insight review articles may once have controlled Lyme tick-bearing mice by out-competing Box 2 them for food".The loss of the passenger pigeon to nineteenth- Ecosystem services century over-hunting may,therefore,have contributed to the rise of Lyme disease in humans in the twentieth century.The economic Ecosystem services are defined as the processes and conditions of impacts of invasions of novel species are particularly well document- natural ecosystems that support human activity and sustain human ed.The introduction and spread of single pests such as the golden life.Such services include the maintenance of soil fertility,climate apple snail (Pomacea canaliculata)and the European corn borer regulation and natural pest control,and provide flows of ecosystem (Ostrinia nubilalis)have had major impacts on food production and goods such as food,timber and fresh water.They also provide farm incomes.Estimates of the overall cost of invasions by exotic intangible benefits such as aesthetic and cultural values species in the United States range widely from $1.1 to $137 billion Ecosystem services are generated by the biodiversity present in annually.70.In Australia,plant invasions alone entail an annual cost natural ecosystems.Ecologists and economists have begun to ofUS$2.1 billion" quantify the impacts of changes in biodiversity on the delivery of The provision of tangible ecosystem goods and services by ecosystem services and to attach monetary value to these changes. natural systems depends not only on species'presence or absence Techniques used to attach value to biodiversity change range from but also on their abundance.Large populations ofthe white-footed direct valuation based on market prices to estimates of what mouse (Peromyscus leucopus)in the northeastern United States individuals are willing to pay to protect endangered wildlife control outbreaks of gypsy moth (Lymatria dispar)but spread Although there are estimates of the global values of ecosystem Lyme disease,whereas small populations of the mouse decrease the services,valuation of the marginal losses that accompany specific incidence of Lyme disease but allow gypsy moth defoliation2.An biodiversity changes are most relevant to policy decisions. analysis of the costs of changes in biodiversity thus involves more Predicting the value of such losses involves uncertainty,because than just analysis ofextinctions and invasions.Theloss ofa species ecological and societal systems interact in nonlinear ways and to extinction is of special societal concern,however,because it is because human preferences change through time.Assumptions irreversible.Future opportunities to learn and derive newly recog- today about future values may underestimate the values placed on nized benefits from an extinct species are lost forever.Preventing natural systems by future generations.Therefore,minimizing loss such a loss preserves an 'option value'for society-the value of of biodiversity offers a conservative strategy for maintaining this attaining more knowledge about species and their contribution to value. human well being in order to make informed decisions in the future4 For example,significant value($230-330 million)has been attributed to genetic information gained from preventing Global environmental changes have the potential to exacerbate land conversion in Jalisco,Mexico,in an area containing a wild the ecological and societal impacts of changes in biodiversity In grass,teosinte (Euchlaena mexicana),that can be used to develop many regions,land conversion forces declining populations towards viral-resistant strains ofperennial corn.Ifthis land had been con- the edges of their species range,where they become increasingly verted to agriculture or human settlements,the societal benefits of vulnerable to collapse if exposed to further human impact?.Warm- development would have come at the expense ofan irreversible loss ing allows the poleward spread of exotics and pathogens,such as in genetic material that could be used for breeding viral resistance dengue-and malaria-transmitting mosquitoes(Aedesand Anopheles in one of the most widely consumed cereal crops in the world.The sp.)%6and pests of key food crops,such as corn-boring insects perceived costs of diversity loss in this situation might have been Warming can also exacerbate the impacts of water-consuming small-especially relative to the development benefits-whereas invasive plant species in water-scarce areas by increasing regional the actual (unrecognized)costs of losing genetic diversity would water losses.The Tamarix-invaded Colorado River in the United have been significant(Fig.8).Decisions to preserve land to gain States currently has a mean annual flow that is 10%less than regional further information about the societal value of species diversity or water allocations for human use.Warming by4'Cwould reduce the ecosystem function typically involve a large degree of uncertainty, flow of the Colorado River by more than 20%,further increasing the which often leads to myopic decisions regarding land use. marginal costs of water losses to Tamarix?s.Similar impacts ofglobal change in regions such as Sahelian Africa,which have less water and less well developed distribution mechanisms,might directly affect Figure 7 Water losses to human survival.In many cases,accelerated biodiversity loss is the invasive,deep-rooted alreadyjeopardizing thelivelihoods oftraditionalpeoples star thistle,C.solstitialis, The combination of irreversible species losses and positive provides an example of feedbacks between biodiversity changes and ecosystem processes are the financial impacts of likely to cause nonlinear cost increases to society in the future,partic- introducing exotic species ularly when thresholds of ecosystem resilience are exceeded.For on ecosystem example,Imperata cylindrica,an aggressive indigenous grass, composition.(Photograph colonizes forest lands of Asia that are cleared for slash-and-burn courtesy of P.Collins/A-Z agriculture,forminga monoculture grassland with no vascular plant Botanical Collection.) diversity and many fewer mammalian species than the native forest. The total area of Imperata in Asia is currently about 35 million ha(4% of land area).Once in place,Imperata is difficult and costly to remove and enhances fire,which promotes the spread of the grass. The annual cost of reversing this conversion in Indonesia,where 4% of the nation's area(8.6 million ha)is now in Imperata grasslands, would be over $400 million if herbicides are used,and $1.2 billion if labour is used to remove the grass manually.Farmers typically burn the fields because herbicides and labour are too expensive.Burning these grasslands,however,increases losses of soil nitrogen and carbon,which erode agricultural productivity,and enhances regen- eration of Imperata.This positive feedback with nonlinear changes in land cover will probably continue in the future aslands are deforested 240 2000 Macmillan Magazines Ltd NATURE VOL 405|11 MAY 2000 www.nature.commay once have controlled Lyme tick-bearing mice by out-competing them for food66. The loss of the passenger pigeon to nineteenth￾century over-hunting may, therefore, have contributed to the rise of Lyme disease in humans in the twentieth century. The economic impacts of invasions of novel species are particularly well document￾ed. The introduction and spread of single pests such as the golden apple snail (Pomacea canaliculata) and the European corn borer (Ostrinia nubilalis) have had major impacts on food production and farm incomes67,68. Estimates of the overall cost of invasions by exotic species in the United States range widely from $1.1 to $137 billion annually69,70. In Australia, plant invasions alone entail an annual cost of US$2.1 billion71. The provision of tangible ecosystem goods and services by natural systems depends not only on species’ presence or absence but also on their abundance. Large populations of the white-footed mouse (Peromyscus leucopus) in the northeastern United States control outbreaks of gypsy moth (Lymatria dispar) but spread Lyme disease, whereas small populations of the mouse decrease the incidence of Lyme disease but allow gypsy moth defoliation72. An analysis of the costs of changes in biodiversity thus involves more than just analysis of extinctions and invasions. The loss of a species to extinction is of special societal concern, however, because it is irreversible. Future opportunities to learn and derive newly recog￾nized benefits from an extinct species are lost forever. Preventing such a loss preserves an ‘option value’ for society — the value of attaining more knowledge about species and their contribution to human well being in order to make informed decisions in the future73,74. For example, significant value ($230–330 million) has been attributed to genetic information gained from preventing land conversion in Jalisco, Mexico, in an area containing a wild grass, teosinte (Euchlaena mexicana), that can be used to develop viral-resistant strains of perennial corn73. If this land had been con￾verted to agriculture or human settlements, the societal benefits of development would have come at the expense of an irreversible loss in genetic material that could be used for breeding viral resistance in one of the most widely consumed cereal crops in the world. The perceived costs of diversity loss in this situation might have been small — especially relative to the development benefits — whereas the actual (unrecognized) costs of losing genetic diversity would have been significant (Fig. 8). Decisions to preserve land to gain further information about the societal value of species diversity or ecosystem function typically involve a large degree of uncertainty, which often leads to myopic decisions regarding land use. Global environmental changes have the potential to exacerbate the ecological and societal impacts of changes in biodiversity6 . In many regions, land conversion forces declining populations towards the edges of their species range, where they become increasingly vulnerable to collapse if exposed to further human impact75. Warm￾ing allows the poleward spread of exotics and pathogens, such as dengue- and malaria-transmitting mosquitoes (Aedes and Anopheles sp.)76 and pests of key food crops, such as corn-boring insects68. Warming can also exacerbate the impacts of water-consuming invasive plant species in water-scarce areas by increasing regional water losses. The Tamarix-invaded Colorado River in the United States currently has a mean annual flow that is 10% less than regional water allocations for human use77. Warming by 4˚C would reduce the flow of the Colorado River by more than 20%, further increasing the marginal costs of water losses to Tamarix78. Similar impacts of global change in regions such as Sahelian Africa, which have less water and less well developed distribution mechanisms, might directly affect human survival. In many cases, accelerated biodiversity loss is already jeopardizing the livelihoods of traditional peoples79. The combination of irreversible species losses and positive feedbacks between biodiversity changes and ecosystem processes are likely to cause nonlinear cost increases to society in the future, partic￾ularly when thresholds of ecosystem resilience are exceeded80. For example, Imperata cylindrica, an aggressive indigenous grass, colonizes forest lands of Asia that are cleared for slash-and-burn agriculture, forming a monoculture grassland with no vascular plant diversity and many fewer mammalian species than the native forest. The total area of Imperata in Asia is currently about 35 million ha (4% of land area)81. Once in place, Imperata is difficult and costly to remove and enhances fire, which promotes the spread of the grass. The annual cost of reversing this conversion in Indonesia, where 4% of the nation’s area (8.6 million ha) is now in Imperata grasslands, would be over $400 million if herbicides are used, and $1.2 billion if labour is used to remove the grass manually. Farmers typically burn the fields because herbicides and labour are too expensive. Burning these grasslands, however, increases losses of soil nitrogen and carbon, which erode agricultural productivity, and enhances regen￾eration of Imperata. This positive feedback with nonlinear changes in land cover will probably continue in the future as lands are deforested insight review articles 240 NATURE | VOL 405 | 11 MAY 2000 | www.nature.com Figure 7 Water losses to the invasive, deep-rooted star thistle, C. solstitialis, provides an example of the financial impacts of introducing exotic species on ecosystem composition. (Photograph courtesy of P. Collins/A-Z Botanical Collection.) Ecosystem services are defined as the processes and conditions of natural ecosystems that support human activity and sustain human life. Such services include the maintenance of soil fertility, climate regulation and natural pest control, and provide flows of ecosystem goods such as food, timber and fresh water. They also provide intangible benefits such as aesthetic and cultural values88. Ecosystem services are generated by the biodiversity present in natural ecosystems. Ecologists and economists have begun to quantify the impacts of changes in biodiversity on the delivery of ecosystem services and to attach monetary value to these changes. Techniques used to attach value to biodiversity change range from direct valuation based on market prices to estimates of what individuals are willing to pay to protect endangered wildlife89. Although there are estimates of the global values of ecosystem services64, valuation of the marginal losses that accompany specific biodiversity changes are most relevant to policy decisions. Predicting the value of such losses involves uncertainty, because ecological and societal systems interact in nonlinear ways and because human preferences change through time. Assumptions today about future values may underestimate the values placed on natural systems by future generations89. Therefore, minimizing loss of biodiversity offers a conservative strategy for maintaining this value. Box 2 Ecosystem services © 2000 Macmillan Magazines Ltd