Chapter 55. Ecosystems. Concept 55.1 Physical laws govern energy flow and chemical cycling in ecosystems.

Transcription

1 Chapter 55 Ecsystems Lecture Outline Overview: Observing Ecsystems An ecsystem cnsists f all the rganisms that live in a cmmunity as well as all the abitic factrs with which they interact. An ecsystem can encmpass a large area, such as a frest, r a micrcsm, such as the area under a fallen lg. Sme eclgists view the entire bisphere as a glbal ecsytem. The dynamics f an ecsystem invlve tw prcesses that cannt be fully described by ppulatin r cmmunity phenmena: energy flw and chemical cycling. Energy enters mst ecsystems in the frm f sunlight, which is cnverted t chemical energy by auttrphs, passed t hetertrphs in the rganic cmpunds f fd, and dissipated as heat. Chemical elements are cycled amng the abitic and bitic cmpnents f the ecsystem. Phtsynthetic rganisms assimilate chemical elements in rganic frm frm the air, sil, and water, and incrprate them int their bimass. Sme f these chemical elements are cnsumed by animals. The elements are returned in rganic frm t the envirnment by the metablism f plants and animals and by decmpsers such as bacteria and fungi, which break dwn rganic wastes and dead rganisms. Energy, unlike matter, cannt be recycled. An ecsystem must be pwered by a cntinuus influx f energy frm an external surce, usually the sun. Energy flws thrugh ecsystems, whereas matter cycles within and thrugh them. Cncept 55.1 Physical laws gvern energy flw and chemical cycling in ecsystems. Ecsystem eclgists view ecsystems as transfrmers f energy and prcessrs f matter. We can fllw the transfrmatin f energy in an ecsystem and the mvements f chemical elements thrugh a cmmunity by gruping the species in a cmmunity int trphic levels f feeding relatinships. Ecsystems bey physical laws. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 55-1

2 The first law f thermdynamics states that energy cannt be created r destryed but nly transfrmed. Plants and ther phtsynthetic rganisms cnvert slar energy t chemical energy, but the ttal amunt f energy des nt change. The ttal amunt f energy stred in rganic mlecules plus the amunts reflected and dissipated as heat must equal the ttal slar energy intercepted by the plant. One area f ecsytem eclgy cmputes such energy budgets and traces energy flw thrugh ecsystems in rder t understand the factrs that cntrl these energy transfers. Transfers help determine hw many rganisms a habitat can supprt and the amunt f fd that humans can harvest frm a site. The secnd law f thermdynamics states that every exchange f energy increases the entrpy f the universe. Sme energy is lst as heat in any cnversin prcess. The efficiency f eclgical energy cnversins can be measured. Accrding t the law f cnservatin f mass, matter, like energy, cannt be created r destryed. Because mass is cnserved, we can determine hw much f an element cycles within an ecsystem r is gained r lst by that ecsystem ver time. Unlike energy, chemical elements are cntinuusly recycled within ecsystems. A carbn r nitrgen atm mves frm ne trphic level t anther and eventually t the decmpsers and back again. Chemical elements can mve between ecsystems as inputs and utputs. Nutrients enter a frest ecsystem as dust r as slutes disslved in rainwater r leached frm rcks in the grund. Nitrgen is als supplied thrugh the bilgical prcess f nitrgen fixatin. On the utput side, gases return elements t the atmsphere and water carries materials away. Like rganisms, ecsystems are pen systems, absrbing energy and mass and releasing heat and waste prducts. Mst inputs and utputs are small cmpared t the amunts recycled within ecsystems. The balance between inputs and utputs determines whether an ecsystem is a surce r a sink fr an element. If an element s utputs exceed its inputs, it will eventually limit prductin in that system. Human activities may change the balance f inputs and utputs cnsiderably. Trphic relatinships determine the rutes f energy flw and chemical cycling in ecsystems. Eclgists assign species t trphic levels n the basis f their main surce f nutritin and energy. Auttrphs, the primary prducers f the ecsystem, ultimately supprt all ther rganisms. Mst auttrphs are phtsynthetic plants, algae, r prkarytes that use light energy t synthesize sugars and ther rganic cmpunds. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 55-2

3 Chemsynthetic prkarytes are the primary prducers in deep-sea hydrthermal vents. Hetertrphs are at trphic levels abve the primary prducers and depend n their bisynthetic utput. Herbivres that eat primary prducers are called primary cnsumers. Carnivres that eat herbivres are called secndary cnsumers. Carnivres that eat ther carnivres are called tertiary cnsumers. Anther imprtant grup f hetertrphs is the detritivres, r decmpsers. Detritivres get energy frm detritus, nnliving rganic material such as the remains f dead rganisms, feces, fallen leaves, and wd. Many detritivres are in turn eaten by secndary and tertiary cnsumers. Tw imprtant grups f detritivres are prkarytes and fungi. These rganisms secrete enzymes that digest rganic material and then absrb the breakdwn prducts, linking the primary prducers and cnsumers in an ecsystem. Decmpsitin cnnects all trphic levels. Detritivres recycle chemical elements back t primary prducers. Detritivres cnvert rganic materials frm all trphic levels t inrganic cmpunds usable by primary prducers, which then recycle these elements int rganic cmpunds. If decmpsitin stpped, all life n Earth wuld cease as detritus piled up and the supply f chemical ingredients fr the synthesis f new rganic matter was exhausted. Cncept 55.2 Energy and ther limiting factrs cntrl primary prductin in ecsystems. The amunt f light energy cnverted t chemical energy by an ecsystem s auttrphs in a given time perid is an ecsystem s primary prductin. An ecsystem s energy budget depends n its primary prductin. Mst primary prducers use light energy t synthesize rganic mlecules, which can be brken dwn t prduce adensine triphsphate (ATP). The amunt f phtsynthetic prductin sets the spending limit f the entire ecsystem. A glbal energy budget can be analyzed. Every day, Earth s atmsphere is bmbarded by apprximately jules f slar radiatin. The intensity f slar energy striking Earth varies with latitude, with the trpics receiving the greatest input. Mst f this radiatin is scattered, absrbed, r reflected by cluds and dust in the atmsphere. Much f the slar radiatin that reaches Earth s surface lands n bare grund, bdies f water, r ice, which either absrb r reflect the energy. Only a small fractin f this slar radiatin actually strikes algae, phtsynthetic prkarytes, r plants. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 55-3

4 Only certain wavelengths are absrbed by phtsynthetic pigments; the rest is transmitted, reflected, r lst as heat. Thus, nly abut 1% f the visible light that reaches phtsynthetic rganisms is cnverted t chemical energy by phtsynthesis. Despite this small amunt, Earth s primary prducers prduce abut 150 billin metric tns ( kg) f rganic material per year. The ttal primary prductin in an ecsystem is knwn as grss primary prductin (GPP), the amunt f light energy that is cnverted t chemical energy by phtsynthesis per unit time. Plants use sme f these mlecules as fuel in their wn cellular respiratin. Net primary prductin (NPP) is equal t grss primary prductin minus the energy used by the primary prducers fr respiratin (R): NPP = GPP R In many ecsystems, NPP is abut half f GPP. T eclgists, net primary prductin is the key measurement because it represents the stred chemical energy that is available t cnsumers in the ecsystem. Net primary prductin can be expressed as energy per unit area per unit time (J/m 2 yr), r as bimass f vegetatin added t the ecsystem per unit area per unit time (g/m 2 yr). Net primary prductin shuld nt be cnfused with the ttal bimass f phtsynthetic auttrphs present in a given time, which is called the standing crp. Net primary prductin is the amunt f new bimass added in a given perid f time. Althugh a frest has a large standing crp bimass, its primary prductin may actually be less than that f sme grasslands, which d nt accumulate vegetatin because animals cnsume the plants rapidly and because grasses and herbs decmpse mre quickly than trees d. Different ecsystems vary greatly in their net primary prductin. Trpical rain frests are amng the mst prductive terrestrial ecsystems, and they cntribute a large prtin f Earth s verall net primary prductin. Estuaries and cral reefs als have very high net primary prductin, but they cver nly abut ne-tenth the area cvered by trpical rain frests. The pen cean has a relatively lw prductin per unit area but cntributes as much glbal net primary prductin as terrestrial systems because f its vast size. What limits primary prductin in ecsystems? What factrs culd we change t increase r decrease primary prductin fr a given ecsystem? In aquatic ecsystems, light and nutrients limit primary prductin. Light is a key variable cntrlling primary prductin in ceans and lakes because slar radiatin can penetrate t nly a certain depth knwn as the phtic zne. The first 15 m f water absrbs mre than half f the slar radiatin. Even in clear water, nly 5 10% f the radiatin may reach a depth f 75 m. If light were the main variable limiting primary prductin in the cean, we wuld expect prductin t increase alng a gradient frm the ples tward the equatr, which receives the greatest intensity f light. There is n such gradient, hwever. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 55-4

5 Sme parts f the cean in the trpics and subtrpics exhibit lw primary prductin, whereas sme high-latitude cean regins are relatively prductive. Mre than light, nutrients limit primary prductin in aquatic ecsystems. A limiting nutrient is an element that must be added fr prductin t increase in a particular area. The nutrient that mst ften limits marine prductin is either nitrgen r phsphrus. Nitrgen and phsphrus levels are very lw in the phtic zne because these nutrients are rapidly taken up by phytplanktn and because detritus tends t sink. Nutrient levels are higher in deeper water, where light des nt penetrate. Nutrient enrichment experiments cnfirmed that nitrgen is limiting phytplanktn grwth ff the suth shre f Lng Island, New Yrk. This knwledge can be used t prevent algal blms by limiting pllutin that fertilizes phytplanktn. Eliminating phsphates frm sewage will nt slve the prblem unless nitrgen pllutin is als cntrlled. Sme areas f the cean have lw phytplanktn density despite their relatively high nitrgen cncentratins. Fr example, the Sargass Sea has a very lw density f phytplanktn. Nutrient-enrichment experiments shwed that irn availability limits primary prductin in this area. Windblwn dust frm the land is the main input f irn t the cean, but relatively little windblwn dust reaches the centers f ceans. Marine eclgists carried ut large-scale field experiments in the Pacific Ocean, spreading lw cncentratins f disslved irn ver 72 km 2 f cean and measuring the change in phytplanktn density ver a seven-day perid. A massive phytplanktn blm ccurred, with increased cncentratins in water samples frm test sites. Adding irn stimulates the grwth f cyanbacteria that fix atmspheric nitrgen, and the extra nitrgen stimulates the prliferatin f phytplanktn. In areas f upwelling, nutrient-rich deep waters circulate t the cean surface. These areas have exceptinally high primary prductin, supprting the hypthesis that nutrient availability determines marine primary prductin. Because the supply f available nutrients stimulates the grwth f the phytplanktn ppulatins that frm the base f marine fd webs, upwelling areas are prime fishing lcatins. The largest areas f upwelling ccur in the Suthern Ocean (als called the Antarctic Ocean) and the castal waters ff Peru, Califrnia, and parts f western Africa. Nutrient limitatin is als cmmn in freshwater lakes. Sewage and fertilizer pllutin can add large amunts f nutrients t lakes. Cyanbacteria and algae grw rapidly in respnse t these added nutrients, ultimately reducing xygen cncentratins and visibility in the water. This prcess is called eutrphicatin and has a wide range f eclgical impacts, including the lss f mst fish species. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 55-5

6 A series f whle-lake experiments identified phsphrus as the nutrient that limited cyanbacterial grwth. This research led t the use f phsphate-free detergents and ther water quality refrms. In terrestrial ecsystems, temperature and misture are the key factrs limiting primary prductin. Trpical rain frests, with their warm, wet cnditins, are the mst prductive f all terrestrial ecsystems. By cntrast, lw-prductivity ecsystems are generally dry (deserts) r dry and cld (arctic tundra). Between these extremes lie temperate frest and grassland ecsystems with mderate climates and intermediate prductivity. These cntrasts in climate can be represented by a measure called actual evaptranspiratin, which is the annual amunt f water transpired by plants and evaprated frm a landscape. Actual evaptranspiratin increases with precipitatin and the amunt f slar energy available t drive evapratin and transpiratin. On a mre lcal scale, mineral nutrients in the sil can limit terrestrial primary prductin. Nitrgen and phsphrus are the sil nutrients that mst cmmnly limit terrestrial prductin. Adding mre f a limiting nutrient t sil will increase prductin until sme ther nutrient becmes limiting. Studies relating nutrients t terrestrial primary prductin have practical applicatins in agriculture. Farmers can maximize crp yields by using fertilizers with the right balance f nutrients fr the lcal sil and type f crp. Cncept 55.3 Energy transfer between trphic levels is typically nly 10% efficient. The amunt f chemical energy in cnsumers fd that is cnverted t their wn new bimass during a given time perid is called the secndary prductin f an ecsystem. In mst ecsystems, herbivres eat nly a small fractin f the plant material that is prduced. Mrever, herbivres cannt digest all the plant material that they d eat. Thus, much f primary prductin is nt used by cnsumers. We can measure the efficiency f animals as energy transfrmers using the fllwing equatin: Prductin efficiency = Net secndary prductin 100% / Assimilatin f primary prductin Net secndary prductin is the energy stred in bimass represented by grwth and reprductin. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 55-6

7 Assimilatin is the ttal energy taken in and used fr grwth, reprductin, and respiratin. Prductin efficiency is thus the fractin f fd energy that is nt used fr respiratin. Prductin efficiencies differ amng rganisms. Birds and mammals typically have lw prductin efficiencies f 1 3% because they use s much energy t maintain a cnstant bdy temperature. Fishes, which are ecttherms, have prductin efficiencies f arund 10%. Insects and micrrganisms are even mre efficient, with prductin efficiencies averaging 40%. Trphic efficiency is the percentage f prductin transferred frm ne trphic level t the next. Trphic efficiencies must always be less than prductin efficiencies because they take int accunt nt nly the energy lst thrugh respiratin and cntained in feces, but als the energy in rganic material at lwer trphic levels that is nt cnsumed. Trphic efficiencies are generally abut 10% and range frm apprximately 5% t 20%, depending n the type f ecsystem. In ther wrds, 90% f the energy available at ne trphic level is typically nt transferred t the next. This lss is multiplied ver the length f a fd chain. If 10% f energy is transferred frm primary prducers t primary cnsumers, and 10% f that energy is transferred t secndary cnsumers, then nly 1% f net primary prductin is available t secndary cnsumers. The prgressive lss f energy alng a fd chain limits the abundance f tp-level carnivres. Only abut 0.1% f the chemical energy fixed by phtsynthesis can flw all the way thrugh a fd web t a tertiary cnsumer, such as a snake r a shark. This limits mst fd webs t fur r five trphic levels. Pyramids f net prductin represent the multiplicative lss f energy in a fd chain. The width f each tier in the pyramid is prprtinal t the net prductin f each trphic level, expressed in jules. The highest level, which represents tp-level predatrs, cntains relatively few individuals. Because ppulatins f tp predatrs are typically small and the animals may be widely spaced within their habitats, many predatr species are highly susceptible t extinctin. Bimass pyramids represent the eclgical cnsequences f lw trphic efficiencies. Each tier represents the standing crp (the ttal dry mass f all rganisms) in ne trphic level. Mst bimass pyramids narrw sharply frm primary prducers t tp-level carnivres because energy transfers are s inefficient. In sme aquatic ecsystems, the bimass pyramid is inverted and primary cnsumers utweigh prducers. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 55-7

8 Such inverted bimass pyramids ccur because the prducers phytplanktn grw, reprduce, and are cnsumed by zplanktn s rapidly that they never develp a large standing crp. Phytplanktn have a shrt turnver time, which means they have a small standing crp bimass cmpared t their prductin: Turnver time = Standing crp bimass (g/m 2 ) / Prductin (g/m 2 day) Because phytplanktn replace their bimass at such a rapid rate, they can supprt a bimass f zplanktn much greater than their wn bimass. Hwever, the pyramid f net prductin fr this ecsystem is still bttm-heavy. The dynamics f energy flw thrugh ecsystems have imprtant implicatins fr the human ppulatin. Eating meat is an inefficient way f tapping phtsynthetic prductin. Wrldwide agriculture culd feed many mre peple if all humans fed as primary cnsumers, eating nly plant material. Estimates f Earth s human carrying capacity depend greatly n ur diet and n the amunt f resurces each f us cnsumes. Herbivres cnsume a small percentage f vegetatin: the green-wrld hypthesis. Hw green is ur wrld? metric tns f carbn are stred in the plant bimass f terrestrial ecsystems. Glbal terrestrial primary prductin is abut metric tns per year. Herbivres annually cnsume less than ne-sixth f the glbal net primary prductin. Mst f the rest is eventually cnsumed by detritivres. Accrding t the green-wrld hypthesis, terrestrial herbivres cnsume relatively little plant bimass because they are held in check by a variety f factrs. Plants have defenses, such as spines r nxius chemicals. Lw nutrient cncentratins in plant tissues frce herbivres t prcess large quantities f bimass t extract small amunts f nutrients. Abitic factrs such as temperature and misture may limit the number f herbivres. Bitic factrs such as intraspecific cmpetitin, including territrial behavir, and interspecific cmpetitin, particularly frm predatrs, parasites, and pathgens, may limit the grwth f herbivre ppulatins. Cncept 55.4 Bilgical and gelgic prcesses cycle nutrients between rganic and inrganic parts f an ecsystem. Chemical elements are available t ecsystems in nly limited amunts. Life n Earth depends n the recycling f essential chemical elements. Bigechemical cycles invlve bth bitic and abitic cmpnents f ecsystems. There are tw general categries f bigechemical cycles: glbal and lcal. Gaseus frms f carbn, xygen, sulfur, and nitrgen ccur in the atmsphere, and cycles f these elements are glbal. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 55-8

9 Elements such as phsphrus, ptassium, and calcium are t heavy t ccur as gases. In terrestrial ecsystems, these elements cycle lcally, absrbed frm the sil by plant rts and eventually returned t the sil by decmpsers. In aquatic systems, these elements cycle mre bradly as disslved frms carried in currents. We will cnsider a general mdel f chemical cycling that includes the main reservirs f elements and the prcesses that transfer elements between reservirs. Each reservir is defined by tw characteristics: whether it cntains rganic r inrganic materials and whether r nt the materials are directly available fr use by rganisms. Reservir a includes the nutrients in living rganisms and in detritus. These nutrients are available t ther rganisms when cnsumers feed and when detritivres cnsume nnliving rganic material. Reservir b includes materials that mve t the fssilized rganic reservir as dead rganisms and are buried by sedimentatin ver millins f years. Sme fssilized rganisms becme cal, il, r peat. Nutrients in fssilized depsits cannt be assimilated directly but may mve int the available reservir f inrganic nutrients when fssil fuels are burned, releasing exhaust int the atmsphere. Reservir c includes inrganic elements and cmpunds that are disslved in water r present in sil r air. These materials are available fr use by rganisms. Reservir d includes inrganic elements present in rcks. These nutrients are nt directly available fr use by rganisms, but they may gradually becme available thrugh ersin and weathering. Describing bigechemical cycles in general terms is much simpler than trying t trace elements thrugh these cycles. Eclgists study chemical cycling by adding tiny amunts f radiactive istpes t the elements they are tracing and by fllwing the mvement f naturally ccurring stable, nnradiactive istpes thrugh the varius bitic and abitic cmpnents f an ecsystem. There are several imprtant bigechemical cycles. We will cnsider the cycling f water, carbn, nitrgen, and phsphrus. The water cycle Bilgical imprtance Water is essential t all rganisms, and its availability influences the rates f ecsystem prcesses. Bilgically available frms Liquid water is the primary frm in which water is used. Reservirs The ceans cntain 97% f the water in the bisphere. Tw percent is bund as ice. One percent is in lakes, rivers, and grundwater. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 55-9

10 A negligible amunt is in the atmsphere. Key prcesses The main prcesses driving the water cycle are evapratin f liquid water by slar energy, cndensatin f water vapr int cluds, and precipitatin. Transpiratin by terrestrial plants mves significant amunts f water. Surface and grundwater flw returns water t the ceans. The carbn cycle Bilgical imprtance Organic mlecules have a carbn framewrk. Bilgically available frms Auttrphs cnvert carbn dixide t rganic mlecules that are used by hetertrphs. Reservirs The majr reservirs f carbn are fssil fuels, sils, aquatic sediments, the ceans, plant and animal bimass, and the atmsphere (CO2). Key prcesses Phtsynthesis by plants and phytplanktn fixes atmspheric CO2. CO2 is added t the atmsphere by cellular respiratin f prducers and cnsumers. Vlcanes and the burning f fssil fuels add CO2 t the atmsphere. The nitrgen cycle Bilgical imprtance Nitrgen is a cmpnent f amin acids, prteins, and nucleic acids. Nitrgen may be a limiting plant nutrient. Bilgically available frms Plants and algae can use ammnium (NH4 + ) r nitrate (NO3 ). Varius bacteria can use NH4 +, NO3, r NO2. Animals can use nly rganic frms f nitrgen. Reservirs The majr reservir f nitrgen is the atmsphere, which is 80% nitrgen gas (N2). Nitrgen is als bund in sils and the sediments f lakes, rivers, and ceans. Sme nitrgen is disslved in surface water and grundwater. Nitrgen is stred in living bimass. Key prcesses Nitrgen enters ecsystems primarily thrugh bacterial nitrgen fixatin. Sme nitrgen is fixed by lightning and industrial fertilizer prductin. Ammnificatin by bacteria decmpses rganic nitrgen. In nitrificatin, bacteria cnvert NH4 + t NO3. In denitrificatin, bacteria use NO3 fr metablism instead f O2, thus releasing N2. The phsphrus cycle Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

11 Bilgical imprtance Phsphrus is a cmpnent f nucleic acids, phsphlipids, and ATP and ther energy-string mlecules. Phsphrus is a mineral cnstituent f bnes and teeth. Bilgically available frms The nly bilgically imprtant inrganic frm f phsphrus is phsphate (PO4 3 ), which plants absrb and use t synthesize rganic cmpunds. Reservirs The majr reservir f phsphrus is sedimentary rcks f marine rigin. There are als large quantities f phsphrus in sils, disslved in the ceans, and in rganisms. Key prcesses Weathering f rcks gradually adds phsphate t sil. Sme phsphate leaches int grundwater and surface water and mves t the sea. Phsphate may be taken up by prducers and incrprated int rganic material. Phsphate is returned t sil r water thrugh decmpsitin f bimass r excretin by cnsumers. Decmpsitin rates largely determine the rates f nutrient cycling. The rates at which nutrients cycle in different ecsystems are extremely variable as a result f variable rates f decmpsitin. Decmpsitin is cntrlled by the same factrs that limit primary prductin in aquatic and terrestrial ecsystems: temperature, misture, and nutrient availability. Decmpsitin takes an average f fur t six years in temperate frests, whereas in trpical rain frests, mst rganic material decmpses in a few mnths t a few years. The difference is largely due t the warmer temperatures and mre abundant precipitatin in trpical rain frests. In trpical rain frests, relatively little rganic material accumulates as leaf litter n the frest flr. The wdy trunks f trees cntain 75% f the nutrients, and the sil cntains 10%. Thus, the relatively lw cncentratins f sme nutrients in the sil f trpical rain frests result frm a shrt cycling time, nt frm a lack f these elements in the ecsystem. In temperate frests, where decmpsitin is slwer, the sil may cntain 50% f the rganic material. The nutrients present in temperate frest detritus and sil may remain there fr a lng time befre plants assimilate them. Decmpsitin n land slws when cnditins are t dry fr decmpsers t thrive r t wet t supply them with enugh xygen. Ecsystems that are bth cld and wet, such as peat lands, stre large amunts f rganic matter. Decmpsers grw prly mst f the year there, and net primary prductin greatly exceeds decmpsitin. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

12 In aquatic ecsystems, decmpsitin in the anaerbic mud f bttm sediments can take 50 years r lnger. Algae and aquatic plants usually assimilate nutrients directly frm the water. Aquatic sediments may cnstitute a nutrient sink. Interchange between the bttm layers f water and the surface increases aquatic prductivity. Nutrient cycling is strngly regulated by vegetatin. Since 1963, Herbert Brmann and Gene Likens have been studying nutrient cycling in a frest ecsystem at the Hubbard Brk Experimental Frest in the White Muntains f New Hampshire. The study site is a deciduus frest with several valleys, each drained by a small creek that is a tributary f Hubbard Brk. Bedrck impenetrable t water is clse t the surface f the sil, and each valley cnstitutes a watershed that can drain nly thrugh its creek. The research team first determined the mineral budget fr each valley by measuring the input and utflw f several key nutrients. They cllected rainfall at several sites t measure the amunt f water and disslved minerals added t the ecsystem. T mnitr the lss f water and minerals, they cnstructed a small cncrete dam with a V-shaped spillway acrss the creek at the bttm f each valley. Abut 60% f the water added t the ecsystem as rainfall and snw exits thrugh the stream, and the remaining 40% is lst by evaptranspiratin. Preliminary studies cnfirmed that internal cycling within a terrestrial ecsystem cnserves mst f the mineral nutrients. Only abut 0.3% mre calcium (Ca 2+ ) left a valley via its creek than was added by rainwater, and this small net lss was prbably replaced by chemical decmpsitin f the bedrck. During mst years, the frest actually registered small net gains f a few mineral nutrients, including nitrgen. In ne experiment, the trees in ne valley were cut dwn and then the valley was sprayed with herbicides fr three years t prevent regrwth f plants. All the riginal plant material was left in place t decmpse. The inflw and utflw f water and minerals in this experimentally altered watershed were cmpared with thse in a cntrl watershed. Over the three years, water runff frm the altered watershed increased by 30 40%, with n plants t absrb and transpire water frm the sil. Net lsses f minerals frm the altered watershed were huge. The cncentratin f Ca 2+ in the creek increased furfld, fr example, and the cncentratin f K + increased by a factr f 15. Mst remarkable was the lss f nitrate, whse cncentratin in the creek increased 60-fld, reaching levels cnsidered unsafe fr drinking water. This study demnstrates that the amunt f nutrients leaving an intact frest ecsystem is cntrlled mainly by the plants. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

13 The effects f defrestatin ccur within a few mnths and cntinue as lng as living plants are absent. The 45 years f data frm Hubbard Brk reveal ther trends as well. In the last half-century, acid rain and snw have disslved mst f the Ca 2+ in the frest sil, and the streams have carried it away. By the 1990s, the frest bimass at Hubbard Brk had stpped increasing, apparently because f a lack f Ca 2+. T test this idea, eclgists at Hubbard Brk began a massive experiment in They first established a cntrl and an experimental watershed, which they mnitred ver tw years befre using a helicpter t add Ca 2+ t the experimental watershed. By 2006, sugar maple trees grwing in the Ca 2+ -enriched lcatin had higher Ca 2+ cncentratins in their fliage, healthier crwns, and greater seedling establishment than thse grwing in the cntrl watershed. These data suggest that sugar maple declines in the nrtheastern United States and suthern Canada are attributable at least in part t the cnsequences f sil acidificatin. The Hubbard Brk studies, as well as 25 ther lng-term eclgical research prjects funded by the Natinal Science Fundatin, assess natural ecsystem prcesses and prvide imprtant insight int the mechanisms by which human activities affect these prcesses. Cncept 55.5 Human activities nw dminate mst chemical cycles n Earth. Human activities and technlgies have disrupted the trphic structure, energy flw, and chemical cycling f ecsystems wrldwide. Human activities have greater influence n chemical cycles than natural prcesses d. The human ppulatin mves nutrients frm ne part f the bisphere t anther. Human activity intrudes in nutrient cycles. Nutrients frm farm sil may run ff int streams and lakes, depleting nutrients in ne area, causing excesses in anther, and altering chemical cycles in bth places. Humans als add entirely new materials sme txic t ecsystems. In agricultural ecsystems, large amunts f nutrients are remved frm the area as crp bimass. After a while, the natural stre f nutrients can becme exhausted. Then the sil cannt be used t grw crps withut nutrient supplementatin. Nitrgen is the main nutrient lst thrugh agriculture. Plwing mixes the sil and increases the decmpsitin rate f rganic matter, releasing usable nitrgen that is then remved frm the ecsystem when crps are harvested. Applied fertilizers make up fr the lss f usable nitrgen frm agricultural ecsystems. As shwn in the Hubbard Brk studies, withut plants t take up nitrates frm the sil, the nitrates are likely t be leached frm the ecsystem. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

14 Recent studies indicate that human activities have apprximately dubled the wrldwide supply f fixed nitrgen, due t the use f fertilizers, cultivatin f legumes, and fssil fuel cmbustin. A key measure f the amunt f nitrgen in an ecsystem is the critical lad, the amunt f added nutrient that can be absrbed by plants withut causing damage. Nitrgenus minerals in the sil that exceed the critical lad eventually leach int grundwater r run ff int freshwater and marine ecsystems, cntaminating water supplies, chking waterways, and killing fish. Nitrate cncentratins in grundwater are increasing in mst agricultural regins, smetimes exceeding safe levels fr drinking. Agricultural runff and sewage frm nrthern Eurpe and the central United States cntaminate many rivers that drain int the Atlantic Ocean. The Mississippi River carries nitrgen pllutin t the Gulf f Mexic, fueling a phytplanktn blm each summer. When phytplanktns die, their decmpsitin creates an extensive dead zne f lw xygen availability alng the cast. Fish, shrimp, and ther marine animals have disappeared frm these ecnmically imprtant waters. T reduce the size f the dead zne, farmers have begun using fertilizers mre efficiently, and managers are restring wetlands in the Mississippi watershed. Nutrient runff can als lead t the eutrphicatin f lakes. The blm and subsequent die-ff f algae and cyanbacteria in a lake and the depletin f xygen are similar t what ccurs in a marine dead zne. Eutrphicatin threatens the survival f rganisms. Fr example, eutrphicatin f Lake Erie cupled with verfishing wiped ut cmmercially imprtant fishes such as blue pike, whitefish, and lake trut by the 1960s. Tighter regulatins n waste dumping int the lake have enabled sme fish ppulatins t rebund, but many native species f fishes and invertebrates have nt recvered. Cmbustin f fssil fuels is the main cause f acid precipitatin. The burning f fssil fuels releases xides f sulfur and nitrgen that react with water in the atmsphere t prduce sulfuric and nitric acids. These acids fall back t earth as acid precipitatin rain, snw, sleet, r fg with a ph less than 5.2. Acid precipitatin lwers the ph f streams and lakes and affects sil chemistry and nutrient availability. Acid precipitatin is a reginal prblem arising frm lcal emissins. The tall exhaust stacks built fr smelters and generating plants exprt the prblem far dwnwind. In the 1960s, eclgists bserved that rganisms in eastern Canadian lakes were dying because f air pllutin frm factries in the midwestern United States. Lakes and streams in suthern Nrway and Sweden were lsing fish because f acid rain frm pllutants generated in Great Britain and central Eurpe. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

15 By 1980, the ph f precipitatin in large areas f Nrth America and Eurpe averaged and ccasinally drpped as lw as 3.0. In terrestrial ecsystems, the change in sil ph due t acid precipitatin causes calcium and ther nutrients t leach frm the sil. The resulting nutrient deficiencies limit the grwth f plants. Acid precipitatin can als damage plants directly, mainly by leaching nutrients frm leaves. Freshwater ecsystems are particularly sensitive t acid precipitatin. Lakes with pr buffering capacity because f lw bicarbnate levels are mst vulnerable. Fish ppulatins have declined in many lakes in Nrway, Sweden, and Canada, where ph levels have drpped belw 5.0. Lake trut, keystne predatrs in many Canadian lakes, die when the ph drps belw 5.4. When the fish are replaced by acid-tlerant species, the dynamics f fd webs in the lakes change dramatically. Several large ecsystem experiments have been carried ut t test the feasibility f reversing the effects f acid precipitatin. One is the Ca 2+ additin experiment at Hubbard Brk. Anther is a 17-year experiment in Nrway in which scientists built a glass rf ver a frest and then shwered the frest with precipitatin frm which acids had been remved. This clean precipitatin quickly increased the ph and decreased the nitrate, ammnium, and sulfate cncentratins in stream water in the frest. Leaders f mre than 40 Eurpean natins have signed a treaty t reduce air pllutin. Envirnmental regulatins and new industrial technlgies have enabled many develped cuntries t reduce sulfur dixide emissins during the past 40 years. In the United States, sulfur dixide emissins decreased 31% between 1993 and As a result, precipitatin in the nrtheastern United States is gradually becming less acidic. Even if sulfur dixide emissins cntinue t decrease, eclgists estimate that it will take decades fr aquatic ecsystems t recver. Currently, emissins f nitrgen xides in the United States are increasing, and emissins f sulfur dixide and acid precipitatin in central and eastern Eurpe cntinue t damage frests. Txins can becme cncentrated in successive trphic levels f fd webs. Humans release many txic chemicals, including thusands f synthetics previusly unknwn in nature. Organisms acquire txic substances frm the envirnment alng with nutrients and water. Sme f the pisns are metablized and excreted, but thers accumulate in specific tissues, especially fat. Fat-sluble txins becme mre cncentrated in successive trphic levels f a fd web, a prcess called bilgical magnificatin. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

16 Magnificatin ccurs because the bimass at any given trphic level is prduced frm a much larger bimass ingested frm the level belw. Thus, tp-level carnivres tend t be the rganisms mst severely affected by txic cmpunds in the envirnment. Chlrinated hydrcarbns, including the industrial chemicals called PCBs (plychlrinated biphenyls) and many pesticides, such as DDT, demnstrate bilgical magnificatin. Many f these cmpunds disrupt the endcrine systems f a large number f animal species, including humans. In the fd web f the Great Lakes, the cncentratin f PCBs in herring gull eggs at the tp f the fd web is nearly 5,000 times the cncentratin in phytplanktn at the base f the fd web. An infamus case f bilgical magnificatin that harmed tp-level carnivres invlved DDT, a chemical used t cntrl insects such as msquites and agricultural pests in the decade after Wrld War II. By the 1950s, scientists were learning that DDT persists in the envirnment and is transprted by water t areas far frm where it is applied. One f the first signs that DDT was a serius envirnmental prblem was a decline in the ppulatins f pelicans, spreys, and eagles, birds that feed at the tp f fd webs. The accumulatin f DDT (and DDE, a prduct f its partial breakdwn) in the tissues f these birds interfered with the depsitin f calcium in their eggshells. When these birds tried t incubate their eggs, the weight f the parents brke the shells f affected eggs, resulting in catastrphic declines in their reprductin rates. Rachel Carsn s bk, Silent Spring, helped bring the prblem t public attentin in the 1960s, leading t the banning f DDT in the United States in After the ban, the ppulatins f the affected bird species recvered. In the trpics, DDT is still used t cntrl the msquites that spread malaria. Scieties there face a trade-ff between saving human lives and prtecting ther species. The best apprach seems t be t apply DDT sparingly and t cuple its use with msquit netting and ther lw-technlgy slutins. The cmplicated histry f DDT illustrates the imprtance f understanding the eclgical cnnectins between diseases and cmmunities. Many txins that cannt be degraded by micrrganisms persist in the envirnment fr years r even decades. In ther cases, chemicals released int the envirnment may be relatively harmless but are cnverted t mre txic prducts thrugh reactins with ther substances, by expsure t light, r by the metablism f micrrganisms. Fr example, insluble mercury has been rutinely expelled int rivers and the sea. Bacteria in the bttm mud cnvert the waste t methylmercury, an extremely txic sluble cmpund that accumulates in the tissues f rganisms, including humans wh cnsume fish frm the cntaminated waters. Human activities may be causing climate change by increasing levels f atmspheric carbn dixide. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

17 Since the Industrial Revlutin, the cncentratin f CO2 in the atmsphere has increased greatly as a result f burning fssil fuels and defrestatin. Befre 1850, the average CO2 cncentratin in the envirnment was 274 ppm. Measurements in 1958 read 316 ppm and have increased t 380 ppm tday. If CO2 emissins cntinue t increase at the present rate, the atmspheric cncentratin f CO2 will mre than duble frm the start f the Industrial Revlutin t Increased prductivity by vegetatin is ne cnsequence f increasing CO2 levels. Because C3 plants are mre limited than C4 plants by CO2 availability, ne effect f increasing CO2 levels may be the spread f C3 species int terrestrial habitats previusly favring C4 plants. Fr example, crn may be replaced n farms by wheat and sybeans. T assess the effect f rising levels f atmspheric CO2 n temperate frests, scientists at Duke University began the Frest-Atmsphere Carbn Transfer and Strage (FACTS-I) experiment in The FACTS-I study is testing hw elevated CO2 levels influence tree grwth, carbn cncentratin in sils, insect ppulatins, sil misture, understry plant grwth, and ther factrs ver a ten-year perid. After ten years, trees in the experimental plts (with elevated CO2 levels) prduced abut 15% mre wd each year than thse in the cntrl plts. This increased grwth is far lwer than predicted frm the results f greenhuse experiments. The availability f nitrgen and ther nutrients apparently limits the ability f the trees t use the extra CO2. Researchers at FACTS-I began remving this limitatin in 2005 by fertilizing half f each plt with ammnium nitrate. In mst f the wrld s ecsystems, nutrients limit ecsystem prductivity and fertilizers are unavailable. The results f FACTS-I and ther experiments suggest that increased atmspheric CO2 levels will increase plant prductin smewhat, but far less than scientists predicted even a decade ag. Rising atmspheric CO2 levels are changing Earth s heat budget. When light energy hits Earth, much f it is reflected ff the surface. Water vapr, CO2, and ther greenhuse gases are transparent t visible light, but they intercept and absrb infrared light, reflecting sme f it back tward Earth. This prcess retains slar heat, prducing the greenhuse effect. If it were nt fr the greenhuse effect, the average air temperature n Earth wuld be 18 C. The increasing cncentratin f atmspheric CO2 is a great cncern because f its link t increased glbal temperature. Fr mre than a century, scientists have studied hw greenhuse gases warm Earth and hw burning fssil fuels culd cntribute t the warming. Mst envirnmental scientists are cnvinced that such warming has already begun and will increase rapidly during this century. Glbal mdels predict that by the end f the 21st century, the atmspheric CO2 cncentratin will mre than duble and the average glbal temperature will rise by 3 C. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

18 A crrelatin between CO2 levels and temperatures in prehistric times supprts these mdels. Climatlgists estimate past CO2 cncentratins by measuring CO2 levels in bubbles trapped in glacial ice, sme f which are half a millin years ld. Prehistric temperatures are inferred by several methds, including analysis f past vegetatin based n fssils and the chemical istpes in sediments and crals. An increase f nly 1.3 C wuld make the wrld warmer than at any time in the past 100,000 years. The ecsystems where the greatest warming has already ccurred are thse in the far nrth, particularly nrthern cniferus frests and tundra. As snw and ice melt and uncver darker, mre absrptive surfaces, these systems reflect less radiatin back t the atmsphere and warm further. Arctic sea ice in the summer f 2005 cvered the smallest area n recrd. There may be n summer ice in the Arctic by the end f this century, decreasing habitat fr plar bears, seals, and seabirds. Higher temperatures increase the likelihd f fires. In breal frests f western Nrth America and Russia, fires have burned twice the usual area in recent decades. A warming trend wuld als alter the gegraphic distributin f precipitatin, making majr agricultural areas f the central United States much drier. Varius mathematical mdels disagree abut the details f hw the climate in each regin will be affected. By studying hw past perids f glbal warming and cling affected plant cmmunities, eclgists are trying t predict the cnsequences f future temperature changes. Analysis f fssilized pllen indicates that plant cmmunities change dramatically with changes in temperature. Hwever, past climate changes ccurred gradually, and plant and animal ppulatins culd migrate int areas where abitic cnditins allwed them t survive. Many rganisms, especially plants that cannt disperse rapidly ver lng distances, may nt be able t survive the fast rates f climate change prjected t result frm glbal warming. Furthermre, many habitats tday are much mre fragmented than they were in the past, further limiting the ability f many rganisms t migrate. Quick prgress t slw glbal warming can be made by using energy mre efficiently and by replacing fssil fuels with renewable slar and wind pwer and, mre cntrversially, with nuclear pwer. Stabilizing CO2 emissins will require cncerted internatinal effrt and the acceptance f changes in bth persnal lifestyles and industrial prcesses. Many eclgists think this effrt suffered a majr setback in 2001, when the United States pulled ut f the Kyt Prtcl, a 1997 pledge by industrialized natins t reduce their CO2 utput by 5% ver a ten-year perid. Human activities are depleting atmspheric zne. Life n Earth is prtected frm the damaging effects f ultravilet (UV) radiatin by a layer f O3, r zne, in the lwer stratsphere. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

19 Studies suggest that the zne layer has been gradually thinning since The destructin f zne results mainly frm the accumulatin f CFCs (chlrflurcarbns) chemicals used in refrigeratin and in manufacturing prcesses. The breakdwn prducts frm these chemicals rise t the stratsphere, where the chlrine they cntain reacts with zne t reduce it t O2. Subsequent reactins liberate the chlrine, allwing it t react with ther zne mlecules in a catalytic chain reactin. The thinning f the zne layer is mst apparent ver Antarctica in the spring, where cld, stable air allws the chain reactin t cntinue. Scientists first described the zne hle ver Antarctica in The magnitude f zne depletin and the size f the zne hle have generally increased in recent years. The hle smetimes extends as far as the suthernmst prtins f Australia, New Zealand, and Suth America. At middle latitudes, zne levels have decreased by 2 10% during the past 20 years. As a result f decreased zne levels in the stratsphere, increased amunts f UV radiatin are reaching the surface f Earth. The cnsequences f zne depletin fr life n Earth may be severe fr plants, animals, and micrbes. Sme scientists expect increases in the incidence f skin cancer and cataracts, as well as unpredictable effects n crps and natural cmmunities, especially phytplanktn. Eclgists have cnducted field experiments in which they used filters t decrease r blck natural UV rays. An experiment perfrmed n a scrub ecsystem near the tip f Suth America shwed that, when the zne hle passed ver the area, the amunt f UV radiatin reaching the grund increased sharply, causing DNA damage in plants that were nt prtected by filters. Scientists have shwn similar DNA damage and less phytplanktn grwth when the zne hle pens ver the Suthern Ocean each year. The gd news abut the zne hle is hw quickly many cuntries respnded t it. Since 1987, apprximately 190 natins, including the United States, have signed the Mntreal Prtcl, a treaty that regulates the use f zne-depleting chemicals. Many natins, again including the United States, have ended the prductin f chlrflurcarbns. As a cnsequence f these actins, chlrine cncentratins in the stratsphere have stabilized and zne depletin is slwing. Even if all chlrflurcarbns were glbally banned tday, hwever, the chlrine mlecules that are already in the atmsphere wuld cntinue t influence stratspheric zne levels fr at least 50 years. Destructin f Earth s zne shield is ne mre example f hw much humans have been able t disrupt the dynamics f ecsystems and the bisphere. It als highlights ur ability t slve envirnmental prblems. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc