted in a -

1. Pond ecosystem
2. Grassland ecosystem
3. Forest ecosystem
4. Parasitic ecosystem

Solution: In a parasitic food chain, the pyramid of numbers is inverted as the number of parasites are more than their hosts. For example, a single tree supports a large number of fruit eating birds which in turn harbour a larger number of parasites such as bugs and lice. Thus, the correct option is d.

FAQs

1. Why are ecological pyramids also known as Eltonian pyramids?

Answer: Ecological pyramids were first developed by Charles Elton in 1927 and are hence also known as Eltonian pyramids.

2. Why can a vegetarian diet support a larger population than a non-vegetarian diet?

Answer: The biomass of producers in any terrestrial ecosystem is much higher than that of the primary consumers and the successive higher trophic levels. Thus, a large biomass is available for herbivores to feed on and hence producers can support a large population of primary consumers. But the biomass of herbivores or primary carnivores available for feeding primary carnivores, secondary carnivores or top carnivores is comparatively much less and hence can support only smaller populations

3. Can the pyramid of biomass be inverted?

Answer: The pyramid of biomass in an aquatic ecosystem is inverted because the biomass of producers, that is, phytoplanktons is much less compared to that of the primary consumers, that is, zooplanktons. This is because phytoplanktons are microscopic with negligible mass and they reproduce and die quickly.

4. Which trophic level is occupied by humans in an ecological pyramid?

Answer: Humans are omnivores and can hence function at multiple trophic levels in an ecosystem. They function as herbivores or primary consumers when they feed on plants, secondary consumers or primary carnivores when they feed on herbivores such as cows, goats, sheep, etc., and top carnivores when they feed on other carnivores such as chicken, pigs, snakes, sharks etc.

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