Net primary production provides the energy for all heterotrophic activity. Consumers capture the energy stored within the organic molecules of their food sources. Therefore, each trophic level acquires the energy represented by the biomass consumed from the lower trophic level. The rate of conversion of NPP into heterotroph tissues is secondary productivity. As with primary productivity, we can distinguish the total rate of energy consumption by secondary producers from the energy incorporated into consumer tissues (net secondary productivity) after expenditure of energy through respiration. Secondary productivity is limited by the amount of net primary production because only the net energy stored in plants is available for consumers, secondary producers cannot consume more matter than is available, and energy is lost during each transfer between trophic levels.
Not all food energy removed by consumers is ingested. Consumer feeding often is wasteful. Scraps of food are dropped, or damaged plant parts are abscissed (Faeth et al. 1981, Risley and Crossley 1993), making this material available to decomposers. Of the energy contained in ingested material, some is not assimilable and is egested, becoming available to reducers. A portion of assimilated energy must be used to support metabolic work (e.g., for maintenance, food acquisition, and various other activities) and is lost through respiration (see Chapter 4). The remainder is available for growth and reproduction (secondary production).
Secondary production can vary widely among heterotrophs and ecosystems. Herbivores generally have lower efficiencies of food conversion (ingestion/GPP <10%) than do predators (<15%) because the chemical composition of animal food is more digestible than is plant food (Whittaker 1970). Heterotherms have higher efficiencies than do homeotherms because of the greater respiratory losses associated with maintaining constant body temperature (Golley 1968; see also Chapter 4). Therefore, ecosystems dominated by invertebrates or heterothermic vertebrates (e.g., most freshwater aquatic ecosystems dominated by insects and fish) will have higher rates of secondary production, relative to net primary production, than will ecosystems with greater representation of homeothermic vertebrates.
Eventually, all plant and animal matter enters the detrital pool as organisms die. The energy in detritus then becomes available to reducers (detritivores and decomposers). Detritivores fragment detritus and inoculate homogenized detritus with microbial decomposers during gut passage. Detrital material consists primarily of lignin and cellulose, but detritivores often improve their efficiency of energy assimilation by association with gut microorganisms or by reingestion of feces (coprophagy) following microbial decay of cellulose and lignin (e.g., Breznak and Brune 1994).
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