<P> Energy pyramids begin with producers on the bottom (such as plants) and proceed through the various trophic levels (such as herbivores that eat plants, then carnivores that eat herbivores, then carnivores that eat those carnivores, and so on). The highest level is the top of the food chain . </P> <P> An energy pyramid of biomass shows the relationship between biomass and trophic level by quantifying the biomass present at each trophic level of an energy community at a particular time . It is a graphical representation of biomass (total amount of living or organic matter in an ecosystem) present in unit area in different tropic levels . Typical units are grams per meter, or calories per meter . The pyramid of biomass may be "inverted". For example, in a pond ecosystem, the standing crop of phytoplankton, the major producers, at any given point will be lower than the mass of the heterotrophs, such as fish and insects . This is explained as the phytoplankton reproduce very quickly, but have much shorter individual lives . </P> <P> One problem with biomass pyramids is that they can make a trophic level appear to contain more energy than it actually does . For example, all birds have beaks and skeletons, which despite having mass are not typically digested by the next trophic level . </P> <P> An' ecological pyramid of productivity' is often more useful, showing the production or turnover of biomass at each trophic level . Instead of showing a single snapshot in time, productivity pyramids show the flow of energy through the food chain . Typical units are grams per meter per year or calories per meter per year . As with the others, this graph shows producers at the bottom and higher trophic levels on top . </P>

The concept of ecological pyramid was first proposed by