Energy Flow through Ecosystems

Modeling Ecosystems Energy Flow: Ecological Pyramids

The structure of ecosystems can be visualized with ecological pyramids, which were first described by the pioneering studies of Charles Elton in the 1920s. Ecological pyramids show the relative amounts of various parameters (such as number of organisms, energy, and biomass) across trophic levels.

Pyramids of numbers can be either upright or inverted, depending on the ecosystem. As shown in Figure, typical grassland during the summer has a base of many plants, and the numbers of organisms decrease at each trophic level. However, during the summer in a temperate forest, the base of the pyramid consists of few trees compared with the number of primary consumers, mostly insects. Because trees are large, they have great photosynthetic capability, and dominate other plants in this ecosystem to obtain sunlight. Even in smaller numbers, primary producers in forests are still capable of supporting other trophic levels.

Another way to visualize ecosystem structure is with pyramids of biomass. This pyramid measures the amount of energy converted into living tissue at the different trophic levels. Using the Silver Springs ecosystem example, this data exhibits an upright biomass pyramid (Figure), whereas the pyramid from the English Channel example is inverted. The plants (primary producers) of the Silver Springs ecosystem make up a large percentage of the biomass found there. However, the phytoplankton in the English Channel example make up less biomass than the primary consumers, the zooplankton. As with inverted pyramids of numbers, this inverted pyramid is not due to a lack of productivity from the primary producers, but results from the high turnover rate of the phytoplankton. The phytoplankton are consumed rapidly by the primary consumers, thus, minimizing their biomass at any particular point in time. However, phytoplankton reproduce quickly, thus they are able to support the rest of the ecosystem.

Pyramid ecosystem modeling can also be used to show energy flow through the trophic levels. Notice that these numbers are the same as those used in the energy flow compartment diagram in (). Pyramids of energy are always upright, and an ecosystem without sufficient primary productivity cannot be supported. All types of ecological pyramids are useful for characterizing ecosystem structure. However, in the study of energy flow through the ecosystem, pyramids of energy are the most consistent and representative models of ecosystem structure (Figure).

Art Connection

 Part A: on the left is a pyramid diagram of the number of individuals per 0.1 hectare in a summer grassland. There are 1,500,000 grass plants, 200,000 herbivorous insects, 90,000 predatory insects, and 1 bird. Part A: on the right is a pyramid diagram of organisms per 0.1 hectare in a temperate forest. There are 200 trees, 150,000 herbivorous insects, 120,000 predatory insects, and 5 birds. Part B: on the left is a pyramid diagram of dry biomass in grams per meter squared in the English Channel. The biomass is 4 phytoplankton and 21 zooplankton. Part B: on the right is a pyramid diagram of dry biomass in grams per meter squared in Silver Springs, Florida. The biomass of plants is 809. The biomass of primary consumers, including herbivorous insects and snails is 37. The biomass of secondary consumer fishes is 11, and the biomass of tertiary consumer fishes is 5. Primary, secondary and tertiary decomposers have a combined biomass of 5. Part C is a pyramid diagram of energy in kilocalories per meter squared per year. The energy of plants is 20,810. The energy of primary consumers, including insects and snails, is 3,368. The energy of primary consumer fishes is 383, and the energy of secondary consumer fishes is 21. The energy of decomposers, including fungi and bacteria, is 5,060.
Ecological pyramids depict the (a) biomass, (b) number of organisms, and (c) energy in each trophic level.

Pyramids depicting the number of organisms or biomass may be inverted, upright, or even diamond-shaped. Energy pyramids, however, are always upright. Why?

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