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Thursday, December 25, 2008

Ecology 6

6. What are ecological pyramids?


(ii) Describe the different types of ecological pyramis. (KILLLLLER QN!)

(i) Ecological pyramids are used to compare the trophic levels in a food chain.

(ii) There are 3 types of Ecological Pyramids. The three being
(1) Pyramid of Numbers
(2) Pyramid of Biomass
(3)Pyramid of Energy

1. The Pyramid of Numbers

A pyramid of numbers allows us to compare the number of organisms present in each trophic level at a particular time.

The bottom of the pyramid is Trophic Level 1. The base of the pyramid is more often than not, a producer and mostly the highest in number amongst the other organims and the trophic levels above it. This is so as energy is lost when it is passed from one organism to another. The producers get the chemical energy directly from the sun thus they tend to be the highest in number as the posses the most energy ( as would be seen in an Energy-Pyramid).

In a pyramid of numbers, all the organisms at each trophic level are taken into account, regardless of their size and stage of development.

Tree->Aphids->Ladybirds

The tree is huge and many aphids would feed on that one tree that contains alot of chemical energy and the ladybirds would feed on the many aphids that are feeding on the tree. This pyramid of numbers would be inaccurate because at Trophic Level 1, the quantity should be the greatest.

Variation in Ecological Pyramids; Pyramid of Numbers

A pyramid of numbers may be inverted if:

• Organisms at one trophic level are parasitic on organisms of another trophic level
• Many small organisms of one trophic level feed on a large number of organism of another trophic level.

2. Pyramid of Biomass

A pyramid of biomass aloows us to compare the mass of organisms present in each trophic level at a particular time.

The pyramid of biomass is constructed based on the dry mass of organisms in each trophic level at any one time. This is called the standing mass of organisms.

Variation in Pyramid of Biomass:
Pyramids of biomass for rapidly reproducing organisms are not pyramid-shaped. Since the pyramid of biomass is based on standing mass, it does not take into account the rate of reproduction. This is a disadvantage when considering organisms that reproduce rapidly.

3.Pyramid of Energy
The total energy in the various trophic levels of a food chain can be represented in the form of a pyramid. This is called the pyramid of energy.

Unlike the pyramid of biomass, we need to determine the total enery content in each trophic level over a period of time. This means that we have to take into account the rate at which the organisms in each trophic level reproduce.

A lot of enery is lost to the environment as food is transferred from one trophic level to the next. Energy may be lost to the environment:
- As heat during respiration at each tophic level;
-In uneaten body parts by the predator
-Through undigested matter egested by consumers, and
-Through waste products excreted by consumers, i.e.: Urea (Protein De Animation)

Thus, more and more energy is lost as we go down a food chain. A large number of produces might be required to support just a few consumers at the end. The greatest amount of energy is lost during its transfer from producer to primary consumer.