Ecological pyramid
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An Ecological Pyramid (or Trophic pyramid) is a graphical representation designed to show the biomass or productivity at each trophic level in a given ecosystem. Biomass pyramids show the abundance or biomass of organisms at each trophic level, while productivity pyramids show the production or turn-over in biomass. Ecological Pyramids begin with producers on the bottom and proceed through the various trophic levels, the highest of which is on top.
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[edit] Pyramid of Biomass
An Ecological Pyramid of Biomass shows the relationship between biomass and trophic level by quantifying the amount of biomass present at each trophic level. Typical units for a biomass pyramid could be grams per meter2, or calories per meter2. Biomass pyramids provide a single snapshot in time of an ecological community.
One problem with biomass pyramids is that they can make a trophic level look like it contains more energy than it actually does. For example, all birds have beaks and skeletons, which despite taking up mass are not eaten by the next trophic level. In a Pyramid of Biomass the skeletons and beaks would still be quantified even though they do not contribute to the overall flow of energy into the next trophic level.
Biomass pyramids can also be misleading, in that they do not include the turn-over in biomass at a given trophic level. This can lead to inverted pyramids, in which a small biomass of autotrophs supports a larger biomass of herbivores. This often occurs in aquatic ecosystems, where algae and phytoplankton can maintain a high level of productivity despite being grazed by longer lived fish and zooplankton.
[edit] Pyramid of Productivity
An Ecological Pyramid of Productivity is often more useful, showing the production or turnover of biomass at each trophic level. Instead of showing a single snapshot in time, productivity pyramids show the flow of energy through the food chain. Typical units would be grams per meter2 per year or calories per meter2 per year. As with the others, this graph begins with producers at the bottom and places higher trophic levels on top.
When an ecosystem is healthy, this graph generally look like the standard Ecological Pyramid. This is because in order for the ecosystem to sustain itself, there must be more energy at lower trophic levels than there is at higher trophic levels. This allows for organisms on the lower levels to maintain a stable population, but to also feed the organisms on higher trophic levels, thus transferring energy up the pyramid. The exception to this generalization is when portions of a food web are supported by inputs of resources from outside of the local community. In small, forested streams, for example, many consumers feed on dead leaves which fall into the stream. The productivity at the second trophic level is therefore greater than could be supported by the local primary production.
When energy is transferred to the next trophic level, typically only 10% of it is used to build new biomass, becoming stored energy (the rest going to metabolic processes). As such, in a Pyramid of Productivity each step will be 10% the size of the previous step (100, 10, 1, 0.1, 0.01, 0.001 etc.).
The advantages of the Pyramid of Productivity:
- It takes account of the rate of production over a period of time.
- Two species of comparable biomass may have very different life spans. Therefore their relative biomasses is misleading, but their productivity is directly comparable.
- The relative energy flow within an ecosystem can be compared using pyramids of energy; also different ecosystems can be compared.
- There are no inverted pyramids.
- The input of solar energy can be added.
The disadvantages of the Pyramid of Productivity:
- The rate of biomass production of an organism is required, which involves measuring growth and reproduction through time.
- There is still the difficulty of assigning the organisms to a specific trophic level. As well as the organism in the food chains there is the problem of assigning the decomposers and detritivores to a particular trophic level.
Nonetheless, productivity pyramids usually provide more insight into a ecological community when the necessary information is available.
