insight review articles Figure 6 Trophic interactions can affect ecosystem processes by influencing species'abundances. a,Removal of sea otters by Russian fur traders caused an explosion in the population of sea urchins that overgrazed kelp.(Photographs courtesy of M. Sewell/Still Pictures and J.Rotman/BBC Natural History Unit.)b.Similarly,changes in the species balance and the abundance of fish can deplete phytoplankton grazers and cause algal blooms.(Photograph courtesy of J. Foott/BBC Natural History Unit.) native plant diversity(for example,soil type and disturbance regime) changes in community composition and vulnerability to invasion. also promote species invasions37 Introduction ofexotic species or changes in community composition Experimentalstudies with plantsor soil microorganisms often can affect ecosystem goods or services either by directly reducing show that vulnerability to invasion is governed more strongly by the abundances of useful species (by predation or competition),or by traits of resident and invading species than by species richness per se. altering controls on critical ecosystem processes(Fig.4). Both competition and trophic interactions contribute to these effects These impacts can be wide-ranging and costly.For example,the of community composition on invasibility.For example,in its native introduction of deep-rooted species in arid regions reduces supplies range,the Argentine ant (Linepithaema humile)is attacked by and increases costs of water for human use.Marginal water losses to species-specific parasitoids that modify its behaviour and reduce its the invasive star thistle,Centaurea solstitialis,in the Sacramento River ability to dominate food resources and competitively exclude other valley,California,have been valued at US$16-56million per year(J.D. ant species.These parasitoids are absent from the introduced range Gerlach,unpublished results)(Fig.7).In South Africa's Cape region, of Argentine ants,which may explain their success at eliminating the presence of rapidly transpiring exotic pines raises the unit cost of native ant communities in North America.Observational and water procurement by nearly 30%(ref.62).Increased evapotranspi- experimental studies together indicate that the effect of species ration due to the invasion of Tamarix in the United States costs an diversity on vulnerability to invasion depends on the components of estimated $65-180 million per year in reduced municipaland agricul- diversity involved (richness,evenness,composition and species tural water supplies In addition to raising water costs,the presence interactions)and their interactions with other ecological factors such of sediment-trapping Tamarix stands has narrowed river channels as disturbance regime,resource supply and rate of propagule arrival. and obstructed over-bank flows throughout the western United Humans significantly affect all of these factors (Figs 1,4),thereby States,increasing flood damages by as much as $50 million annually3 dramatically increasing the incidence of invasions worldwide. Those species changes that have greatest ecological impact frequently incur high societal costs.Changes in traits maintaining Societal consequences of altered diversity regional climate2 constitute an ecosystem service whose value in Biodiversity and its links to ecosystem properties have cultural, tropical forests has been estimated at $220 hayr(ref.64).The loss intellectual,aesthetic and spiritual values that are important to or addition of species that alter disturbance regimes can also be society.In addition,changes in biodiversity that alter ecosystem func- costly.The increased fire frequency resulting from the cheatgrass tioning have economic impacts through the provision of ecosystem invasion in the western United States has reduced rangeland values goods and services to society(Fig.I and Box 2).Changes in diversity and air quality and led to increased expenditures on fire suppres- can directly reduce sources of food,fuel,structural materials,medici- sion.The disruption of key species interactions can also have large nals or genetic resources.These changes can also alter the abundance societal and ecological consequences.Large populations ofpassenger of other species that control ecosystem processes,leading to further pigeons (Ectopistes migratorius)in the northeastern United States NATURE|VOL 40511 MAY 2000www.nature.com 2000 Macmillan Magazines Ltd 239native plant diversity (for example, soil type and disturbance regime) also promote species invasions57. Experimental studies with plants58 or soil microorganisms59 often show that vulnerability to invasion is governed more strongly by the traits of resident and invading species than by species richness per se. Both competition and trophic interactions contribute to these effects of community composition on invasibility. For example, in its native range, the Argentine ant (Linepithaema humile) is attacked by species-specific parasitoids that modify its behaviour and reduce its ability to dominate food resources and competitively exclude other ant species60. These parasitoids are absent from the introduced range of Argentine ants, which may explain their success at eliminating native ant communities in North America61. Observational and experimental studies together indicate that the effect of species diversity on vulnerability to invasion depends on the components of diversity involved (richness, evenness, composition and species interactions) and their interactions with other ecological factors such as disturbance regime, resource supply and rate of propagule arrival. Humans significantly affect all of these factors (Figs 1, 4), thereby dramatically increasing the incidence of invasions worldwide. Societal consequences of altered diversity Biodiversity and its links to ecosystem properties have cultural, intellectual, aesthetic and spiritual values that are important to society. In addition, changes in biodiversity that alter ecosystem func￾tioning have economic impacts through the provision of ecosystem goods and services to society (Fig. 1 and Box 2). Changes in diversity can directly reduce sources of food, fuel, structural materials, medici￾nals or genetic resources. These changes can also alter the abundance of other species that control ecosystem processes, leading to further changes in community composition and vulnerability to invasion. Introduction of exotic species or changes in community composition can affect ecosystem goods or services either by directly reducing abundances of useful species (by predation or competition), or by altering controls on critical ecosystem processes (Fig. 4). These impacts can be wide-ranging and costly. For example, the introduction of deep-rooted species in arid regions reduces supplies and increases costs of water for human use. Marginal water losses to the invasive star thistle, Centaurea solstitialis, in the Sacramento River valley, California, have been valued at US$16–56 million per year (J. D. Gerlach, unpublished results) (Fig. 7). In South Africa’s Cape region, the presence of rapidly transpiring exotic pines raises the unit cost of water procurement by nearly 30% (ref. 62). Increased evapotranspi￾ration due to the invasion of Tamarix in the United States costs an estimated $65–180 million per year in reduced municipal and agricul￾tural water supplies63. In addition to raising water costs, the presence of sediment-trapping Tamarix stands has narrowed river channels and obstructed over-bank flows throughout the western United States, increasing flood damages by as much as $50 million annually63. Those species changes that have greatest ecological impact frequently incur high societal costs. Changes in traits maintaining regional climate22 constitute an ecosystem service whose value in tropical forests has been estimated at $220 ha–1 yr–1 (ref. 64). The loss or addition of species that alter disturbance regimes can also be costly. The increased fire frequency resulting from the cheatgrass invasion in the western United States has reduced rangeland values and air quality and led to increased expenditures on fire suppres￾sion65. The disruption of key species interactions can also have large societal and ecological consequences. Large populations of passenger pigeons (Ectopistes migratorius) in the northeastern United States insight review articles NATURE | VOL 405 | 11 MAY 2000 | www.nature.com 239 a b Figure 6 Trophic interactions can affect ecosystem processes by influencing species’ abundances. a, Removal of sea otters by Russian fur traders caused an explosion in the population of sea urchins that overgrazed kelp. (Photographs courtesy of M. Sewell/Still Pictures and J. Rotman/BBC Natural History Unit.) b, Similarly, changes in the species balance and the abundance of fish can deplete phytoplankton grazers and cause algal blooms. (Photograph courtesy of J. Foott/BBC Natural History Unit.) © 2000 Macmillan Magazines Ltd