In contrast to GPP, NPP is easier to assess, largely by measuring the production of new autotrophic biomass, or through gas-exchange measurements of canopy stands. NPP is GPP minus respiratory loss. The great utility of NPP to ecologists is that it reflects the amount of biomass potentially available for consumption by heterotrophs, and thus is a measure of energy and biomass available to support foodweb and ecosystem processes. High levels of NPP enhance ecosystem complexity by increasing the resource availability for trophic interactions. Because of the energy costs needed for growth and maintenance of an organism, there is typically a large (75-99%) reduction of energy content in each successive trophic level in an ecosystem. A rough approximation is that successive trophic levels differ in energy by order of magnitude. In large and highly productive ecosystems, sufficient energy may flow from primary producers to support primary consumers (herbivores and detritovores) with two or three trophic levels of secondary consumers (carnivores) (Figure 2). In unproductive systems, insufficient energy is available in primary producers to support large trophic pyramids with multiple levels of carnivores. Consequently, unproductive systems are simpler with less overall diversity.
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