| Lecture 38: Energy Efficiency and Biological Magnification
Reading: Economy of Nature, pp. 624-631.
Efficiency Within a Trophic Level Production Efficiency
Whole community data are difficult to collect but individual species data are known, and production efficiency varies between different groups of animals. Birds and Mammals (endotherms) 1% - 3% Habitat differences do not account for this 10x difference in efficiency. This is a fundamental difference between endothermic and ectothermic organisms, birds and mammals compared to all other organisms.
Efficiency Between Trophic Levels
In terrestrial systems, Lindeman’s Efficiency is approximately 10%. In aquatic and marine systems, Lindeman’s Efficiency is 15%-20%. These energy transfer efficiencies can be expressed in graphic forms called Eltonian Pyramids (Ricklefs, 1996, p 137, Fig. 6.8). Eltonian (trophic) Pyramids can be based on evaluation of numbers, standing crop biomass, energy standing crop, or energy flow rates.
Numbers (numbers per unit area): Generally, organisms at lower trophic levels (primary producers) are more abundant than those at higher trophic levels (top carnivores). Inverted trophic pyramids are possible for forest ecosystems, since tree are the largest organisms in the system, but an inverted pyramid does not correctly reflect the flow of energy in those ecosystems. Bias in pyramid structure due to individual body size occurs (after Odum, 1971, p 80, Fig. 3-15).
Biomass (biomass per unit area): As with numbers, there may be bias due to size differences between organisms at different trophic levels (after Odum, 1971, p 80, Fig. 3-15).
Standing crop energy (kcal per unit area): The same biases possible for biomass and numbers may occur in standing crop energy (after Odum, 1971, p 80, Fig. 3-15).
Energy flow rate (kcal per unit area per unit time): These are the most accurate representations of trophic relationships (after Odum, 1971, p 80, Fig. 3-15).
Trophic relations in terms of energy flow rates are shown for a tallgrass prairie site in the United States (data from Krebs, 1994, p 649, Fig. 27.12).
Note that more energy flow is below ground than above ground in this ecosystem.
The energy flow rate decreases with each trophic level by at least an order of magnitude. This is generally true regardless of the form of energy flow evaluated as seen in data for Cedar Bog Lake, MN (Ricklefs, 1996, p 143, Table 6.2).
Similar patterns are seen in total energy flow rates in a Georgia salt marsh (Ricklefs, 1996, p 186, Fig. 8.17).
Biological Accumulation and Magnification Energy is dispersed in food chains but matter is often concentrated. Many nutrients (minerals nutrients) are accumulated by primary producers, so the concentrations in living tissues is greater than the concentrations of the same materials in the environment (examples are Ca, P, K, Mg). These materials are incorporated in organic compounds which are broken down and reused by consumers, or broken down and recycled in biogeochemical cycles (the next major topic to be considered). Some materials are accumulated by primary producers but persist and are passed to consumers intact. Such persistent materials are magnified as they are passed up a food chain because consumers receive all the persistent matter but only a fraction of the energy from the organism in the trophic level beneath them. Dichloro-diphenyl-trichloroethane (DDT) is a potent insecticide that was used for mosquito control on the coastal waters of Long Island, NY between 1942 and 1960. The chemical was applied to the water at concentrations that were non-lethal to fishes and other wildlife but effective against mosquito larvae. However, DDT appeared in top carnivore animals at concentrations as high as 100,000x greater than the application concentrations as a result of biological magnification by the food chain. Similar magnification was observed with the application of DDD to Clear Lake, CA for control of gnats between 1949 and 1954. DDD concentration in the water began at 0.02 ppm and was collected from the carcasses of fish eating birds (western grebe) at 1600 ppm (in body fat). Similar biological magnification occurs with any material that is taken-up by organisms and persists, this is not a unique characteristic of organophosphate pesticides.
Biological magnification of DDT in coastal waters of Long Island, NY. DDT concentrations (parts per million) from organisms are in fatty tissues.
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