Net Ecosystem Productivity

In most ecosystems, the vast majority of NPP is ultimately consumed by heterotrophs, either from living vegetation by herbivores (including parasites of plants and algae), or from dead vegetation by detritovores. Most of the NPP leaves the system as respired CO2, yet a fraction remains as accrued carbon. To describe carbon accumulation in an

Secondary carnivores Primary carnivores

Herbivores id detritovores

Productive ecosystem with high NPP

Consumers >- or secondary producers

Primary carnivores Herbivores, detritovores Primary producers

Productive ecosystem with high NPP

Unproductive ecosystem with low NPP

Figure 2 The energy content in a hypothetical trophic pyramid from a productive and unproductive ecosystem. It is assumed that energy content declines tenfold in successively higher trophic levels. In an unproductive system, the amount of energy is too small to support a viable population of secondary carnivores, and hence the system can only support primary carnivores.

ecosystem, the concept net ecosystem productivity (NEP) is used. NEP is a measure of the carbon accumulation in an ecosystem over time, typically, on an annual basis. It reflects the difference between GPP and all heterotrophic processes in a system. In ecosystems in early stages of development (i.e., in early-successional stages), NEP can be consistently high from year to year, because much of the biomass will remain in the system in the bodies of long-lived organisms, increasing populations of recently arrived species, detritus, and organic carbon in the soil or sediments. In mature ecosystems, NEP is typically negligible when averaged over multiple years, because the maintenance costs and death of existing organisms balances the production of new biomass by autotrophs. In old, senescent ecosystems, NEP is often negative. Potential GPP in senescent ecosystems often declines because the standing biomass may lock up mineral nutrients, and overcrowding of the leaf canopy can lead to excessive self-shading. With a loss in GPP, respiration by the standing biomass typically exceeds autotrophy and NEP declines.

GPP and NPP typically have no consistent relationship to NEP. High GPP and NPP can potentially enhance NEP in a direct proportion, but this requires all forms of heterotrophic activity to be relatively low. The best correlations between NPP and NEP are present in early to mid-successional stages in ecosystems, when resource availability is relatively high, and auto-trophic biomass can exceed heterotrophic activity. Paradoxically, some of the highest sustained levels of NEP occur in stressed environments where heterotro-phy is low. Peat bogs in boreal regions ofthe planet have high NEP despite low photosynthetic potential, because the anaerobic, low temperature and acidic water of the bogs prevents decay. Since the end of the ice ages, peat has steadily accumulated such that the high-latitude bogs now form some of the largest stores of organic carbon on the surface of the Earth.

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