The term ecosystem is used to denote the biological community (primary producers, decomposers, detritivores, herbivores, etc.) together with the abiotic environment in which it is set. Lindemann laid the foundations of a science of ecological energetics by considering the efficiency of transfer between trophic levels - from incident radiation received by a community through its capture by green plants in photosynthesis to its subsequent use by heterotrophs. This is the topic of the present chapter.
The bodies of the living organisms within a unit area constitute a standing crop of biomass. Primary productivity is the rate at which biomass is produced per unit area by plants. The total fixation of energy by photosynthesis is gross primary productivity (GPP), a proportion of which is respired by the plants as autotrophic respiration (RA). The difference between GPP and RA is net primary productivity (NPP) and represents the actual rate of production of new biomass that is available for consumption by heterotrophic organisms. The rate of production of biomass by heterotrophs is secondary productivity, and their respiration is heterotrophic respiration (HE). Net ecosystem productivity (NEP) is GPP minus total respiration (RA + RH).
We discuss the broad patterns in primary productivity across the face of the globe and in relation to seasonal and annual variations in conditions, and note that primary productivity : biomass ratios are higher in aquatic than terrestrial communities.
The factors that limit terrestrial primary productivity are solar energy (and particularly its inefficient use by plants), water and temperature (and their complex interactions), soil texture and drainage, and mineral nutrient availability. The length of the growing season is particularly influential. In aquatic environments, primary productivity depends in particular on the availability of solar radiation (with strong patterns related to water depth) and nutrients (especially important are human inputs to lakes, estuarine inputs to oceans and ocean upwelling zones).
Unlike plants, heterotrophic bacteria, fungi and animals cannot manufacture from simple molecules the complex, energy-rich compounds they need. They derive their matter and energy either directly by consuming plant material or indirectly from plants by eating other heterotrophs. There is a general positive relationship between primary and secondary productivity in ecosystems, but most primary production passes, when dead, through the detritus system rather than as living material through the grazing system. The pathways traced by energy through communities are determined by three energy transfer efficiencies (consumption, assimilation and production efficiencies). Grazer consumption efficiencies are highest where plants have little structural support tissue and low C : N and C : P ratios. We discuss temporal patterns in the balance between primary productivity and its consumption by heterotrophs, and show that broad climatic patterns (such as El Niño) can profoundly influence ecosystem energetics.
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