Human Domination of Earth's Ecosystems - School of Environmental ...

Alterations of theBiogeochemical CyclesCarbon. Life on Earth is based on carbon,and the CO 2in the atmosphere is the primaryresource for photosynthesis. Humanityadds CO 2to the atmosphere by mining andburning fossil fuels, the residue of life fromthe distant past, and by converting forestsand grasslands to agricultural and otherlow-biomass ecosystems. The net result ofboth activities is that organic carbon fromrocks, organisms, and soils is released intothe atmosphere as CO 2.The modern increase in CO 2representsthe clearest and best documented signal ofhuman alteration of the Earth system.Thanks to the foresight of Roger Revelle,Charles Keeling, and others who initiatedcareful and systematic measurements of atmosphericCO 2in 1957 and sustained themthrough budget crises and changes in scientificfashions, we have observed the concentrationof CO 2as it has increased steadilyfrom 315 ppm to 362 ppm. Analysis of airbubbles extracted from the Antarctic andGreenland ice caps extends the record backmuch further; the CO 2concentration wasmore or less stable near 280 ppm for thousandsof years until about 1800, and hasincreased exponentially since then (17).NP60°30°EQThere is no doubt that this increase hasbeen driven by human activity, today primarilyby fossil fuel combustion. The sourcesof CO 2can be traced isotopically; beforethe period of extensive nuclear testing inthe atmosphere, carbon depleted in 14 C wasa specific tracer of CO 2derived from fossilfuel combustion, whereas carbon depletedin 13 C characterized CO 2from both fossilfuels and land transformation. Direct measurementsin the atmosphere, and analysesof carbon isotopes in tree rings, show thatboth 13 C and 14 CinCO 2were diluted inthe atmosphere relative to 12 C as the CO 2concentration in the atmosphere increased.Fossil fuel combustion now adds 5.5 0.5 billion metric tons of CO 2-Ctotheatmosphere annually, mostly in economicallydeveloped regions of the temperatezone (18) (Fig. 4). The annual accumulationof CO 2-C has averaged 3.2 0.2billion metric tons recently (17). The othermajor terms in the atmospheric carbonbalance are net ocean-atmosphere flux,net release of carbon during land transformation,and net storage in terrestrial biomassand soil organic matter. All of theseterms are smaller and less certain thanfossil fuel combustion or annual atmosphericaccumulation; they represent richareas of current research, analysis, andsometimes contention.The human-caused increase in atmosphericCO 2already represents nearly a30% change relative to the pre-industrialera (Fig. 2), and CO 2will continue to increasefor the foreseeable future. IncreasedCO 2represents the most important humanenhancement to the greenhouse effect; theconsensus of the climate research communityis that it probably already affects climatedetectably and will drive substantialclimate change in the next century (1). Thedirect effects of increased CO 2on plantsand ecosystems may be even more important.The growth of most plants is enhancedby elevated CO 2, but to very different extents;the tissue chemistry of plants thatrespond to CO 2is altered in ways thatdecrease food quality for animals and microbes;and the water use efficiency ofplants and ecosystems generally is increased.The fact that increased CO 2affectsspecies differentially means that it is likelyto drive substantial changes in the speciescomposition and dynamics of all terrestrialecosystems (19).Water. Water is essential to all life. Itsmovement by gravity, and through evaporationand condensation, contributes to drivingEarth’s biogeochemical cycles and tocontrolling its climate. Very little of theDownloaded from www.sciencemag.org on September 17, 201230°60°SP180° 120°W 60°W 0°60°E120°E180°50 100 150 200 250 300 350 400 450Emissions (g m -2 year -1 )Fig. 4. Geographical distribution of fossil fuel sources of CO 2 as of 1990. The global mean is 12.2 g m 2 year 1 ; most emissions occur in economicallydeveloped regions of the north temperate zone. EQ, equator; NP, North Pole; SP, South Pole. [Prepared by A. S. Denning, from information in (18)]496SCIENCE VOL. 277 25 JULY 1997 www.sciencemag.org

HUMAN-DOMINATED ECOSYSTEMS: ARTICLESwater on Earth is directly usable by humans;most is either saline or frozen. Globally, humanitynow uses more than half of the runoffwater that is fresh and reasonably accessible,with about 70% of this use in agriculture(20) (Fig. 2). To meet increasing demandsfor the limited supply of fresh water, humanityhas extensively altered river systemsthrough diversions and impoundments. Inthe United States only 2% of the rivers rununimpeded, and by the end of this centurythe flow of about two-thirds of all of Earth’srivers will be regulated (21). At present, asmuch as 6% of Earth’s river runoff is evaporatedas a consequence of human manipulations(22). Major rivers, including the Colorado,the Nile, and the Ganges, are used soextensively that little water reaches the sea.Massive inland water bodies, including theAral Sea and Lake Chad, have been greatlyreduced in extent by water diversions foragriculture. Reduction in the volume of theAral Sea resulted in the demise of nativefishes and the loss of other biota; the loss ofa major fishery; exposure of the salt-laden seabottom, thereby providing a major source ofwindblown dust; the production of a drierand more continental local climate and adecrease in water quality in the general region;and an increase in human diseases(23).Impounding and impeding the flow ofrivers provides reservoirs of water that can beused for energy generation as well as foragriculture. Waterways also are managed fortransport, for flood control, and for the dilutionof chemical wastes. Together, these activitieshave altered Earth’s freshwater ecosystemsprofoundly, to a greater extent thanterrestrial ecosystems have been altered. Theconstruction of dams affects biotic habitatsindirectly as well; the damming of the DanubeRiver, for example, has altered the silicachemistry of the entire Black Sea. The largenumber of operational dams (36,000) in theworld, in conjunction with the many thatare planned, ensure that humanity’s effectson aquatic biological systems will continue(24). Where surface water is sparse or overexploited,humans use groundwater—and inmany areas the groundwater that is drawnupon is nonrenewable, or fossil, water (25).For example, three-quarters of the water supplyof Saudi Arabia currently comes fromfossil water (26).Alterations to the hydrological cycle canaffect regional climate. Irrigation increasesatmospheric humidity in semiarid areas, oftenincreasing precipitation and thunderstormfrequency (27). In contrast, landtransformation from forest to agriculture orpasture increases albedo and decreases surfaceroughness; simulations suggest that thenet effect of this transformation is to increasetemperature and decrease precipitationregionally (7, 26).Conflicts arising from the global use ofwater will be exacerbated in the yearsahead, with a growing human populationand with the stresses that global changeswill impose on water quality and availability.Of all of the environmental securityissues facing nations, an adequate supply ofclean water will be the most important.Nitrogen. Nitrogen (N) is unique amongthe major elements required for life, in thatits cycle includes a vast atmospheric reservoir(N 2) that must be fixed (combinedwith carbon, hydrogen, or oxygen) before itcan be used by most organisms. The supplyof this fixed N controls (at least in part) theproductivity, carbon storage, and speciescomposition of many ecosystems. Before theextensive human alteration of the N cycle,90 to 130 million metric tons of N (Tg N)were fixed biologically on land each year;rates of biological fixation in marine systemsare less certain, but perhaps as muchwas fixed there (28).Human activity has altered the globalcycle of N substantially by fixing N 2—deliberatelyfor fertilizer and inadvertentlyduring fossil fuel combustion. Industrial fixationof N fertilizer increased from 10Tg/year in 1950 to 80 Tg/year in 1990; aftera brief dip caused by economic dislocationsin the former Soviet Union, it is expectedto increase to 135 Tg/year by 2030 (29).Cultivation of soybeans, alfalfa, and otherlegume crops that fix N symbiotically enhancesfixation by another 40 Tg/year,and fossil fuel combustion puts 20 Tg/yearof reactive N into the atmosphere globally—someby fixing N 2, more from the mobilizationof N in the fuel. Overall, humanactivity adds at least as much fixed N toterrestrial ecosystems as do all naturalsources combined (Fig. 2), and it mobilizes50 Tg/year more during land transformation(28, 30).Alteration of the N cycle has multipleconsequences. In the atmosphere, these include(i) an increasing concentration of thegreenhouse gas nitrous oxide globally; (ii)substantial increases in fluxes of reactive Ngases (two-thirds or more of both nitric oxideand ammonia emissions globally are humancaused);and (iii) a substantial contributionto acid rain and to the photochemical smogthat afflicts urban and agricultural areasthroughout the world (31). Reactive N thatis emitted to the atmosphere is depositeddownwind, where it can influence the dynamicsof recipient ecosystems. In regionswhere fixed N was in short supply, added Ngenerally increases productivity and C storagewithin ecosystems, and ultimately increaseslosses of N and cations from soils, ina set of processes termed “N saturation” (32).Where added N increases the productivity ofecosystems, usually it also decreases theirbiological diversity (33).Human-fixed N also can move from agriculture,from sewage systems, and fromN-saturated terrestrial systems to streams,rivers, groundwater, and ultimately theoceans. Fluxes of N through streams andrivers have increased markedly as humanalteration of the N cycle has accelerated;river nitrate is highly correlated with thehuman population of river basins and withthe sum of human-caused N inputs to thosebasins (8). Increases in river N drive theeutrophication of most estuaries, causingblooms of nuisance and even toxic algae,and threatening the sustainability of marinefisheries (16, 34).Other cycles. The cycles of carbon, water,and nitrogen are not alone in being alteredby human activity. Humanity is also thelargest source of oxidized sulfur gases in theatmosphere; these affect regional air quality,biogeochemistry, and climate. Moreover,mining and mobilization of phosphorusand of many metals exceed their naturalfluxes; some of the metals that are concentratedand mobilized are highly toxic (includinglead, cadmium, and mercury) (35).Beyond any doubt, humanity is a majorbiogeochemical force on Earth.Synthetic organic chemicals. Synthetic organicchemicals have brought humanitymany beneficial services. However, manyare toxic to humans and other species, andsome are hazardous in concentrations as lowas 1 part per billion. Many chemicals persistin the environment for decades; some areboth toxic and persistent. Long-lived organochlorinecompounds provide the clearestexamples of environmental consequencesof persistent compounds. Insecticidessuch as DDT and its relatives, and industrialcompounds like polychlorinated biphenyls(PCBs), were used widely in North Americain the 1950s and 1960s. They were transportedglobally, accumulated in organisms,and magnified in concentration throughfood chains; they devastated populations ofsome predators (notably falcons and eagles)and entered parts of the human food supplyin concentrations higher than was prudent.Domestic use of these compounds wasphased out in the 1970s in the UnitedStates and Canada, and their concentrationsdeclined thereafter. However, PCBsin particular remain readily detectable inmany organisms, sometimes approachingthresholds of public health concern (36).They will continue to circulate throughorganisms for many decades.Synthetic chemicals need not be toxicto cause environmental problems. The factthat the persistent and volatile chlorofluorocarbons(CFCs) are wholly nontoxic contributedto their widespread use as refriger-Downloaded from www.sciencemag.org on September 17, 2012www.sciencemag.org SCIENCE VOL. 277 25 JULY 1997 497

ants and even aerosol propellants. The subsequentdiscovery that CFCs drive thebreakdown of stratospheric ozone, and especiallythe later discovery of the Antarcticozone hole and their role in it, representgreat surprises in global environmental science(37). Moreover, the response of theinternational political system to those discoveriesis the best extant illustration thatglobal environmental change can be dealtwith effectively (38).Particular compounds that pose serioushealth and environmental threats can beand often have been phased out (althoughPCB production is growing in Asia). Nonetheless,each year the chemical industryproduces more than 100 million tons oforganic chemicals representing some 70,000different compounds, with about 1000 newones being added annually (39). Only asmall fraction of the many chemicals producedand released into the environmentare tested adequately for health hazards orenvironmental impact (40).Biotic ChangesHuman modification of Earth’s biologicalresources—its species and genetically distinctpopulations—is substantial and growing.Extinction is a natural process, but thecurrent rate of loss of genetic variability, ofpopulations, and of species is far abovebackground rates; it is ongoing; and it representsa wholly irreversible global change.At the same time, human transport of speciesaround Earth is homogenizing Earth’sbiota, introducing many species into newareas where they can disrupt both naturaland human systems.Losses. Rates of extinction are difficultto determine globally, in part because themajority of species on Earth have not yetbeen identified. Nevertheless, recent calculationssuggest that rates of species extinctionare now on the order of 100 to 1000times those before humanity’s dominance ofEarth (41). For particular well-knowngroups, rates of loss are even greater; asmany as one-quarter of Earth’s bird specieshave been driven to extinction by humanactivities over the past two millennia,particularly on oceanic islands (42) (Fig. 2).At present, 11% of the remaining birds,18% of the mammals, 5% of fish, and 8% ofplant species on Earth are threatened withextinction (43). There has been a disproportionateloss of large mammal speciesbecause of hunting; these species played adominant role in many ecosystems, andtheir loss has resulted in a fundamentalchange in the dynamics of those systems(44), one that could lead to further extinctions.The largest organisms in marinesystems have been affected similarly, byfishing and whaling. Land transformation isthe single most important cause of extinction,and current rates of land transformationeventually will drive manymore species to extinction, although witha time lag that masks the true dimensionsof the crisis (45). Moreover, the effects ofother components of global environmentalchange—of altered carbon and nitrogencycles, and of anthropogenic climatechange—are just beginning.As high as they are, these losses of speciesunderstate the magnitude of loss ofgenetic variation. The loss to land transformationof locally adapted populations withinspecies, and of genetic material withinpopulations, is a human-caused change thatreduces the resilience of species and ecosystemswhile precluding human use of thelibrary of natural products and genetic materialthat they represent (46).Although conservation efforts focusedon individual endangered species haveyielded some successes, they are expensive—andthe protection or restoration ofwhole ecosystems often represents the mosteffective way to sustain genetic, population,and species diversity. Moreover, ecosystemsthemselves may play important roles inboth natural and human-dominated landscapes.For example, mangrove ecosystemsprotect coastal areas from erosion and providenurseries for offshore fisheries, but theyare threatened by land transformation inmany areas.Invasions. In addition to extinction, humanityhas caused a rearrangement ofEarth’s biotic systems, through the mixingof floras and faunas that had long beenisolated geographically. The magnitude oftransport of species, termed “biological invasion,”is enormous (47); invading speciesare present almost everywhere. On manyislands, more than half of the plant speciesare nonindigenous, and in many continentalareas the figure is 20% or more (48) (Fig.2).As with extinction, biological invasionoccurs naturally—and as with extinction,human activity has accelerated its rate byorders of magnitude. Land transformationinteracts strongly with biological invasion,in that human-altered ecosystems generallyprovide the primary foci for invasions, whilein some cases land transformation itself isdriven by biological invasions (49). Internationalcommerce is also a primary causeof the breakdown of biogeographic barriers;trade in live organisms is massive and global,and many other organisms are inadvertentlytaken along for the ride. In freshwatersystems, the combination of upstreamland transformation, altered hydrology, andnumerous deliberate and accidental speciesintroductions has led to particularly widespreadinvasion, in continental as well asisland ecosystems (50).In some regions, invasions are becomingmore frequent. For example, in the SanFrancisco Bay of California, an average ofone new species has been established every36 weeks since 1850, every 24 weeks since1970, and every 12 weeks for the last decade(51). Some introduced species quickly becomeinvasive over large areas (for example,the Asian clam in the San Francisco Bay),whereas others become widespread only aftera lag of decades, or even over a century(52).Many biological invasions are effectivelyirreversible; once replicating biological materialis released into the environment andbecomes successful there, calling it back isdifficult and expensive at best. Moreover,some species introductions have consequences.Some degrade human health andthat of other species; after all, most infectiousdiseases are invaders over most of theirrange. Others have caused economic lossesamounting to billions of dollars; the recentinvasion of North America by the zebramussel is a well-publicized example. Somedisrupt ecosystem processes, altering thestructure and functioning of whole ecosystems.Finally, some invasions drive losses inthe biological diversity of native species andpopulations; after land transformation, theyare the next most important cause of extinction(53).ConclusionsThe global consequences of human activityare not something to face in the future—asFig. 2 illustrates, they are with us now. Allof these changes are ongoing, and in manycases accelerating; many of them were entrainedlong before their importance wasrecognized. Moreover, all of these seeminglydisparate phenomena trace to a singlecause—the growing scale of the human enterprise.The rates, scales, kinds, and combinationsof changes occurring now are fundamentallydifferent from those at any othertime in history; we are changing Earthmore rapidly than we are understanding it.We live on a human-dominated planet—and the momentum of human populationgrowth, together with the imperative forfurther economic development in most ofthe world, ensures that our dominance willincrease.The papers in this special section summarizeour knowledge of and provide specificpolicy recommendations concerningmajor human-dominated ecosystems. In addition,we suggest that the rate and extentof human alteration of Earth should affecthow we think about Earth. It is clear thatwe control much of Earth, and that ourDownloaded from www.sciencemag.org on September 17, 2012498SCIENCE VOL. 277 25 JULY 1997 www.sciencemag.org

HUMAN-DOMINATED ECOSYSTEMS: ARTICLESactivities affect the rest. In a very real sense,the world is in our hands—and how wehandle it will determine its compositionand dynamics, and our fate.Recognition of the global consequencesof the human enterprise suggests three complementarydirections. First, we can work toreduce the rate at which we alter the Earthsystem. Humans and human-dominated systemsmay be able to adapt to slower change,and ecosystems and the species they supportmay cope more effectively with the changeswe impose, if those changes are slow. Ourfootprint on the planet (54) might then bestabilized at a point where enough spaceand resources remain to sustain most of theother species on Earth, for their sake andour own. Reducing the rate of growth inhuman effects on Earth involves slowinghuman population growth and using resourcesas efficiently as is practical. Often itis the waste products and by-products ofhuman activity that drive global environmentalchange.Second, we can accelerate our efforts tounderstand Earth’s ecosystems and howthey interact with the numerous componentsof human-caused global change. Ecologicalresearch is inherently complex anddemanding: It requires measurement andmonitoring of populations and ecosystems;experimental studies to elucidate the regulationof ecological processes; the development,testing, and validation of regionaland global models; and integration with abroad range of biological, earth, atmospheric,and marine sciences. The challenge ofunderstanding a human-dominated planetfurther requires that the human dimensionsof global change—the social, economic,cultural, and other drivers of human actions—beincluded within our analyses.Finally, humanity’s dominance of Earthmeans that we cannot escape responsibilityfor managing the planet. 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Naylor for constructivecomments on this paper, A. S. Denning andS. M. Garcia for assistance with illustrations, and C.Nakashima and B. Lilley for preparing text and figuresfor publication.Downloaded from www.sciencemag.org on September 17, 2012www.sciencemag.org SCIENCE VOL. 277 25 JULY 1997 499