Re: where should we put the omnivores in the pyramid of biomass?

Hi Lim,

The biomass pyramid explains why animals in higher trophic levels have
lower biomass than organisms in lower trophic levels.  A certain biomass of
primary producers can support a smaller biomass of primary consumers
(herbivores) due to the inefficiency of energy transfer.  When a herbivore
consumes a primary producer, only a small proportion of the energy is
transferred into herbivore biomass - the rest is used for metabolic
processes and is lost as heat.  Similarly, when a carnivore consumes a
herbivore, only a fraction of the herbivore biomass is converted to
carnivore biomass, and the carnivores are represented at the top of the
biomass pyramid with the smallest amount of biomass in the food web.   This
information can be found in any general biology text book (such as W.K.
Purves, G.H. Orians, and H.C. Heller's Life: The Science of Biology). 
You're correct to note that the question of the place of omnivores in
biomass pyramids is rarely discussed in textbooks.  Given the above
information, however, we can predict where omnivores should be located in
these biomass pyramids.

An omnivore, therefore, fits in between the herbivores and carnivores.  You
can imagine that different animals will exhibit different degrees of
omnivory - some animals will eat mostly plants, with the occasional meal
containing meat (like a vegetarian who will eat turkey with his family at
Thanksgiving), while others will consume primarily meat, but with much
smaller quantities of primary producers (the beef farmer who reluctantly
eats a salad with his steak).  The omnivore that eats mostly plants will be
similar to the herbivore, and the biomass of this type of omnivore will
only be slightly less than that of the herbivores in this food web.  The
omnivore that consumes meat preferentially will be more similar to the
carnivore in the biomass pyramid, but the addition of some plant material
in their diets will allow them to obtain higher biomass than pure
carnivores.  An omnivore consuming equal amounts herbivore and primary
producer should sit right between the herbivores and the carnivores in the
biomass pyramid.