Ecosystem
Ecological Pyramids
You must be familiar with the shape of a pyramid
The base of a pyramid is broad and it narrows towards the apex
One gets a similar shape, whether you express the food or energy relationship between organisms at different trophic levels
This, relationship is expressed in terms of number, biomass or energy
The base of each pyramid represents the producers or the first trophic level while the apex represents tertiary or top level consumer
(a) pyramid of number;
(b) pyramid of biomass and
(c) pyramid of energy
Any calculations of energy content, biomass or numbers, has to include all organisms at that trophic level
No generalisations we make will be true if we take only a few individuals at any trophic level into account
Also a given organism may occupy more than one trophic level simultaneously
One must remember that the trophic level represents a functional level, not a species as such
A given species may occupy more than one trophic level in the same ecosystem at the same time; for example, a sparrow is a primary consumer when it eats seeds, fruits, peas, and a secondary consumer when it eats insects and worms
Can you work out how many trophic levels human beings function at in a food chain?
In most ecosystems, all the pyramids, of number, of energy and biomass are upright, i.e., producers are more in number and biomass than the herbivores, and herbivores are more in number and biomass than the carnivores
Also energy at a lower trophic level is always more than at a higher level
There are exceptions to this generalisation: If you were to count the number of insects feeding on a big tree what kind of pyramid would you get?
Now add an estimate of the number of small birds depending on the insects, as also the number of larger birds eating the smaller
Draw the shape you would get
The pyramid of biomass in sea is generally inverted because the biomass of fishes far exceeds that of phytoplankton
Isn’t that a paradox?
How would you explain this?
Pyramid of energy is always upright, can never be inverted, because when energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step
Each bar in the energy pyramid indicates the amount of energy present at each trophic level in a given time or annually per unit area
However, there are certain limitations of ecological pyramids such as it does not take into account the same species belonging to two or more trophic levels
It assumes a simple food chain, something that almost never exists in nature; it does not accommodate a food web