Vitousek Et Al., Human Domination of Earth's Ecosystems

Home , Dominance (ecology), Largest organisms

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Humanalteration of Earthis substantialand growing. Between one-thirdand one-half interactwith the atmosphere,with aquatic of the land surface has been transformedby human action; the carbon dioxide con- systems,and with surroundingland. More- centrationin the atmosphere has increased by nearly30 percent since the beginningof over, land transformationinteracts strongly the IndustrialRevolution; more atmospheric nitrogen is fixed by humanitythan by all with most other componentsof global en- naturalterrestrial sources combined;more than halfof all accessible surface fresh water vironmentalchange. is put to use by humanity;and about one-quarterof the birdspecies on Earthhave been The measurementof land transforma- drivento extinction. By these and other standards, it is clear that we live on a human- tion on a globalscale is challenging;chang- dominated planet. es can be measuredmore or less straightfor- wardly at a given site, but it is difficult to aggregatethese changesregionally and globally. In contrastto analysesof human al- Aii organismsmodify their environment, reasonablywell quantified;all are ongoing. teration of the global carbon cycle, we and humans are no exception. As the hu- These relativelywell-documented changes cannot install instrumentson a tropical man populationhas grownand the powerof in turn entrain further alterationsto the mountainto collect evidence of land trans- technology has expanded, the scope and functioningof the Earthsystem, most no- formation. Remote sensing is a most useful nature of this modification has changed tably by drivingglobal climatic change (1) technique,but only recentlyhas there been drastically.Until recently, the term "hu- and causingirreversible losses of biological a seriousscientific effortto use high-resolu- man-dominated ecosystems" would have diversity(2). tion civilian satellite imagery to evaluate elicited images of agricultural fields, pas- even the more visible forms of land trans- tures, or uLrbanlandscapes; now it applies Land Transformation formation, such as deforestation, on conti- with greater or lesser force to all of Earth. nental to global scales (3). Many ecosystems are dominated directly by The use of land to yield goods and services Land transformation encompasses a humanity, and no ecosystem on Earth's sur- representsthe most substantialhuman al- wide variety of activities that vary sub- face is free of pervasive human influence. terationof the Earthsystem. Human use of stantially in their intensity and conse- This article provides an overview of hu- land alters the structure and functioning of quences. At one extreme, 10 to 15% of man effects on Earth's ecosystems. It is not ecosystems,and it alters how ecosystems Earth's land surface is occupied by row- intended as a litany of environmental disas- ters, though some disastrous situations are described; nor is it intended either to down- Fig. 1. A conceptual play or to celebrate environmental success- model illustrating hu- Human population manity's direct and indi- Size Resource use es, of which there have been many. Rather, rect effects on the Earth we explore how large humanity looms as a system [modified from presence on the globe-how, even on the (56)]. Human enterprises grandest scale, most aspects of the structure and functioning of Earth's ecosystems can- Agriculture Industry Recreation Internationalcommerce not be understood without accounting for the strong, often dominant influence of humanity. Land Biotic additions We view human alterations to the Earth transformationc andlosses system as operating through the interacting Landclearing Ib Invasion processes summarized in Fig. 1. The growth Forestry En Hunting Grazing Global Fishing of the human population, and growth in the Intensification K biogeochemistry p a resource base used by humanity, is main- Carbon tained by a suite of human enterprises such Nitrogen as agriculture, industry, fishing, and inter- Water Synthetic chemicals national commerce. These enterprises Other elements transform the land surface (through crop- ping, forestry, and urbanization), alter the major biogeochemical cycles, and add or remove species and genetically distinct populations in most of Earth's ecosystems. Many of these changes are substantial and IT ,' IFll Climate change Loss of biological diversity P. M. Vitousek and H. A. Mooney are in the Department Enhanced of Biological Sciences, Stanford University,Stanford, CA PI Extinctionof species 94305, USA. J. Lubchenco is in the Department of Zool- greenhouse populations ogy, Oregon State University, Corvallis, OR 97331, Aerosols ~~~~~~and USA. J. M. Melillois at the U.S. Office of Science and of ecosystems Technology Policy, Old Executive Office Building, Room Land cover ~~~~~Loss 443, Washington, DC 20502, USA.

494 SCIENCE * VOL. 277 * 25 JULY 1997 * www.sciencemag.org m HUMAN-DOMINATED ECOSYSTEMS: ARTICLES crop agricultureor by urban-industrialar- the reactive chemistryof the troposphere, tion growsto morethan 25%for upwelling eas, and another 6 to 8% has been con- bringing elevated carbon monoxide con- areasand to 35% for temperatecontinen- verted to pastureland (4); these systems centrations and episodes of urban-like tal shelf systems (13). are wholly changed by human activity. At photochemical air pollution to remote Many of the fisheries that capturema- the other extreme,every terrestrialecosys- tropical areas of Africa and South Amer- rine productivityare focused on top pred- tem is affected by increased atmospheric ica; and it causes runoff of sediment and ators,whose removalcan alter marineeco- carbon dioxide (CO2), and most ecosys- nutrientsthat drive substantialchanges in systems out of proportionto their abun- tems have a history of hunting and other stream,lake, estuarine,and coral reef eco- dance. Moreover, many such fisheries low-intensity resource extraction. Be- systems (7-10). have proved to be unsustainable,at least tween these extremes lie grassland and The central importanceof land trans- at our present level of knowledge and semiaridecosystems that are grazed (and formation is well recognized within the control. As of 1995, 22% of recognized sometimesdegraded) by domestic animals, communityof researchersconcerned with marine fisheries were overexploitedor al- and forests and woodlands from which global environmentalchange. Several re- ready depleted, and 44% more were at wood products have been harvested; to- search programsare focused on aspects of their limit of exploitation (14) (Figs. 2 gether, these represent the majority of it (9, 11); recent and substantialprogress and 3). The consequencesof fisheries are Earth'svegetated surface. toward understandingthese aspects has not restricted to their target organisms; The variety of human effects on land been made (3), and much more progress commercial marine fisheries around the makesany attemptto summarizeland trans- can be anticipated. Understanding land worlddiscard 27 million tons of nontarget formationsglobally a matterof semanticsas transformationis a difficult challenge; it animals annually, a quantity nearly one- well as substantialuncertainty. Estimates of requiresintegrating the social, economic, third as largeas total landings(15). More- the fractionof land transformedor degraded and culturalcauses of land transformation over, the dredgesand trawlsused in some by humanity(or its corollary,the fractionof with evaluations of its biophysicalnature fisheries damage habitats substantiallyas the land'sbiological production that is used and consequences. This interdisciplinary they are draggedalong the sea floor. or dominated)fall in the rangeof 39 to 50% approach is essential to predicting the A recent increase in the frequency, (5) (Fig. 2). These numbershave largeun- course, and to any hope of affecting the extent, and duration of harmful algal certainties,but the fact that they arelarge is consequences, of human-caused land blooms in coastal areas (16) suggeststhat not at all uncertain.Moreover, if anything transformation. human activity has affected the base as these estimates understatethe global im- well as the top of marine food chains. pact of land transformation,in that land Oceans Harmfulalgal blooms are sudden increases that has not been transformedoften has in the abundanceof marine phytoplank- been divided into fragmentsby human al- Human alterations of marine ecosystems ton that produce harmful structures or terationof the surroundingareas. This frag- are moredifficult to quantifythan those of chemicals. Some but not all of these phy- mentation affects the species composition terrestrialecosystems, but several kinds of toplankton are stronglypigmented (red or and functioningof otherwiselittle modified informationsuggest that they are substan- brown tides). Algal blooms usually are ecosystems(6). tial. The human populationis concentrat- correlated with changes in temperature, Overall, land transformationrepresents ed near coasts-about 60% within 100 nutrients, or salinity; nutrients in coastal the primarydriving force in the loss of km-and the oceans' productive coastal waters,in particular,are much modifiedby biological diversity worldwide.Moreover, margins have been affected strongly by human activity. Algal blooms can cause the effects of land transformationextend humanity. Coastal wetlands that mediate extensive fish kills through toxins and by far beyond the boundariesof transformed interactions between land and sea have causing anoxia; they also lead to paralytic lands. Land transformationcan affect cli- been alteredover large areas;for example, shellfish poisoning and amnesic shellfish mate directly at local and even regional approximately50% of mangrove ecosys- poisoning in humans. Although the exis- scales. It contributes -20% to current tems globally have been transformedor tence of harmful algal blooms has long anthropogenic CO2 emissions, and more destroyedby human activity (12). More- been recognized,they have spreadwidely substantiallyto the increasingconcentra- over, a recent analysis suggestedthat al- in the past two decades (16). tions of the greenhousegases methane and though humans use about 8% of the pri- nitrous oxide; fires associatedwith it alter maryproduction of the oceans, that frac- 100 Fig. 2. Human domi- 100 nance or alteration of a ;g 80 several major compo- ' 80 , 60 nents of the Earth sys- < 60 tem, expressed as (from & 0 40 left to right) percentage X 40 = 20 . Phase II- of the land surface trans- a - | 1 # = _10 Developing formed (5); percentage t 20 01 LO 0 LO 0 LO 0 LO 0 I' of the current atmo- o l II I LO LO CD N- N- CO co 0) 0) spheric C02 concentra- Land Water Plant Marine tion that results from hu- transformation CO2 use Nitrogen invasion Bird fisheries Year man action (17); per- concentration fixation extinction Fig. 3. Percentage of major world marine fish centage of accessible resources in different phases of development, surface fresh water used (20); percentage of terrestrialN fixationthat is human-caused (28); percentage 1951 to 1994 [from (57)]. Undeveloped = a low of plant species in Canada that humanity has introduced from elsewhere (48); percentage of bird and relativelyconstant level of catches; develop- species on Earth that have become extinct in the past two millennia, almost all of them as a conse- ing = rapidlyincreasing catches; mature = a high quence of human activity (42); and percentage of major marine fisheries that are fully exploited, and plateauinglevel of catches; senescent = overexploited, or depleted (14). catches decliningfrom higher levels.

www.sciencemag.org * SCIENCE * VOL. 277 * 25 JULY 1997 495 Alterations of the There is no doubt that this increasehas sometimes contention. Biogeochemical Cycles been driven by human activity, today pri- The human-caused increase in atmo- marilyby fossilfuel combustion.The sourc- spheric CO2 already represents nearly a Carbon.Life on Earth is based on carbon, es of CO2 can be tracedisotopically; before 30% change relative to the pre-industrial and the CO2 in the atmosphereis the pri- the period of extensive nuclear testing in era (Fig. 2), and CO2 will continue to in- maryresource for photosynthesis. Humanity the atmosphere,carbon depleted in 14Cwas crease for the foreseeable future. Increased adds CO2 to the atmosphere by mining and a specific tracerof CO2 derivedfrom fossil CO2 represents the most important human burningfossil fuels, the residueof life from fuel combustion,whereas carbon depleted enhancement to the greenhouse effect; the the distant past, and by convertingforests in 13C characterized CO2 from both fossil consensus of the climate research commu- and grasslandsto agriculturaland other fuels and land transformation.Direct mea- nity is that it probably already affects cli- low-biomassecosystems. The net result of surementsin the atmosphere,and analyses mate detectably and will drive substantial both activities is that organiccarbon from of carbon isotopes in tree rings, show that climate change in the next century (1 ). The rocks, organisms,and soils is releasedinto both 13C and 14C in CO2 were diluted in direct effects of increased CO2 on plants the atmosphereas CO2. the atmosphererelative to l2C as the CO2 and ecosystems may be even more impor- The modernincrease in CO2 represents concentrationin the atmosphereincreased. tant. The growth of most plants is enhanced the clearestand best documentedsignal of Fossil fuel combustionnow adds 5.5 + by elevated CO2, but to very different ex- human alteration of the Earth system. 0.5 billion metric tons of C02-C to the tents; the tissue chemistry of plants that Thanks to the foresightof Roger Revelle, atmosphereannually, mostly in economi- respond to CO2 is altered in ways that Charles Keeling, and others who initiated cally developed regions of the temperate decrease food quality for animals and mi- carefuland systematicmeasurements of at- zone (18) (Fig. 4). The annual accumula- crobes; and the water use efficiency of mosphericCO2 in 1957 and sustainedthem tion of C02-C has averaged 3.2 + 0.2 plants and ecosystems generally is in- throughbudget crises and changesin scien- billion metric tons recently (17). The oth- creased. The fact that increased CO2 affects tific fashions, we have observedthe con- er majorterms in the atmosphericcarbon species differentially means that it is likely centrationof CO2 as it has increasedsteadi- balance are net ocean-atmosphereflux, to drive substantial changes in the species ly from315 ppmto 362 ppm.Analysis of air net release of carbonduring land transfor- composition and dynamics of all terrestrial bubbles extractedfrom the Antarctic and mation, and net storagein terrestrialbio- ecosystems (19). Greenlandice caps extendsthe recordback mass and soil organic matter. All of these Water. Water is essential to all life. Its much further;the CO2 concentrationwas terms are smaller and less certain than movement by gravity, and through evapora- more or less stable near 280 ppm for thou- fossil fuel combustion or annual atmo- tion and condensation, contributes to driv- sands of years until about 1800, and has spheric accumulation;they representrich ing Earth's biogeochemical cycles and to increasedexponentially since then (17). areas of current research, analysis, and controlling its climate. Very little of the

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Fig. 4. Geographical distributionof fossil fuel sources of C02 as of 1990. The global mean is 12.2 g m-2 year-1; most emissions occur in economically developed regions of the north temperate zone. EQ, equator; NP, North Pole; SP, South Pole. [Prepared by A. S. Denning, from informationin (18)]

496 SCIENCE * VOL. 277 * 25 JULY 1997 * www.sciencemag.org a HUMAN-DOMINATED ECOSYSTEMS: ARTICLES wateron Earthis directlyusable by humans; tion regionally(7, 26). ecosystems,usually it also decreasestheir most is eithersaline or frozen.Globally, hu- Conflicts arisingfrom the global use of biologicaldiversity (33). manitynow usesmore than half of the runoff water will be exacerbated in the years Human-fixedN also can move from ag- waterthat is freshand reasonablyaccessible, ahead, with a growinghuLman population riculture,from sewage systems, and from with about 70% of this use in agriculture and with the stressesthat global changes N-saturatedterrestrial systems to streams, (20) (Fig. 2). To meet increasingdemands will imposeon waterquality and availabil- rivers, groundwater,and ultimately the for the limitedsupply of freshwater, humanity. Of all of the environmentalsecurity oceans. Fluxes of N through streamsand ity has extensively altered river systems issuesfacing nations, an adequatesupply of rivers have increasedmarkedly as human throughdiversions and impoundments.In clean waterwill be the most important. alterationof the N cycle has accelerated; the United States only 2% of the riversrun Nitrogen.Nitrogen (N) is uniqueamong river nitrate is highly correlatedwith the unimpeded,and by the end of this century the majorelements required for life, in that human populationof riverbasins and with the flow of abouttwo-thirds of all of Earth's its cycle includesa vast atmosphericreser- the sum of human-causedN inputsto those riverswill be regulated(21). At present,as voir (N2) that must be fixed (combined basins (8). Increasesin river N drive the much as 6% of Earth'sriver runoff is evapo- with carbon,hydrogen, or oxygen) beforeit eutrophicationof most estuaries, causing ratedas a consequenceof humanmanipula- can be used by most organisms.The supply blooms of nuisance and even toxic algae, tions (22). Majorrivers, including the Col- of this fixed N controls(at least in part)the and threateningthe sustainabilityof marine orado,the Nile, and the Ganges,are used so productivity,carbon storage, and species fisheries(16, 34). extensivelythat little waterreaches the sea. compositionof manyecosystems. Before the Othercycles. The cyclesof carbon,water, Massiveinland water bodies, includingthe extensive human alterationof the N cycle, and nitrogenare not alone in being altered Aral Sea and LakeChad, have been greatly 90 to 130 million metric tons of N (Tg N) by human activity. Humanity is also the reducedin extent by water diversionsfor were fixed biologicallyon land each year; largestsource of oxidizedsulfur gases in the agriculture.Reduction in the volumeof the rates of biological fixation in marine sys- atmosphere;these affect regionalair quali- Aral Sea resultedin the demise of native tems are less certain, but perhapsas much ty, biogeochemistry,and climate. More- fishesand the loss of other biota;the loss of was fixed there (28). over, mining and mobilizationof phospho- a majorfishery; exposure of the salt-ladensea Human activity has altered the global rusand of manymetals exceed their natural bottom,thereby providing a majorsource of cycle of N substantiallyby fixing N2-de- fluxes;some of the metalsthat are concen- windblowndust; the productionof a drier liberately for fertilizer and inadvertently trated and mobilizedare highly toxic (in- and more continental local climate and a duringfossil fuel combustion.Industrial fix- cludinglead, cadmium,and mercury)(35). decreasein waterquality in the generalre- ation of N fertilizerincreased from <10 Beyond any doubt, humanity is a major gion; and an increase in human diseases Tg/yearin 1950 to 80 Tg/yearin 1990;after biogeochemicalforce on Earth. (23). a briefdip causedby economic dislocations Syntheticorganic chemicals. Synthetic or- Impoundingand impeding the flow of in the formerSoviet Union, it is expected ganic chemicals have brought humanity riversprovides reservoirs of waterthat can be to increaseto >135 Tg/yearby 2030 (29). many beneficial services. However, many used for energy generationas well as for Cultivation of soybeans,alfalfa, and other are toxic to humansand other species,and agriculture.Waterways also are managedfor legume crops that fix N symbioticallyen- some arehazardous in concentrationsas low transport,for flood control,and for the dilu- hances fixation by another -40 Tg/year, as 1 partper billion. Manychemicals persist tion of chemicalwastes. Together, these ac- andfossil fuel combustionputs >20 Tg/year in the environmentfor decades;some are tivities have alteredEarth's freshwater eco- of reactive N into the atmosphereglobal- both toxic and persistent.Long-lived or- systemsprofoundly, to a greaterextent than ly-some by fixing N2, more from the mo- ganochlorinecompounds provide the clear- terrestrialecosystems have been altered.The bilizationof N in the fuel. Overall,human est examplesof environmentalconsequenc- constructionof dams affectsbiotic habitats activity adds at least as much fixed N to es of persistent compounds. Insecticides indirectlyas well; the dammingof the Dan- terrestrial ecosystems as do all, natural such as DDT and its relatives,and industrial ube River,for example,has alteredthe silica sourcescombined (Fig. 2), and it mobilizes compoundslike polychlorinatedbiphenyls chemistryof the entireBlack Sea. The large >50 Tg/yearmore duringland transforma- (PCBs),were used widely in North America numberof operationaldams (36,000) in the tion (28, 30). in the 1950s and 1960s. They were trans- world, in conjunctionwith the many that Alteration of the N cycle has multiple ported globally,accumulated in organisms, are planned,ensure that humanity'seffects consequences.In the atmosphere,these in- and magnified in concentration through on aquaticbiological systems will continue clude (i) an increasingconcentration of the food chains;they devastatedpopulations of (24). Where surfacewater is sparseor over- greenhousegas nitrous oxide globally;(ii) some predators(notably falcons and eagles) exploited,humans use groundwater-andin substantialincreases in fluxes of.reactive N and enteredparts of the humanfood supply many areasthe groundwaterthat is drawn gases(two-thirds or moreof both nitricoxide in concentrationshigher than was prudent. upon is nonrenewable,or fossil,water (25). and ammoniaemissions globally are human- Domestic use of these compounds was Forexample, three-quarters of the watersup- caused);and (iii) a substantialcontribution phased out in the 1970s in the United ply of Saudi Arabia currentlycomes from to acid rain and to the photochemicalsmog States and Canada, and their concentra- fossil water(26). that afflicts urban and agriculturalareas tions declined thereafter.However, PCBs Alterationsto the hydrologicalcycle can throughoutthe world(31). ReactiveN that in particularremain readily detectable in affect regionalclimate. Irrigationincreases is emitted to the atmosphereis deposited many organisms, sometimes approaching atmospherichumidity in semiaridareas, of- downwind,where it can influence the dy- thresholdsof public health concern (36). ten increasingprecipitation and thunder- namics of recipient ecosystems.In regions They will continue to circulate through storm frequency (27). In contrast, land wherefixed N was in shortsupply, added N organismsfor many decades. transformationfrom forest to agricultureor generallyincreases productivity and C stor- Synthetic chemicals need not be toxic pastureincreases albedo and decreasessur- age within ecosystems,and ultimatelyin- to cause environmentalproblems. The fact face roughness;simulations suggest that the creaseslosses of N and cationsfrom soils, in that the persistentand volatile chlorofluo- net effect of this transformation is to in- a set of processes termed "N saturation"(32). rocarbons (CFCs) are wholly nontoxic con- crease temperature and decrease precipita- Where added N increases the productivity of tributed to their widespread use as refriger-

www.sciencemag.org * SCIENCE * VOL. 277 * 25 JULY 1997 497 ants and even aerosolpropellants. The sub- fishing and whaling.Land transformation is spreadinvasion, in continental as well as sequent discovery that CFCs drive the the single most important cause of ex- islandecosystems (50). breakdownof stratosphericozone, and es- tinction, and current rates of land trans- In some regions,invasions are becoming peciallythe laterdiscovery of the Antarctic formation eventually will drive many more frequent. For example, in the San ozone hole and their role in it, represent more species to extinction, although with FranciscoBay of California,an averageof greatsurprises in global environmentalsci- a time lag that masksthe true dimensions one new specieshas been establishedevery ence (37). Moreover,the responseof the of the crisis (45). Moreover,the effects of 36 weeks since 1850, every 24 weeks since internationalpolitical system to those dis- other components of global environmen- 1970, and every 12 weeksfor the last decade coveries is the best extant illustrationthat tal change-of altered carbon and nitro- (51). Some introducedspecies quickly be- global environmentalchange can be dealt gen cycles, and of anthropogenicclimate come invasiveover largeareas (for example, with effectively(38). change-are just beginning. the Asian clam in the San FranciscoBay), Particularcompounds that pose serious As high as they are, these losses of spe- whereasothers become widespreadonly af- health and environmentalthreats can be cies understatethe magnitude of loss of ter a lag of decades,or even over a century and often have been phasedout (although genetic variation.The loss to land transfor- (52). PCB productionis growingin Asia). None- mation of locally adaptedpopulations with- Manybiological invasions are effectively theless, each year the chemical industry in species, and of genetic materialwithin irreversible;once replicatingbiological ma- produces more than 100 million tons of populations,is a human-causedchange that terial is releasedinto the environmentand organicchemicals representing some 70,000 reducesthe resilienceof speciesand ecosys- becomes successfulthere, calling it back is differentcompounds, with about 1000 new tems while precludinghuman use of the difficult and expensive at best. Moreover, ones being added annually (39). Only a libraryof naturalproducts and genetic ma- some species introductions have conse- small fractionof the many chemicalspro- terial that they represent(46). quences. Some degradehuman health and duced and releasedinto the environment Although conservation efforts focused that of other species;after all, most infec- are tested adequatelyfor health hazardsor on individual endangered species have tiousdiseases are invaders over most of their environmentalimpact (40). yielded some successes, they are expen- range.Others have causedeconomic losses sive-and the protection or restorationof amountingto billions of dollars;the recent Biotic Changes whole ecosystemsoften representsthe most invasion of North America by the zebra effectiveway to sustaingenetic, population, mussel is a well-publicizedexample. Some Human modificationof Earth'sbiological and speciesdiversity. Moreover, ecosystems disrupt ecosystem processes, altering the resources-its species and genetically dis- themselves may play important roles in structureand functioningof whole ecosys- tinct populations-is substantialand grow- both natural and human-dominatedland- tems. Finally,some invasionsdrive losses in ,ing. Extinctionis a naturalprocess, but the scapes. For example, mangroveecosystems the biologicaldiversity of native speciesand currentrate of loss of genetic variability,of protect coastal areasfrom erosion and pro- populations;after land transformation,they populations, and of species is far above vide nurseriesfor offshorefisheries, but they are the next most importantcause of ex- backgroundrates; it is ongoing;and it rep- are threatened by land transformationin tinction (53). resentsa wholly irreversibleglobal change. many areas. At the same time, human transportof spe- Invasions.In additionto extinction, hu- Conclusions cies aroundEarth is homogenizingEarth's manity has caused a rearrangementof biota, introducingmany species into new Earth'sbiotic systems,through the mixing The globalconsequences of humanactivity areaswhere they can disruptboth natural of floras and faunas that had long been are not somethingto face in the future-as and human systems. isolated geographically.The magnitudeof Fig. 2 illustrates,they are with us now. All Losses.Rates of extinction are difficult transportof species, termed"biological in- of these changesare ongoing, and in many to determineglobally, in part because the vasion,"is enormous(47); invadingspecies cases accelerating;many of them were en- majorityof species on Earthhave not yet are present almost everywhere.On many trained long before their importancewas been identified.Nevertheless, recent calcu- islands,more than half of the plant species recognized.Moreover, all of these seeming- lations suggestthat rates of species extinc- are nonindigenous,and in many continen- ly disparatephenomena trace to a single tion are now on the orderof 100 to 1000 tal areasthe figureis 20%or more(48) (Fig. cause-the growingscale of the humanen- times those beforehumanity's dominance of 2). terprise.The rates, scales, kinds, and com- Earth (41). For particular well-known As with extinction, biological invasion binationsof changesoccurring now arefun- groups, rates of loss are even greater;as occurs naturally-and as with extinction, damentallydifferent from those at any oth- many as one-quarterof Earth'sbird species human activity has acceleratedits rate by er time in history;we are changing Earth have been driven to extinction by human ordersof magnitude.Land transformation more rapidlythan we are understandingit. activities over the past two millennia, interactsstrongly with biological invasion, We live on a human-dominatedplanet- particularlyon oceanic islands(42) (Fig.2). in that human-alteredecosystems generally and the momentumof human population At present, 11% of the remainingbirds, providethe primaryfoci for invasions,while growth, together with the imperativefor 18%of the mammals,5% of fish, and 8%of in some cases land transformationitself is furthereconomic developmentin most of plant species on Earthare threatenedwith driven by biological invasions (49). Inter- the world,ensures that our dominancewill extinction (43). There has been a dispro- national commerceis also a primarycause increase. portionate loss of large mammal species of the breakdownof biogeographicbarriers; The papersin this special section sum- because of hunting; these species played a tradein live organismsis massiveand glob- marizeour knowledgeof and provide spe- dominant role in many ecosystems, and al, and many other organismsare inadver- cific policy recommendationsconcerning their loss has resulted in a fundamental tently taken along for the ride. In freshwa- majorhuman-dominated ecosystems. In ad- change in the dynamics of those systems ter systems, the combination of upstream dition, we suggestthat the rate and extent (44), one that could lead to further ex- land transformation,altered hydrology, and of human alterationof Earthshould affect tinctions. The largest organisms in marine numerous deliberate and accidental species how we think about Earth. It is clear that systems have been affected similarly, by introductions has led to particularly wide- we control much of Earth, and that our

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