Biomass Vs. Energy Pyramids

Biomass pyramids and energy pyramids are two types of ecological "infographics" used by scientists to represent the relationships among elements in a food chain. Scientists can use these kinds of biological pyramids to determine the health of plant and animal populations by using pictures to represent concrete measurements of an ecosystem's parts.

"Trophic levels" are the units that biologists visualize in ecological pyramids. A trophic level is the place an organism (a plant or an animal) holds in a food chain — in other words, what it eats and what eats it. The lowest level is occupied by plants that get their energy directly from the sun — grass, for example. The next level is occupied by herbivores, such as rabbits that eat the grass. The upper levels are occupied by carnivores that feed on animals in lower trophic levels. Because each level gets energy from the level below it, you can say that it "rests" on the level below. That's what makes the pyramid such a valuable tool for depicting complex ecosystems in simple, easy-to-understand images.

"Biomass" is the estimated, combined dry mass of all the plants or animals in a population. For example, you might calculate the approximate land area covered in grass in the ecosystem you're studying, estimate the mass of all the grass in one square meter and extrapolate to figure the mass of all the grass in that ecosystem. You would do the same for the mass of the rabbit population that eats the grass, and every higher trophic level that eats the rabbits.

To represent this ecosystem as a biomass pyramid, you would show a bar or block representing the mass of the grass (in grams, kilograms or another acceptable measurement for mass); a proportionally smaller block representing the mass of rabbits resting on the first block; and proportionally smaller blocks representing higher-level predators stacked above those. The result would be a "pyramid" showing the biomass of each trophic level, so that you can directly compare each trophic level to every other.

Energy pyramids use the same kind of graphic representation, but rather than using biomass measurements, they show energy flow in an ecosystem or community. Each level of the pyramid shows a bar or block that represents the amount of energy that trophic level gets by eating members of the level below it. Energy is lost at each level, so (as with biomass pyramids) the upper levels are smaller than the lower levels, resulting in the classic "pyramid" shape.

According to the Annenberg Learner website, up to 90 percent of energy that enters one step of the food chain is lost before the remaining 10 percent gets passed up to the next trophic level. The energy any organism consumes powers life processes (and some is lost to the world as heat), so it makes sense that an animal only passes on a small amount of the energy it has eaten on to the animal that eats it.

The more steps between plant producers at the bottom and higher-order predators at the top, the more energy is lost in the climb up the pyramid. That energy loss also explains why biomass pyramids usually keep the classic pyramid shape — energy loss means the lower trophic levels can only support small numbers of the predators at the top, so there is less overall biomass at higher levels than among producers at the pyramid base.