Ecological pyramid
3574959
222002730
2008-06-27T02:25:09Z
Editor2020
6046894
rv vd
{{Unreferenced|date=February 2008}}
[[image:EcoPyramid.png|thumb|400px|right|An ecological pyramid.]]
An '''Ecological Pyramid''' (or '''Trophic pyramid''') is a graphical representation designed to show the [[biomass]] or [[primary_production|productivity]] at each [[trophic level]] in a given [[ecosystem]]. ''[[biomass (ecology)|Biomass]] pyramids'' show the abundance or biomass of organisms at each trophic level, while ''[[Primary production|productivity]] pyramids'' show the production or turn-over in biomass. ''Ecological Pyramids'' begin with [[autotroph|producers]] on the bottom and proceed through the various trophic levels, the highest of which is on top.
==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 meter<sup>2</sup>, or calories per meter<sup>2</sup>. Biomass pyramids provide a single snapshot in time of an [[biocoenosis|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 [[beak]]s and [[skeleton]]s, 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.
[[image:BioPyramid.png|thumb|300px|left|Pyramid of Biomass]]
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 ecosystem]]s, where [[algae]] and [[phytoplankton]] can maintain a high level of productivity despite being grazed by longer lived [[fish]] and [[zooplankton]].
==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 meter<sup>2</sup> per year or calories per meter<sup>2</sup> 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 [[biocoenosis|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 span]]s. 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 [[detritivore|detritivores]] to a particular trophic level.
Nonetheless, [[productivity]] pyramids usually provide more insight into a [[biocoenosis|ecological community]] when the necessary information is available.
==References==
{{reflist}}
==External links==
*[http://www.eelsinc.org/id43.html Food Chains and Food Webs]
[[Category:Ecology]]
[[de:Nahrungspyramide]]
[[ja:生態ピラミッド]]
[[no:Næringspyramide]]
[[ru:Экологическая пирамида]]
[[sv:Näringspyramid]]
[[uk:Екологічна піраміда]]
[[zh:生態塔]]