Ecological Pyramids

Relationships between numbers, biomass, and energy flow (metabolism) at the biotic community level can be shown graphically by ecological pyramids, in which the first or producer trophic level forms the base, and the successive trophic levels form the tiers. By estimating the number of organisms at each trophic level, ecologists can create a graphic display called a pyramid of numbers for an ecosystem. Because the typical relationship involves many producers to not so many primary consumers to just a few secondary consumers, the graph usually looks something like a pyramid. For example, a million phyto-plankton in a small pond may support 10 000 zooplankton, which in turn may support 100 perch, which might feed one person for a month or so. However, number pyramids are frequently inverted (base smaller than one or more upper tiers) when individual producer organisms are much larger than the average consumers, as in temperate deciduous forests.

Each trophic level in a food chain or web contains a certain amount of biomass, the weight of all organic matter contained in its organisms. Ecologists estimate biomass by harvesting organisms from random patches or narrow strips in an ecosystem. The sample organisms are then sorted according to trophic levels, dried, and weighed. This data is used to plot a pyramid of biomass for the ecosystem and tends to be inverted when individual producers are much smaller than the average consumers, as in aquatic communities dominated by planktonic algae.

The ecological rule for this would be as follows: numbers overemphasize the importance of small organisms while biomass overemphasizes the importance of large organisms. Consequently, neither can be used as a reliable criterion for comparing the functional role of populations that differ widely in size-metabolism relationships, even though of the two, biomass is generally more reliable than numbers.

The energy flow provides a more suitable index for comparing any and all components of an ecosystem. As dictated by the second law of thermodynamics, however, the pyramid of energy must always have a true upright shape considering all sources of food energy. In a food chain or web, biomass (and thus chemical energy) is transferred from one trophic level to another, with some usable energy lost in each transfer. At each successive trophic level, some of the available biomass is neither eaten, digested, nor absorbed. Usable energy is also lost at each level. The percentage of usable energy transferred from one trophic level to the next varies from 5% to 20% (i.e., a loss of 80-95%), depending on the types of species and the ecosystem involved. The energy pyramid shows that the more the trophic levels or steps in a food chain or web, the greater the cumulative loss of usable energy.

Energy flow pyramids explain why the Earth can support more people if they eat at lower trophic levels, for example, by consuming organisms from the first trophic level directly rather than from higher trophic levels. The large loss in energy between successive trophic levels also explains why food chains and webs rarely have more than four or five trophic levels. In most cases, too little energy is left after four or five transfers to support organisms feeding at these high trophic levels. This explains why top carnivores such as eagles, tigers, and white sharks are few in number and are usually the first to suffer when the ecosystems that support them are disrupted.

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