SC BIOL 301 - Ecosystem Ecology (5 pages)

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Ecosystem Ecology

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Ecosystem Ecology


Primary Productivity, Net Productivity, Movement of Energy, Hydrologic Cycle, Carbon Cycle

Lecture number:
Lecture Note
University Of South Carolina-Columbia
Biol 301 - Ecology and Evolution
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Biol 301 1st Edition Lecture 19 Outline of Last Lecture I Community Boundaries II Community Diversity III Species Diversity IV Community food webs V Community Stability Outline of Current Lecture II Primary Productivity III Net Productivity IV Movement of Energy V Hydrologic Cycle VI Carbon Cycle Current Lecture Primary Productivity 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 Standing 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 Respiration Both GPP and NPP are expressed in units of Joules J m2 year These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Primary 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 Researchers 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 Since producers take up CO2 during photosynthesis and produce CO2 during respiration we can measure NPP by recording the rate of CO2 exchange from the leaf Leaves are placed in a sealed chamber with a CO2 sensor When a leaf is exposed to light in the chamber photosynthesis and respiration occur the net uptake of CO2 represents NPP When a leaf is concealed in the dark only CO2 respiration occurs We can then measure GPP GPP NPP Respiration Other techniques can be used to measure CO2 uptake and release Researchers can place a leaf into a sealed container with added CO 2 that contains trace amounts of a rare carbon isotope e g 14C To measure NPP they track the net movement of 14C from the air into the plant tissues and back into the air On a larger scale e g a forest researchers measure CO 2 uptake and release by using towers that sample CO2 concentrations at different heights above the ground The differences in CO2 within the forest and in the atmosphere provides an estimate of photosynthesis and respiration in an area In aquatic systems measuring CO2 does not provide a good estimate of NPP because CO2 is rapidly converted into bicarbonate ions Since producers release O2 during photosynthesis and take up O2 during respiration we can estimate NPP and GPP by measuring changes in concentrations of O2 The process is identical to a light dark bottle experiment in a sealed air chamber except it is done in sealed water chambers In a bottle exposed to light the net increase in O2 is the combined result of photosynthesis and respiration by algae NPP In a dark bottle the decrease in O2 is a result of algal respiration The sum of NPP and respiration is equal to GPP To assess productivity across large spatial scales e g continents oceans we can use remote sensing Remote sensing a technique that allows measurement of conditions on Earth from a distant location typically using satellites or airplanes that take photographs of large areas of the globe Chlorophyll pigments absorb wavelengths in the red and blue range but reflect wavelengths in the green range Satellite images that show a pattern of high absorption of blue and red light and high reflectance of green light indicate ecosystems with high standing crops Net Productivity NPP varies with latitude and across terrestrial and aquatic biomes Tropical areas have high NPP due to intense sunlight warm temperatures abundant precipitation and rapidly recycled nutrients At higher latitudes productivity is limited by shorter periods of sunlight and lower temperatures In deserts NPP is constrained by lack of precipitation Ecosystems that receive more than 3 m of precipitation experience a decline in NPP because nutrients leach from the soil Rates of decomposition are reduced in waterlogged soils When organic matter is broken down slowly fewer nutrients are available in the soil for plant growth Nutrients particularly nitrogen and phosphorus can strongly affect NPP in terrestrial ecosystems Researchers conducted a survey of studies that examined the ratio of NPP in terrestrial ecosystems with added nutrients to NPP in terrestrial ecosystems without added nutrients Grasslands forests and tundra habitats all experienced increased NPP when nitrogen and phosphorus were added In some habitats adding both nutrients causes a greater increase in NPP than adding either alone In addition to temperature precipitation and nutrients aquatic ecosystems are also limited by light Within aquatic ecosystems that have similar temperatures and light levels NPP is largely limited by nutrients Both nitrogen N and phosphorus P can limit NPP of aquatic ecosystems In an hourglass shaped lake in Ontario researchers put in place a plastic curtain that divided the lake into two halves On one side they added carbon and N on the other they added carbon N and P On the side that received carbon N and P there was a large increase in NPP due to growth of algae Movement of Energy Trophic pyramid a chart composed of stacked rectangles representing the amount of energy i e pyramid of energy or biomass i e pyramid of biomass in each

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