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Monday, 22 December 2008!

HandWritten on; 09:14

WHAT ARE ECOLOGICAL PYRAMIDS?

-A graphical representation designed to show the biomass or productivity at each trophic level in a given ecosystem.

PYRAMID OF NUMBERS

-Used to compare the number of organisms present in each trophic level at a particular time.

PYRAMID OF BIOMASS

-Used to compare the mass of organisms present in each trophic level at a particular time.

-Constructed based on the dry mass of organisms in each trophic level at any one time. This is called the standing mass of organisms.

pyramid of biomass (picture from yahoo)

ENERGY OF ENERGY (productivity)

-Shows the total energy in the various trophic levels of a food chain

-The total energy content in each trophic level had to be determined over a period of time.

-Takes into consideration the rate of reproduction of organisms.

A lot of energy is lost to the environment as food is transferred from one trophic level to the next. It may be lost to the environment

as heat during respiration at every trophic level
in uneaten body parts
through undigested matter egested by consumers
through waste products excreted by consumers (urea)

-More and more energy is lost as we go down the food chain.

-The total energy is highest at the first trophic level and lowest at the last trophic level.

pyramid of energy (picture from MSN)

Variations in ecological pyramids

-Most ecological pyramids are pyramid-shaped but there are important exceptions

1. A pyramid of numbers may be upside down or inverted if:
-Organisms of one trophic level are parasitic on organisms of another trophic level.
-Many small organisms of one trophic level feed on a large organism of another trophic level

Tree------> Aphid------> Protozoa

2. The pyramid of biomass may be inverted of not pyramid-shaped.
-Based on the standing mass, not the rate of production of organisms.
-It's a disadvantage when considering organisms that reproduce rapidly.

Phytoplankton------> Zooplankton------> Small fish------> Large fish