Ecological Pyramids

• There are four ways an ecological pyramid can be represented. A Pyramid of Numbers or Population Pyramid 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.
• 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 or Energy Pyramid 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.
  • The last type of pyramid is Biological Magnification (Toxins) Pyramid. This pyramid demonstrates how toxins increase as they move up an ecological pyramid.
  • 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.

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