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ecological pyramid

Ecological Pyramids in an Ecosystem

What is an ecological pyramid? The concept is often associated with descriptions of food chains and food webs (Food Chains and Food Webs in an Ecosystem, Food Chain and Food Web Book Reviews). An ecological pyramid is an illustration of the reduction in energy as you move through each feeding (trophic) level in an ecosystem. The base of the pyramid is large since the ecosystem's energy factories (the producers) are converting solar energy into chemical energy via photosynthesis. A food chain can also depict a reduction in energy at each feeding level if the arrows, drawn between the different levels, continue to be reduced in size.

There are three ways an ecological pyramid can be represented.  A
Pyramid of Numbers can be generated by counting all the organisms at the different feeding levels.  As you might guess, this can be a very difficult task since we are not just identifying each species in the ecosystem. We are also counting how many of each species is present. On occasion, this approach will not work.  For example, one tree (a producer) can represent an ecosystem and harbor numerous populations of herbivores and carnivores. Thus, the bottom of the pyramid would be very small and not flared out as depicted in the diagrams on the right.

Credits: Vole, Owl, Fox, Rodents (Dorling Kindersley) / Owl on Book (Voronin76|Shutterstock)
A second type of pyramid is called a Pyramid of Biomass where organisms are collected from each feeding level, dried and then weighed.  This dry weight (biomass) represents the amount of organic matter (available energy) of the organisms.  [Note that there are alternate, nonlethal ways to determine biomass.] While this approach will generally create a pyramid that illustrates energy flow, its use can also produce an inverted pyramid.  For example, in aquatic ecosystems, phytoplankton could reproduce and then be eaten rapidly by zooplankton.  Therefore, it would be possible to have few herbivores and a lot of carnivores when a collection is taken.  

A third type of pyramid called a
Pyramid of Energy Flow tends to resolve these problems.  This approach necessitates measuring the caloric value of the different organisms that make up the community.  It nicely shows how energy is continually decreasing along the food chain from producers to top level carnivores. 

In summary, there are three models used in ecology - food chains, food webs and ecological pyramids - that can illustrate how energy moves through an ecosystem.

Examples of ecological pyramids

or Primary Consumers
1st Level Carnivores
or Secondary Consumers
2nd Level Carnivores
or Tertiary Consumers