Chapter 20 conceptsPrimary productivity provides energy to the ecosystem.Net primary productivity differs among ecosystems.The movement of energy depends on the efficiency of energy flow.123
Primary productivity1Most energy that moves through ecosystems originates as solar energy that powers photosynthesis.Where sunlight is not available (e.g., deep ocean thermal vents), producers rely on chemosynthesis as their source of energy.Producers harness energy and form the basis of food webs.Producers use energy for respiration, growth, and reproduction.The energy used for growth and reproduction is the energy available to consumers.Primary productivity: the rate at which solar or chemical energy is captured and converted into chemical bonds by photosynthesis or chemosynthesis.
Primary productivity1Standing crop: the biomass of producers present in a given area of an ecosystem at a particular moment in time.Ecosystems with high primary productivity may not have a high standing crop; consumers may eat it as quickly as it grows.Gross primary productivity (GPP):the rate at which energy is captured and assimilated by producers in an area.Net primary productivity (NPP): the rate of energy that is assimilated by producers and converted into producer biomass in an area; includes all energy that is not respired:NPP = GPP - RespirationBoth GPP and NPP are expressed in units of Joules (J) / m2/ year.
Primary productivity1Photosynthesis is not a very efficient process.
Measuring primary productivity1Primary productivity is a rate; the choice of how to measure this rate depends on the particular ecosystem being studied.NPP can be measured as the change in producer biomass over time.Researchers harvest plants to determine mass of growth over a period of time.Substantial amounts of herbivory or tissue mortality will lead to an underestimation of NPP.Researchers may estimate the biomass lost to herbivory or tissue mortality.
Measuring primary productivity1Researchers typically only harvest above-ground plant growth.The amount of below-ground biomass can be substantial (e.g., rhizomes, roots).Harvesting below-ground tissues is challenging because they are deep and tend to break off when harvested.In addition, fine roots frequently die and are replaced, making it difficult to estimate the biomass accumulation.Plants also send energy to mycorrhizal fungi; this energy is included in total NPP.
Light-dark bottle experiments1Since producers take up CO2during photosynthesis and produce CO2during respiration, we can measure NPP by recording the rate of CO2exchange from the leaf.