Ecological pyramids are graphical representations of the trophic structure of ecosystems. Ecological pyramids are organized with the productivity of plants on the bottom, that of herbivores above the plants, and carnivores above the herbivores. If the ecosystem sustains top carnivores, they are represented at the apex of the ecological pyramid of productivity.
Ecological food webs are based on the productivity of green plants (or photoautotrophs), which are the only organisms capable of utilizing diffuse solar radiation to synthesize simple organic compounds from carbon dioxide and water. The fixed energy of the simple organic compounds, plus inorganic nutrients, are then used by plants in more complex metabolic reactions to synthesize a vast diversity of biochemicals. Plants utilize the fixed energy of their biochemicals to achieve growth and reproduction. On average, plant photosynthesis utilizes less than 1% of the solar radiation that is received at the surface of the earth. Higher efficiencies are impossible for a number of reasons, including the second law of thermodynamics, but also other constraining factors such as the availability of nutrients and moisture, appropriate temperatures for growth, and other environmental limitations.
Ecological pyramids are based on the productivity of organisms. Plants account for 90% of the total productivity of the food web, and herbivores account for most of the rest. Carnivores are responsible for less than 1% of ecological productivity.
Courtesy of Gale Research.
However, even relatively fertile plant communities can only achieve conversion efficiencies of 10% or so, and only for relatively short periods of time.
The solar energy fixed by green plants in photosynthesis is, of course, the energetic basis of the productivity of all heterotrophic organisms that can only feed upon living or dead biomass, such as animals and microorganisms. Some of the biomass of plants is consumed as food by animals in the next trophic level, that of herbivores. However, herbivores cannot convert all of the energy of the vegetation that they eat into their own biomass. Depending on the digestibility of the food being consumed, the efficiency of this process is about 1-20%. The rest of the fixed energy of the plant foods is not assimilated by herbivores, or is converted into heat. Similarly, when carnivores eat other animals, only some of the fixed energy of the prey is converted into biomass of the predator. The rest is ultimately excreted, or is converted into heat, in accordance with the requirement for entropy to increase during any energy transformation.
A fact of ecological energetics is that whenever the fixed energy of biomass is passed along a food chain, substantial energy losses occur during each transfer. These energy losses are a necessary consequence of the so-called second law of thermodynamics. This universal principle states that whenever energy is transformed from one state to another, the entropy of the universe must increase (entropy refers to the randomness of distributions of matter and energy). In the context of transfers of fixed biological energy along the trophic chains of ecosystems, increases in entropy are represented by losses of energy as heat (because energy is converted from a highly ordered state in biomass, to a much less-ordered condition as heat). The end result is that transfers of energy between organisms along food chains are inefficient, and this causes the structure of productivity in ecological food webs to always be pyramid shaped.