Modern ecologists view ecosystems in terms of energy flow, carbon flow or nutrient cycles. Living organisms within a unit area constitute a standing crop of biomass. Biomass means the mass of organisms per unit area of ground (or water) and is usually expressed in unit of energy (e.g., m-2) or dry organic matter (e.g., tons ha-1).
The great bulk of biomass in communities is formed by plants. Biomass includes whole bodies of organisms, even though parts of them may be dead. Primary productivity of a community is the rate at which biomass is produced per unit area by plants. It is expressed either in unit of energy (e.g., J m-2 day-1) or of dry organic matter (e.g., kg ha-1 year-1).
Total fixation of energy through photosynthesis is referred to as Gross Primary Productivity (GPP). A proportion of this is lost during respiration by plant and lost from the community as respiratory heat (R). Difference between GPP and R is utilized for production of new biomass that is available for consumption by heterotrophic organisms. Rate of production of biomass by heterotrophs is called secondary production.
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Community trophic structure consists of grazer system alongside a decomposer system. Subcompartments are recognized within each trophic level so that distinctions are made between invertebrates and vertebrates and between microbes and detritivores.
Energy consumed and assimilated by living organisms, some of which is lost from the system as respiratory heat during work. Possible pathways in grazer and decomposer system are same except that feces and dead bodies lost to decomposer system are simply sent back to dead organic matter pool. Energy available as dead organic conditions are unfavourable to decomposition process, leaving incompletely metabolized high energy matter, such as, oil, coal and peat.