• Actual evapotranspiration is the water annually transpired by plants and evaporated from a landscape. Concept 55.3 Energy transfer between trophic levels is typically only 10% efficient • Secondary production of an ecosystem is the amount of chemical energy in food converted to new biomass during a given period of time. Production Efficiency • When a caterpillar feeds on a leaf, only about one-sixth of the leaf’s energy is used for secondary production. • An organism’s production efficiency is the fraction of energy stored in food that is not used for respiration. Production efficiency= net secondary production x 100%/ assimilation of primary production Trophic Efficiency and Ecological Pyramids • Trophic efficiency is the percentage of production transferred from one trophic level to the next. • Trophic efficiency is multiplied over the length of a food chain. • Approximately 0.1% of chemical energy fixed by photosynthesis reaches a tertiary consumer. • A pyramid of net production represents the loss of energy with each transfer in a food chain. • In a biomass pyramid, each tier represents the dry weight of all organisms in one trophic level. Most biomass pyramids show a sharp decrease at successively higher trophic levels. • Certain aquatic ecosystems have inverted biomass pyramids: producers (phytoplankton) are consumed so quickly that they are outweighed by primary consumers. • Turnover time is a ratio of the standing crop biomass to production. Turnover time = standing crop (g/m^2)/ production (g/m^2 x day) • The dynamics of energy flow in ecosystems have important implications for the human population. o Eating meat is a relatively inefficient way of tapping photosynthetic production. o Worldwide agriculture could feed many more people if humans ate only plant material. Concept 55.4 Biological and geochemical processes cycle nutrients and water in ecosystems • Life depends on recycling chemical elements. • Nutrient circuits in ecosystems involve biotic and abiotic components and are often called biogeochemical cycles. Biogeochemical Cycle • Gaseous carbon, oxygen, sulfur, and nitrogen occur in the atmosphere and cycle globally.
• Less mobile elements such as phosphorus, potassium, and calcium cycle on a more local level. • A model of nutrient cycling includes main reservoirs of elements and processes that transfer elements between reservoirs. • All elements cycle between organic and inorganic reservoirs. The Water Cycle Water is essential to all organisms. 97% of the biosphere’s water is contained in the oceans, 2% is in glaciers and polar ice caps, and 1% is in lakes, rivers, and groundwater. Water moves by the processes of evaporation, transpiration, condensation, precipitation, and movement through surface and groundwater. The Carbon Cycle Carbon-based organic molecules are essential to all organisms. Carbon reservoirs include fossil fuels, soils and sediments, solutes in oceans, plant and animal biomass, and the atmosphere. CO 2 is taken up and released through photosynthesis and respiration; additionally, volcanoes and the burning of fossil fuels contribute CO 2 to the atmosphere.
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