<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> A "pyramid of productivity" is often more useful, showing the production or turnover (the rate at which energy or mass is transferred from one trophic level to the next) 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> <P> When an ecosystem is healthy, this graph produces a standard ecological pyramid . This is because in order for the ecosystem to sustain itself, there must be more energy at lower trophic levels than there is at higher trophic levels . This allows organisms on the lower levels to not only to maintain a stable population, but also to transfer energy up the pyramid . The exception to this generalization is when portions of a food web are supported by inputs of resources from outside the local community . In small, forested streams, for example, the volume of higher levels is greater than could be supported by the local primary production . </P> <P> When energy is transferred to the next trophic level, typically only 10% of it is used to build new biomass, becoming stored energy (the rest going to metabolic processes) (Pauly and Christensen, 1995). In this case, in the pyramid of productivity each step will be 10% the size of the previous step (100,000, 10,000, 1,000, 100, 10, 1, . 1, . 01). </P>

When does the species at the top of the pyramid get more energy
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