Ecology Theory

Ecological pyramids

April 29, 2013, 8:09 pm
Content Cover Image

Hypothetical case of a pyramid of numbers. Source: Leo Nomis

Ecological pyramids are diagrams that illustrate how ecologically important factors, such as energy, biomass, and population size, vary between trophic levels in an ecosystem. Traditionally, these diagrams place the primary producers (photosynthetic organisms such as plants) at the bottom and the highest trophic levels at the top of the diagram.  The size of the portion of the diagram associated with each trophic level illustrates the amount of energy, biomass, or number of individuals found in each trophic level. 

General concepts

Energy flows through the food chain in a predictable way, entering at the base of the food chain, by photosynthesis in primary producers, and then moving up the food chain to higher trophic levels.  Because the transfer of energy from one trophic level to the next is inefficient, there is less energy entering higher trophic levels.  Thus, diagrams showing how much energy enters each trophic level  will have a distinct pyramid shape.  Thus, this diagram has become known as the energy pyramid.

It may also be useful and productive to examine how the number and biomass of organisms vary across trophic levels.  Both the number and biomass of organisms at each trophic level should be influenced by the amount of energy entering that trophic level. When there is a direct correlation between energy, numbers, and biomass then biomass pyramids and numbers pyramids will result.  However, the relationship between energy, biomass, and number can be complicated by the growth form and size of organisms and ecological relationships occuring among trophic levels.  Thus, it is possible, and common, that biomass pyramids and numbers pyramids do not look like pyramids at all!

Energy pyramid

The energy pyramid shows how the amount of energy entering each level  varies across trophic levels.  In general, only about 10% of the energy entering a trophic level is transferred to the trophic level above it, so the energy pyramid always has a distinct step-like pattern with less energy entering each trophic level up the food chain.  

The shape of the energy pyramid affects the length of food chains because eventually the amount of energy entering the highest trophic level is not large enough to support a higher trophic level.  

Biomass pyramid

The biomass pyramid shows how the biomass of living organisms varies across trophic levels. The shape of the biomass pyramid in any ecosystem depends on a number of factors. If the amount of biomass in a trophic level depends on the amount of energy entering that trophic level, then, all else being equal, the biomass pyramid should have the same shape as the energy pyramid.  However, when biomass of primary producers is rapidly removed by herbivores the biomass of primary consumers (herbivores) in an ecosystem at any time may be greater than the biomass of primary producers (e.g. plants). Thus, the resulting biomass pyramid is not necessarily shaped like a pyramid.

The shape of the biomass pyramid is also influenced by the growth form of the dominant plants in the ecosystem. For example, woody plants store biomass that has been accumulated for a number of years so there might be much more biomass in the primary producer trophic level in ecosystems dominated by long-lived woody plants (e.g., forests)  than in ecosystems dominated by herbaceous plants (e.g., grasslands).

Numbers pyramid

The numbers pyramid shows how the number of individuals per trophic level varies across trophic levels. if the number of individuals in a trophic level is related to the amount of energy entering that level then the number of individuals per level should also show a pyramidal shape. However, many factors can influence this relationship including the shape of the biomass pyramid and the size of individuals. If the "biomass pyramid" is not shaped like a pyramid then it is unlikely that the "numbers pyramid" will either. Furthermore, when insect herbivores are feeding on large trees, the difference in sizes between individuals in each level will cause there more individual herbivores than individual plants.  

Author's cautionary note

I suggest that students, teachers, and researchers take care when studying ecological pyramids. Diagraming how energy entering a level varies across trophic level will always show a pyramid shape, so that calling this relationship "the energy pyramid" is accurate and therefore a useful concept. However, diagraming how numbers and biomass varies across trophic levels often does not always produce a pyramid shape, so referring to these relationships as "the biomass pyramid" and "the numbers pyramid" is potentially misleading and likely to be a source of confusion. I suggest that the terms "trophic distribution of biomass" and "trophic distribution of numbers" be used instead.  I know that these terms don't roll off of the tongue as easily as  "biomass pyramid" and "numbers pyramid", but they are more accurate and might lead to more clear communication and understanding of this topic.

While searching for figures to use in this article I noticed that many of the figures that people have used to illustrate this concept were drawn incorrectly.  To quantify this, I google searched the term "energy pyramid" and then examined the first 30 images that were relevant to the topic.  I scored whether the diagram properly illustrated the concept or not.  Here are the results.  Sadly, only 2 of the diagrams correctly illustrated the concept in my opinion meaning that over 93% of the figures that people are using to illustrate this concept were incorrect!  More disturbingly, in several cases the diagramatic representation did not correctly illustrate the figures presented in the figure.  

Ecological pyramids appear to be a popular subject for school assignments.  Seven out of the first 10 returns on a google search for "energy pyramid"  were focused on classroom activities or clearly designed for use by educators.  The figures contained in most of these activities were inaccurate including one from an activity identified as "selected and approved by the National Science Teachers Association's network of teacher-webwatchers".   I urge teachers to take care when covering this topic.

See also

References and Further Readings

http://en.wikipedia.org/wiki/Ecological_pyramid

http://science.jrank.org/pages/2259/Ecological-Pyramids-Ecological-pyramids.html

 

Glossary

Citation

McGinley, M. (2013). Ecological pyramids. Retrieved from http://www.eoearth.org/view/article/171523

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