Bài giảng Biology - Chapter 54: Ecosystems

Chapter 54EcosystemsOverview: Ecosystems, Energy, and MatterAn ecosystem consists of all the organisms living in a communityAs well as all the abiotic factors with which they interactEcosystems can range from a microcosm, such as an aquariumTo a large area such as a lake or forestFigure 54.1Regardless of an ecosystem’s sizeIts dynamics involve two main processes: energy flow and chemical cyclingEnergy flows through ecosystemsWhile matter cycles within themConcept 54.1: Ecosystem ecology emphasizes energy flow and chemical cyclingEcosystem ecologists view ecosystemsAs transformers of energy and processors of matterEcosystems and Physical LawsThe laws of physics and chemistry apply to ecosystemsParticularly in regard to the flow of energyEnergy is conservedBut degraded to heat during ecosystem processesTrophic RelationshipsEnergy and nutrients pass from primary producers (autotrophs) To primary consumers (herbivores) and then to secondary consumers (carnivores)Energy flows through an ecosystemEntering as light and exiting as heatFigure 54.2Microorganismsand otherdetritivoresDetritusPrimary producersPrimary consumersSecondaryconsumersTertiary consumersHeatSunKeyChemical cyclingEnergy flowNutrients cycle within an ecosystemDecompositionDecompositionConnects all trophic levelsDetritivores, mainly bacteria and fungi, recycle essential chemical elementsBy decomposing organic material and returning elements to inorganic reservoirsFigure 54.3Concept 54.2: Physical and chemical factors limit primary production in ecosystemsPrimary production in an ecosystemIs the amount of light energy converted to chemical energy by autotrophs during a given time periodEcosystem Energy BudgetsThe extent of photosynthetic productionSets the spending limit for the energy budget of the entire ecosystemThe Global Energy BudgetThe amount of solar radiation reaching the surface of the EarthLimits the photosynthetic output of ecosystemsOnly a small fraction of solar energyActually strikes photosynthetic organismsGross and Net Primary ProductionTotal primary production in an ecosystemIs known as that ecosystem’s gross primary production (GPP)Not all of this productionIs stored as organic material in the growing plantsNet primary production (NPP)Is equal to GPP minus the energy used by the primary producers for respirationOnly NPPIs available to consumersDifferent ecosystems vary considerably in their net primary productionAnd in their contribution to the total NPP on EarthLake and streamOpen oceanContinental shelfEstuaryAlgal beds and reefsUpwelling zonesExtreme desert, rock, sand, iceDesert and semidesert scrubTropical rain forestSavannaCultivated landBoreal forest (taiga)Temperate grasslandTundraTropical seasonal forestTemperate deciduous forestTemperate evergreen forestSwamp and marshWoodland and shrubland010203040506005001,0001,5002,0002,5000510152025Percentage of Earth’s netprimary productionKeyMarineFreshwater (on continents)Terrestrial5.20.30.10.14.73.53.32.92.72.41.81.71.61.51.31.00.40.41253601,5002,5005003.0902,2009006008006007001401,6001,2001,3002,0002505.61.20.90.10.040.9227.99.19.65.43.50.67.14.93.82.30.365.024.4Figure 54.4a–cPercentage of Earth’ssurface area(a)Average net primaryproduction (g/m2/yr)(b)(c)Overall, terrestrial ecosystemsContribute about two-thirds of global NPP and marine ecosystems about one-thirdFigure 54.5180120W60W060E120E180North Pole60N30NEquator30S60SSouth PolePrimary Production in Marine and Freshwater EcosystemsIn marine and freshwater ecosystemsBoth light and nutrients are important in controlling primary productionLight LimitationThe depth of light penetrationAffects primary production throughout the photic zone of an ocean or lakeNutrient LimitationMore than light, nutrients limit primary productionBoth in different geographic regions of the ocean and in lakesA limiting nutrient is the element that must be addedIn order for production to increase in a particular areaNitrogen and phosphorousAre typically the nutrients that most often limit marine productionNutrient enrichment experimentsConfirmed that nitrogen was limiting phytoplankton growth in an area of the oceanEXPERIMENT Pollution from duck farms concentrated near Moriches Bay adds both nitrogen and phosphorus to the coastal water off Long Island. Researchers cultured the phytoplankton Nannochloris atomus with water collected from several bays.Figure 54.6Coast of Long Island, New York. The numbers on the map indicate the data collection stations.Long IslandGreat South BayShinnecock BayMoriches BayAtlantic Ocean3021191511542Figure 54.6(a) Phytoplankton biomass and phosphorus concentration(b) Phytoplankton response to nutrient enrichmentGreatSouth BayMorichesBayShinnecockBayStartingalgaldensity24511301519213024181260Unenriched controlAmmonium enrichedPhosphate enrichedStation numberPhytoplankton(millions of cells per mL)8765432102451130151921876543210Inorganic phosphorus(g atoms/L)Phytoplankton(millions of cells/mL)Station numberCONCLUSION Since adding phosphorus, which was already in rich supply, had no effect on Nannochloris growth, whereas adding nitrogen increased algal density dramatically, researchers concluded that nitrogen was the nutrient limiting phytoplankton growth in this ecosystem.PhytoplanktonInorganicphosphorusRESULTS Phytoplankton abundance parallels the abundance of phosphorus in the water (a). Nitrogen, however, is immediately taken up by algae, and no free nitrogen is measured in the coastal waters. The addition of ammonium (NH4) caused heavy phytoplankton growth in bay water, but the addition of phosphate (PO43) did not induce algal growth (b).Experiments in another ocean regionShowed that iron limited primary productionTable 54.1The addition of large amounts of nutrients to lakesHas a wide range of ecological impactsIn some areas, sewage runoffHas caused eutrophication of lakes, which can lead to the eventual loss of most fish species from the lakesFigure 54.7Primary Production in Terrestrial and Wetland EcosystemsIn terrestrial and wetland ecosystems climatic factorsSuch as temperature and moisture, affect primary production on a large geographic scaleThe contrast between wet and dry climatesCan be represented by a measure called actual evapotranspirationActual evapotranspirationIs the amount of water annually transpired by plants and evaporated from a landscapeIs related to net primary productionFigure 54.8Actual evapotranspiration (mm H2O/yr)Tropical forestTemperate forestMountain coniferous forestTemperate grasslandArctic tundraDesertshrublandNet primary production (g/m2/yr)1,0002,0003,00005001,0001,5000On a more local scaleA soil nutrient is often the limiting factor in primary productionFigure 54.9EXPERIMENT Over the summer of 1980, researchers added phosphorus to some experimental plots in the salt marsh, nitrogento other plots, and both phosphorus and nitrogen to others. Some plots were left unfertilized as controls. RESULTSExperimental plots receiving just phosphorus (P) do not outproduce the unfertilized control plots.CONCLUSIONLive, above-ground biomass(g dry wt/m2)Adding nitrogen (N) boosts net primaryproduction.300250200150100500JuneJulyAugust 1980N  PN onlyControlP only These nutrient enrichment experiments confirmed that nitrogen was the nutrient limiting plant growth in this salt marsh.Concept 54.3: Energy transfer between trophic levels is usually less than 20% efficientThe secondary production of an ecosystemIs the amount of chemical energy in consumers’ food that is converted to their own new biomass during a given period of timeProduction EfficiencyWhen a caterpillar feeds on a plant leafOnly about one-sixth of the energy in the leaf is used for secondary productionFigure 54.10Plant materialeaten by caterpillarCellularrespirationGrowth (new biomass)Feces100 J33 J200 J67 JThe production efficiency of an organismIs the fraction of energy stored in food that is not used for respirationTrophic Efficiency and Ecological PyramidsTrophic efficiencyIs the percentage of production transferred from one trophic level to the nextUsually ranges from 5% to 20%Pyramids of ProductionThis loss of energy with each transfer in a food chainCan be represented by a pyramid of net productionFigure 54.11TertiaryconsumersSecondaryconsumersPrimaryconsumersPrimaryproducers1,000,000 J of sunlight10 J100 J1,000 J10,000 JPyramids of BiomassOne important ecological consequence of low trophic efficienciesCan be represented in a biomass pyramidMost biomass pyramidsShow a sharp decrease at successively higher trophic levelsFigure 54.12a(a) Most biomass pyramids show a sharp decrease in biomass at successively higher trophic levels, as illustrated by data froma bog at Silver Springs, Florida.Trophic levelDry weight(g/m2)Primary producersTertiary consumersSecondary consumersPrimary consumers1.51137809Certain aquatic ecosystemsHave inverted biomass pyramidsFigire 54.12bTrophic levelPrimary producers (phytoplankton)Primary consumers (zooplankton)(b) In some aquatic ecosystems, such as the English Channel, a small standing crop of primary producers (phytoplankton)supports a larger standing crop of primary consumers (zooplankton).Dry weight(g/m2)214Pyramids of NumbersA pyramid of numbersRepresents the number of individual organisms in each trophic levelFigure 54.13Trophic levelNumber of individual organismsPrimary producersTertiary consumersSecondary consumersPrimary consumers3354,904708,6245,842,424The dynamics of energy flow through ecosystemsHave important implications for the human populationEating meatIs a relatively inefficient way of tapping photosynthetic productionWorldwide agriculture could successfully feed many more peopleIf humans all fed more efficiently, eating only plant materialFigure 54.14Trophic levelSecondaryconsumersPrimaryconsumersPrimaryproducersThe Green World HypothesisAccording to the green world hypothesisTerrestrial herbivores consume relatively little plant biomass because they are held in check by a variety of factorsMost terrestrial ecosystemsHave large standing crops despite the large numbers of herbivoresFigure 54.15The green world hypothesis proposes several factors that keep herbivores in checkPlants have defenses against herbivoresNutrients, not energy supply, usually limit herbivoresAbiotic factors limit herbivoresIntraspecific competition can limit herbivore numbersInterspecific interactions check herbivore densitiesConcept 54.4: Biological and geochemical processes move nutrients between organic and inorganic parts of the ecosystemLife on EarthDepends on the recycling of essential chemical elementsNutrient circuits that cycle matter through an ecosystemInvolve both biotic and abiotic components and are often called biogeochemical cyclesA General Model of Chemical CyclingGaseous forms of carbon, oxygen, sulfur, and nitrogenOccur in the atmosphere and cycle globallyLess mobile elements, including phosphorous, potassium, and calciumCycle on a more local levelA general model of nutrient cyclingIncludes the main reservoirs of elements and the processes that transfer elements between reservoirsFigure 54.16Organicmaterialsavailableas nutrientsLivingorganisms,detritusOrganicmaterialsunavailableas nutrientsCoal, oil,peatInorganicmaterialsavailableas nutrientsInorganicmaterialsunavailableas nutrientsAtmosphere,soil, waterMineralsin rocksFormation ofsedimentary rock Weathering,erosionRespiration,decomposition,excretionBurningof fossil fuelsFossilizationReservoir aReservoir bReservoir cReservoir dAssimilation, photosynthesisAll elementsCycle between organic and inorganic reservoirsBiogeochemical CyclesThe water cycle and the carbon cycleFigure 54.17Transportover landSolar energyNet movement ofwater vapor by windPrecipitationover oceanEvaporationfrom oceanEvapotranspirationfrom landPrecipitationover landPercolationthroughsoilRunoff andgroundwaterCO2 in atmospherePhotosynthesisCellularrespirationBurning offossil fuelsand woodHigher-levelconsumersPrimaryconsumersDetritusCarbon compounds in waterDecompositionTHE WATER CYCLETHE CARBON CYCLEWater moves in a global cycleDriven by solar energyThe carbon cycleReflects the reciprocal processes of photosynthesis and cellular respirationThe nitrogen cycle and the phosphorous cycleFigure 54.17N2 in atmosphereDenitrifyingbacteriaNitrifyingbacteriaNitrifyingbacteriaNitrificationNitrogen-fixingsoil bacteriaNitrogen-fixingbacteria in rootnodules of legumesDecomposersAmmonificationAssimilationNH3NH4+NO3NO2 RainPlantsConsumptionDecompositionGeologicupliftWeatheringof rocksRunoffSedimentationPlant uptakeof PO43SoilLeachingTHE NITROGEN CYCLETHE PHOSPHORUS CYCLEMost of the nitrogen cycling in natural ecosystemsInvolves local cycles between organisms and soil or waterThe phosphorus cycleIs relatively localizedDecomposition and Nutrient Cycling RatesDecomposers (detritivores) play a key roleIn the general pattern of chemical cyclingFigure 54.18ConsumersProducersNutrientsavailableto producersAbioticreservoirGeologicprocessesDecomposersThe rates at which nutrients cycle in different ecosystemsAre extremely variable, mostly as a result of differences in rates of decompositionVegetation and Nutrient Cycling: The Hubbard Brook Experimental ForestNutrient cyclingIs strongly regulated by vegetationLong-term ecological research projectsMonitor ecosystem dynamics over relatively long periods of timeThe Hubbard Brook Experimental ForestHas been used to study nutrient cycling in a forest ecosystem since 1963The research team constructed a dam on the siteTo monitor water and mineral lossFigure 54.19a(a) Concrete dams and weirs built across streams at the bottom of watersheds enabled researchers to monitor the outflow of water and nutrients from the ecosystem.In one experiment, the trees in one valley were cut downAnd the valley was sprayed with herbicidesFigure 54.19b(b) One watershed was clear cut to study the effects of the lossof vegetation on drainage and nutrient cycling.Net losses of water and minerals were studiedAnd found to be greater than in an undisturbed areaThese results showed how human activityCan affect ecosystemsFigure 54.19c(c) The concentration of nitrate in runoff from the deforested watershed was 60 times greater than in a control (unlogged) watershed.Nitrate concentration in runoff(mg/L)DeforestedControlCompletion oftree cutting196519661967196880.060.040.020.04.03.02.01.00Concept 54.5: The human population is disrupting chemical cycles throughout the biosphereAs the human population has grown in sizeOur activities have disrupted the trophic structure, energy flow, and chemical cycling of ecosystems in most parts of the worldNutrient EnrichmentIn addition to transporting nutrients from one location to anotherHumans have added entirely new materials, some of them toxins, to ecosystemsAgriculture and Nitrogen CyclingAgriculture constantly removes nutrients from ecosystemsThat would ordinarily be cycled back into the soilFigure 54.20Nitrogen is the main nutrient lost through agricultureThus, agriculture has a great impact on the nitrogen cycleIndustrially produced fertilizer is typically used to replace lost nitrogenBut the effects on an ecosystem can be harmfulContamination of Aquatic EcosystemsThe critical load for a nutrientIs the amount of that nutrient that can be absorbed by plants in an ecosystem without damaging itWhen excess nutrients are added to an ecosystem, the critical load is exceededAnd the remaining nutrients can contaminate groundwater and freshwater and marine ecosystemsSewage runoff contaminates freshwater ecosystemsCausing cultural eutrophication, excessive algal growth, which can cause significant harm to these ecosystemsAcid PrecipitationCombustion of fossil fuelsIs the main cause of acid precipitationNorth American and European ecosystems downwind from industrial regionsHave been damaged by rain and snow containing nitric and sulfuric acidFigure 54.214.64.64.34.14.34.64.64.3EuropeNorth AmericaBy the year 2000The entire contiguous United States was affected by acid precipitationFigure 54.22Field pH5.35.2–5.35.1–5.25.0–5.14.9–5.04.8–4.94.7–4.84.6–4.74.5–4.64.4–4.54.3–4.44.3Environmental regulations and new industrial technologiesHave allowed many developed countries to reduce sulfur dioxide emissions in the past 30 yearsToxins in the EnvironmentHumans release an immense variety of toxic chemicalsIncluding thousands of synthetics previously unknown to natureOne of the reasons such toxins are so harmfulIs that they become more concentrated in successive trophic levels of a food webIn biological magnificationToxins concentrate at higher trophic levels because at these levels biomass tends to be lowerFigure 54.23Concentration of PCBsHerringgull eggs124 ppmZooplankton 0.123 ppmPhytoplankton 0.025 ppmLake trout 4.83 ppmSmelt 1.04 ppmIn some cases, harmful substancesPersist for long periods of time in an ecosystem and continue to cause harmAtmospheric Carbon DioxideOne pressing problem caused by human activitiesIs the rising level of atmospheric carbon dioxideRising Atmospheric CO2Due to the increased burning of fossil fuels and other human activitiesThe concentration of atmospheric CO2 has been steadily increasingFigure 54.24CO2 concentration (ppm)39038037036035034033032031030019601965197019751980198519901995200020051.050.900.750.600.450.300.1500.15 0.30 0.45Temperature variation (C)TemperatureCO2YearHow Elevated CO2 Affects Forest Ecology: The FACTS-I ExperimentThe FACTS-I experiment is testing how elevated CO2Influences tree growth, carbon concentration in soils, and other factors over a ten-year periodFigure 54.25The Greenhouse Effect and Global WarmingThe greenhouse effect is caused by atmospheric CO2But is necessary to keep the surface of the Earth at a habitable temperatureIncreased levels of atmospheric CO2 are magnifying the greenhouse effectWhich could cause global warming and significant climatic changeDepletion of Atmospheric OzoneLife on Earth is protected from the damaging effects of UV radiationBy a protective layer or ozone molecules present in the atmosphereSatellite studies of the atmosphereSuggest that the ozone layer has been gradually thinning since 1975Figure 54.26Ozone layer thickness (Dobson units)Year (Average for the month of October)35030025020015010050019551960196519701975198019851990199520002005The destruction of atmospheric ozoneProbably results from chlorine-releasing pollutants produced by human activityFigure 54.27123Chlorine from CFCs interacts with ozone (O3),forming chlorine monoxide (ClO) and oxygen (O2).Two ClO molecules react, forming chlorine peroxide (Cl2O2).Sunlight causes Cl2O2 to break down into O2 and free chlorine atoms. The chlorine atoms can begin the cycle again.SunlightChlorineO3O2ClOClOCl2O2O2Chlorine atomsScientists first described an “ozone hole”Over Antarctica in 1985; it has increased in size as ozone depletion has increasedFigure 54.28a, b(a) October 1979(b) October 2000