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

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