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lecture+16+Ecosystems

Photosynthesis and transferred inefficiently between

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photosynthesis, and transferred inefficiently between trophic levels We can evaluate patterns of and generalities in energy through ecosystems (beyond plants) by making use of trophic levels. These lump organisms into groups that derive their energy at the same remove from plants (for example, plants are at the first trophic level, herbivores the second, predators that eat herbivores are at the third trophic level). Trophic transfer efficiencies are relatively low. As a general average, about 10% of the energy that is assimilated at one trophic level is assimilated at the next level above.
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Questions The inefficiency of energy transfer allows us to answer questions like: Why are large fierce animals rare? Why do some toxins have disproportionate effects at higher trophic levels?
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Element Supply Organisms require essential elements in characteristic ratios; these ratios are described in terms of stoichiometry. Stoichiometry is a property of chemical reactions, where it represents the characteristic ratio of reactants and products in a reaction like photosynthesis: 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 This stoichiometry can be expressed in molecular terms, as above, or in elemental terms both sides have 6 C, 12 H, and 18 O. The advantage of thinking in terms of elements is that elements are conserved in all but nuclear reactions
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Ecologically Important Elements Most analyses of element (nutrient) cycling emphasize C, N, and P. C embodies energy; it’s the basis of biomass. It is linked to O, in photosynthesis and respiration N makes up much of protein enzymes and is fundamental to metabolism. In plants, the connection between N and photosynthesis is particularly strong.
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photosynthesis and transferred inefficiently between...

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