Sinh học - Chapter 55: Ecosystems

Chapter 55EcosystemsOverview: Observing EcosystemsAn ecosystem consists of all the organisms living in a community, as well as the abiotic factors with which they interactEcosystems range from a microcosm, such as an aquarium, to a large area such as a lake or forestRegardless of an ecosystem’s size, its dynamics involve two main processes: energy flow and chemical cyclingEnergy flows through ecosystems while matter cycles within themFig. 55-1Fig. 55-2Concept 55.1: Physical laws govern energy flow and chemical cycling in ecosystemsEcologists study the transformations of energy and matter within their systemConservation of EnergyLaws of physics and chemistry apply to ecosystems, particularly energy flowThe first law of thermodynamics states that energy cannot be created or destroyed, only transformedEnergy enters an ecosystem as solar radiation, is conserved, and is lost from organisms as heatThe second law of thermodynamics states that every exchange of energy increases the entropy of the universeIn an ecosystem, energy conversions are not completely efficient, and some energy is always lost as heatConservation of MassThe law of conservation of mass states that matter cannot be created or destroyedChemical elements are continually recycled within ecosystemsIn a forest ecosystem, most nutrients enter as dust or solutes in rain and are carried away in waterEcosystems are open systems, absorbing energy and mass and releasing heat and waste productsEnergy, Mass, and Trophic LevelsAutotrophs build molecules themselves using photosynthesis or chemosynthesis as an energy source; heterotrophs depend on the biosynthetic output of other organismsEnergy and nutrients pass from primary producers (autotrophs) to primary consumers (herbivores) to secondary consumers (carnivores) to tertiary consumers (carnivores that feed on other carnivores) Detritivores, or decomposers, are consumers that derive their energy from detritus, nonliving organic matterProkaryotes and fungi are important detritivoresDecomposition connects all trophic levelsFig. 55-3Fig. 55-4Microorganismsand otherdetritivoresTertiary consumersSecondaryconsumersPrimary consumersPrimary producersDetritusHeatSunChemical cyclingKeyEnergy flowConcept 55.2: Energy and other limiting factors control primary production in ecosystemsPrimary production in an ecosystem is the amount of light energy converted to chemical energy by autotrophs during a given time periodEcosystem Energy BudgetsThe extent of photosynthetic production sets the spending limit for an ecosystem’s energy budgetThe Global Energy BudgetThe amount of solar radiation reaching the Earth’s surface limits photosynthetic output of ecosystemsOnly a small fraction of solar energy actually strikes photosynthetic organisms, and even less is of a usable wavelengthGross and Net Primary ProductionTotal primary production is known as the ecosystem’s gross primary production (GPP)Net primary production (NPP) is GPP minus energy used by primary producers for respirationOnly NPP is available to consumersEcosystems vary greatly in NPP and contribution to the total NPP on EarthStanding crop is the total biomass of photosynthetic autotrophs at a given timeFig. 55-5VisibleWavelength (nm)Near-infraredLiquid waterSoilVegetationCloudsSnowPercent reflectance04006008001,0001,20020406080TECHNIQUETropical rain forests, estuaries, and coral reefs are among the most productive ecosystems per unit areaMarine ecosystems are relatively unproductive per unit area, but contribute much to global net primary production because of their volumeFig. 55-6Net primary production (kg carbon/m2·yr)0123·Primary Production in Aquatic EcosystemsIn marine and freshwater ecosystems, both light and nutrients control primary productionLight LimitationDepth of light penetration affects primary production in the photic zone of an ocean or lakeNutrient LimitationMore than light, nutrients limit primary production in geographic regions of the ocean and in lakesA limiting nutrient is the element that must be added for production to increase in an areaNitrogen and phosphorous are typically the nutrients that most often limit marine productionNutrient enrichment experiments confirmed that nitrogen was limiting phytoplankton growth off the shore of Long Island, New YorkFig. 55-7Atlantic OceanMoriches BayShinnecockBayLong IslandGreat South BayABCDEFGEXPERIMENTAmmoniumenrichedPhosphateenrichedUnenrichedcontrolRESULTSABCDEFG3024181260Collection sitePhytoplankton density(millions of cells per mL)Fig. 55-7aAtlantic OceanMoriches BayShinnecockBayLong IslandGreat South BayABCDEFGEXPERIMENTFig. 55-7bAmmoniumenrichedPhosphateenrichedUnenrichedcontrolRESULTSABCDEFG3024181260Collection sitePhytoplankton density(millions of cells per mL)Experiments in the Sargasso Sea in the subtropical Atlantic Ocean showed that iron limited primary productionTable 55-1Upwelling of nutrient-rich waters in parts of the oceans contributes to regions of high primary productionThe addition of large amounts of nutrients to lakes has a wide range of ecological impactsIn some areas, sewage runoff has caused eutrophication of lakes, which can lead to loss of most fish speciesVideo: Cyanobacteria (Oscillatoria)Primary Production in Terrestrial EcosystemsIn terrestrial ecosystems, temperature and moisture affect primary production on a large scaleActual evapotranspiration can represent the contrast between wet and dry climatesActual evapotranspiration is the water annually transpired by plants and evaporated from a landscapeIt is related to net primary productionNet primary production (g/m2·yr)Fig. 55-8Tropical forestActual evapotranspiration (mm H2O/yr)Temperate forestMountain coniferous forestTemperate grasslandArctic tundraDesertshrubland1,5001,000500001,0002,0003,000·On a more local scale, a soil nutrient is often the limiting factor in primary productionConcept 55.3: Energy transfer between trophic levels is typically only 10% efficientSecondary production of an ecosystem is the amount of chemical energy in food converted to new biomass during a given period of timeProduction EfficiencyWhen a caterpillar feeds on a leaf, only about one-sixth of the leaf’s energy is used for secondary productionAn organism’s production efficiency is the fraction of energy stored in food that is not used for respirationFig. 55-9Cellularrespiration100 JGrowth (new biomass)Feces200 J33 J67 JPlant materialeaten by caterpillarTrophic Efficiency and Ecological PyramidsTrophic efficiency is the percentage of production transferred from one trophic level to the nextIt usually ranges from 5% to 20%Trophic efficiency is multiplied over the length of a food chainApproximately 0.1% of chemical energy fixed by photosynthesis reaches a tertiary consumerA pyramid of net production represents the loss of energy with each transfer in a food chainFig. 55-10Primaryproducers100 J1,000,000 J of sunlight10 J1,000 J10,000 JPrimaryconsumersSecondaryconsumersTertiaryconsumersIn a biomass pyramid, each tier represents the dry weight of all organisms in one trophic levelMost biomass pyramids show a sharp decrease at successively higher trophic levelsFig. 55-11(a) Most ecosystems (data from a Florida bog)Primary producers (phytoplankton)(b) Some aquatic ecosystems (data from the English Channel)Trophic levelTertiary consumersSecondary consumersPrimary consumersPrimary producersTrophic levelPrimary consumers (zooplankton)Dry mass(g/m2)Dry mass(g/m2)1.51137809214Certain aquatic ecosystems have inverted biomass pyramids: producers (phytoplankton) are consumed so quickly that they are outweighed by primary consumersTurnover time is a ratio of the standing crop biomass to productionDynamics of energy flow in ecosystems have important implications for the human populationEating meat is a relatively inefficient way of tapping photosynthetic productionWorldwide agriculture could feed many more people if humans ate only plant materialThe Green World HypothesisMost terrestrial ecosystems have large standing crops despite the large numbers of herbivoresFig. 55-12The green world hypothesis proposes several factors that keep herbivores in check:Plant defensesLimited availability of essential nutrientsAbiotic factorsIntraspecific competitionInterspecific interactionsConcept 55.4: Biological and geochemical processes cycle nutrients between organic and inorganic parts of an ecosystemLife depends on recycling chemical elementsNutrient circuits in ecosystems involve biotic and abiotic components and are often called biogeochemical cyclesBiogeochemical CyclesGaseous carbon, oxygen, sulfur, and nitrogen occur in the atmosphere and cycle globallyLess mobile elements such as phosphorus, potassium, and calcium cycle on a more local levelA model of nutrient cycling includes main reservoirs of elements and processes that transfer elements between reservoirsAll elements cycle between organic and inorganic reservoirsFig. 55-13Reservoir AReservoir BOrganicmaterialsavailableas nutrientsFossilizationOrganicmaterialsunavailableas nutrientsReservoir DReservoir CCoal, oil,peatLivingorganisms,detritusBurningof fossil fuelsRespiration,decomposition,excretionAssimilation,photosynthesisInorganicmaterialsavailableas nutrientsInorganicmaterialsunavailableas nutrientsAtmosphere,soil, waterMineralsin rocksWeathering,erosionFormation ofsedimentary rockIn studying cycling of water, carbon, nitrogen, and phosphorus, ecologists focus on four factors:Each chemical’s biological importanceForms in which each chemical is available or used by organismsMajor reservoirs for each chemicalKey processes driving movement of each chemical through its cycleThe Water CycleWater is essential to all organisms97% 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 groundwaterWater moves by the processes of evaporation, transpiration, condensation, precipitation, and movement through surface and groundwater Fig. 55-14aPrecipitationover landTransportover landSolar energyNet movement ofwater vapor by windEvaporationfrom oceanPercolationthroughsoilEvapotranspirationfrom landRunoff andgroundwaterPrecipitationover oceanThe Carbon CycleCarbon-based organic molecules are essential to all organismsCarbon reservoirs include fossil fuels, soils and sediments, solutes in oceans, plant and animal biomass, and the atmosphereCO2 is taken up and released through photosynthesis and respiration; additionally, volcanoes and the burning of fossil fuels contribute CO2 to the atmosphereFig. 55-14bHigher-levelconsumersPrimaryconsumersDetritusBurning offossil fuelsand woodPhyto-planktonCellularrespirationPhoto-synthesisPhotosynthesisCarbon compoundsin waterDecompositionCO2 in atmosphereThe Terrestrial Nitrogen CycleNitrogen is a component of amino acids, proteins, and nucleic acidsThe main reservoir of nitrogen is the atmosphere (N2), though this nitrogen must be converted to NH4+ or NO3– for uptake by plants, via nitrogen fixation by bacteriaOrganic nitrogen is decomposed to NH4+ by ammonification, and NH4+ is decomposed to NO3– by nitrificationDenitrification converts NO3– back to N2Fig. 55-14cDecomposersN2 in atmosphereNitrificationNitrifyingbacteriaNitrifyingbacteriaDenitrifyingbacteriaAssimilationNH3NH4NO2NO3+––AmmonificationNitrogen-fixingsoil bacteriaNitrogen-fixingbacteriaThe Phosphorus CyclePhosphorus is a major constituent of nucleic acids, phospholipids, and ATPPhosphate (PO43–) is the most important inorganic form of phosphorusThe largest reservoirs are sedimentary rocks of marine origin, the oceans, and organismsPhosphate binds with soil particles, and movement is often localizedFig. 55-14dLeachingConsumptionPrecipitationPlantuptakeof PO43–SoilSedimentationUptakePlanktonDecompositionDissolved PO43–RunoffGeologicupliftWeatheringof rocksDecomposition and Nutrient Cycling RatesDecomposers (detritivores) play a key role in the general pattern of chemical cyclingRates at which nutrients cycle in different ecosystems vary greatly, mostly as a result of differing rates of decompositionThe rate of decomposition is controlled by temperature, moisture, and nutrient availabilityRapid decomposition results in relatively low levels of nutrients in the soilFig. 55-15Ecosystem typeEXPERIMENTRESULTSArcticSubarcticBorealTemperateGrasslandMountainPODJRQKB,CE,FH,ILNUSTMGAA80706050403020100–15–10–5051015Mean annual temperature (ºC)Percent of mass lostBCDEFGHIJKLMNOPQRSTUFig. 55-15aEcosystem typeEXPERIMENTArcticSubarcticBorealTemperateGrasslandMountainPODJRQKB,CE,FH,ILNUSTMGAFig. 55-15bRESULTSA80706050403020100–15–10–5051015Mean annual temperature (ºC)Percent of mass lostBCDEFGHIJKLMNOPQRSTUCase Study: Nutrient Cycling in the Hubbard Brook Experimental ForestVegetation strongly regulates nutrient cyclingResearch projects monitor ecosystem dynamics over long periodsThe Hubbard Brook Experimental Forest has been used to study nutrient cycling in a forest ecosystem since 1963The research team constructed a dam on the site to monitor loss of water and mineralsFig. 55-161965(c) Nitrogen in runoff from watershedsNitrate concentration in runoff(mg/L)(a) Concrete dam and weir(b) Clear-cut watershed196619671968ControlCompletion oftree cuttingDeforested0123420406080Fig. 55-16a(a) Concrete dam and weirIn one experiment, the trees in one valley were cut down, and the valley was sprayed with herbicidesFig. 55-16b(b) Clear-cut watershedNet losses of water and minerals were studied and found to be greater than in an undisturbed areaThese results showed how human activity can affect ecosystemsFig. 55-16c1965(c) Nitrogen in runoff from watershedsNitrate concentration in runoff(mg/L)196619671968ControlCompletion oftree cuttingDeforested0123420406080Concept 55.5: Human activities now dominate most chemical cycles on EarthAs the human population has grown, our activities have disrupted the trophic structure, energy flow, and chemical cycling of many ecosystemsNutrient EnrichmentIn addition to transporting nutrients from one location to another, humans have added new materials, some of them toxins, to ecosystemsAgriculture and Nitrogen CyclingThe quality of soil varies with the amount of organic material it containsAgriculture removes from ecosystems nutrients that would ordinarily be cycled back into the soilNitrogen is the main nutrient lost through agriculture; thus, agriculture greatly affects the nitrogen cycleIndustrially produced fertilizer is typically used to replace lost nitrogen, but effects on an ecosystem can be harmfulFig. 55-17Contamination of Aquatic EcosystemsCritical load for a nutrient is the amount that plants can absorb without damaging the ecosystemWhen excess nutrients are added to an ecosystem, the critical load is exceededRemaining nutrients can contaminate groundwater as well as freshwater and marine ecosystemsSewage runoff causes cultural eutrophication, excessive algal growth that can greatly harm freshwater ecosystemsFig. 55-18WinterSummerFig. 55-18aWinterFig. 55-18bSummerAcid PrecipitationCombustion of fossil fuels is the main cause of acid precipitationNorth American and European ecosystems downwind from industrial regions have been damaged by rain and snow containing nitric and sulfuric acidAcid precipitation changes soil pH and causes leaching of calcium and other nutrientsEnvironmental regulations and new technologies have allowed many developed countries to reduce sulfur dioxide emissions Fig. 55-19Year2000199519901985198019751970196519604.04.14.24.34.44.5pHToxins in the EnvironmentHumans release many toxic chemicals, including synthetics previously unknown to natureIn some cases, harmful substances persist for long periods in an ecosystem One reason toxins are harmful is that they become more concentrated in successive trophic levelsBiological magnification concentrates toxins at higher trophic levels, where biomass is lowerPCBs and many pesticides such as DDT are subject to biological magnification in ecosystemsIn the 1960s Rachel Carson brought attention to the biomagnification of DDT in birds in her book Silent SpringFig. 55-20Lake trout4.83 ppmConcentration of PCBsHerringgull eggs124 ppmSmelt1.04 ppmPhytoplankton0.025 ppmZooplankton0.123 ppmGreenhouse Gases and Global WarmingOne pressing problem caused by human activities is the rising level of atmospheric carbon dioxideRising Atmospheric CO2 LevelsDue to the burning of fossil fuels and other human activities, the concentration of atmospheric CO2 has been steadily increasingFig. 55-21CO2CO2 concentration (ppm)Temperature1960300Average global temperature (ºC)1965197019751980Year1985199019952000200513.613.713.813.914.014.114.214.314.414.514.614.714.814.9310320330340350360370380390How Elevated CO2 Levels Affect Forest Ecology: The FACTS-I ExperimentThe FACTS-I experiment is testing how elevated CO2 influences tree growth, carbon concentration in soils, and other factors over a ten-year periodThe CO2-enriched plots produced more wood than the control plots, though less than expectedThe availability of nitrogen and other nutrients appears to limit tree growth and uptake of CO2 Fig. 55-22The Greenhouse Effect and ClimateCO2, water vapor, and other greenhouse gases reflect infrared radiation back toward Earth; this is the greenhouse effect This effect is important for keeping Earth’s surface at a habitable temperatureIncreased levels of atmospheric CO2 are magnifying the greenhouse effect, which could cause global warming and climatic changeIncreasing concentration of atmospheric CO2 is linked to increasing global temperatureNorthern coniferous forests and tundra show the strongest effects of global warmingA warming trend would also affect the geographic distribution of precipitationGlobal warming can be slowed by reducing energy needs and converting to renewable sources of energyStabilizing CO2 emissions will require an international effortDepletion of Atmospheric OzoneLife on Earth is protected from damaging effects of UV radiation by a protective layer of ozone molecules in the atmosphereSatellite studies suggest that the ozone layer has been gradually thinning since 1975Ozone layer thickness (Dobsons)Fig. 55-23Year’052000’95’90’85’80’75’70’65’6019550100250200300350Destruction of atmospheric ozone probably results from chlorine-releasing pollutants such as CFCs produced by human activityFig. 55-24O2SunlightCl2O2ChlorineChlorine atomO3O2ClOClOScientists first described an “ozone hole” over Antarctica in 1985; it has increased in size as ozone depletion has increasedFig. 55-25(a) September 1979(b) September 2006Ozone depletion causes DNA damage in plants and poorer phytoplankton growthAn international agreement signed in 1987 has resulted in a decrease in ozone depletionFig. 55-UN1KeyPrimary producersEnergy flowChemical cyclingPrimary consumersSecondaryconsumersTertiary consumersMicroorganismsand otherdetritivoresDetritusSunHeatFig. 55-UN2FossilizationOrganicmaterialsavailableas nutrientsLivingorganisms,detritusOrganicmaterialsunavailableas nutrientsCoal, oil,peatBurningof fossilfuelsRespiration,decomposition,excretionAssimilation,photosynthesisInorganicmaterialsavailableas nutrientsInorganicmaterialsunavailableas nutrientsAtmosphere,soil, waterMineralsin rocksWeathering,erosionFormation ofsedimentary rockFig. 55-UN3Fig. 55-UN4You should now be able to:Explain how the first and second laws of thermodynamics apply to ecosystemsDefine and compare gross primary production, net primary production, and standing cropExplain why energy flows but nutrients cycle within an ecosystemExplain what factors may limit primary production in aquatic ecosystemsDistinguish between the following pairs of terms: primary and secondary production, production efficiency and trophic efficiencyExplain why worldwide agriculture could feed more people if all humans consumed only plant materialDescribe the four nutrient reservoirs and the processes that transfer the elements between reservoirsExplain why toxic compounds usually have the greatest effect on top-level carnivoresDescribe the causes and consequences of ozone depletion