Ecological pyramid
From Wikipedia, the free encyclopedia
An Ecological Pyramid is a graphical representation designed to show the relationship between energy and trophic levels of a given ecosystem. Most commonly, this relationship is demonstrated through the number of individuals at a given trophic level, the amount of biomass at a given trophic level, or the amount of energy at a given trophic level. It is worth noting that all Ecological Pyramids begin with producers on the bottom and proceed through the various trophic levels, the highest of which is on top.
[edit] Pyramid of Biomass
An Ecological Pyramid of Biomass shows the relationship between energy and trophic level by quantifying the amount of biomass present at each trophic level (dry mass per trophic level). As such, is assumed that there is a direct relationship between biomass and energy. By doing this, the earlier discrepancy is avoided because even though there is only one tree, it is much more massive than the next trophic level.
The main problem with this type of Ecological Pyramid is that it can make a trophic level look like it contains more energy than it actually does. For example, all birds have a beak and skeleton, which despite taking up mass are not eaten by the next trophic level. In a Pyramid of Biomass, the skeleton and beak would still be quantified even though it does not contribute to the overall flow of energy into the next trophic level.
[edit] Pyramid of Energy
An Ecological Pyramid of Energy is the most useful of the three types, showing the direct relationship between energy and trophic level. It measures the number of calories per trophic level. As with the others, this graph begins with producers and ends with a higher trophic level.
When an ecosystem is healthy, this graph will always look like the standard Ecological Pyramid shown at the top of the page. 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.
When energy is transferred to the next trophic level, only 10% of it is used to build bodymass, becoming stored energy (the rest going to metabolic processes). As such, in a Pyramid of Energy, 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 Energy:
- It takes account of the rate of production over a period of time because each rectangle represents energy per unit area / volume per unit time. An example of units might be - kJ/m2/yr.
- Two species weight for weight may not have the same energy content therefore the biomass is misleading but energy 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 Energy:
- The energy value for a given mass of organism is required, which involves complete combustion of a sample.
- 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.
The best way of showing what is happening in the feeding relationships of a community is to use Energy Pyramids.
