insight review articles Figure 1 The role of biodiversity in global change.Human Human Global changes activities that are motivated by activities economic,cultural,intellectual. Biogeochemical cycles -elevated CO,and other aesthetic and spiritual goals(1) 2 greenhouse gases Economic Cultural, are now causing environmental -nutrient loading benefits ◆intellectual, and ecological changes of -water consumption aesthetic and soiritual global significance (2).By a Land use benefits -type variety of mechanisms,these -intensity global changes contribute to Species invasions changing biodiversity,and changing biodiversity feeds Biodiversity Ecosystem goods back on susceptibility to species -richness and services invasions (3.purple arrows;see -evenness -composition text).Changes in biodiversity. -interactions through changes in species Species traits traits,can have direct consequences for ecosystem services and,as a result, human economic and social Ecosystem processes activities (4).In addition, changes in biodiversity can influence ecosystem processes (5).Altered ecosystem processes can thereby influence ecosystem services that benefit humanity (6)and feedback to further alter biodiversity(7,red amrow).Global changes may also directly affect ecosystem processes(8.blue arrows).Depending on the circumstances,the direct effects of global change may be either stronger or weaker than effects mediated by changes in diversity.We argue that the costs of loss of biotic diversity,although traditionally considered to be 'outside the box'of human welfare,must be recognized in our accounting of the costs and benefits of human activities. evenness),the particular species present (species composition),the mycorrhizal species richness also seems to enhance plant production interactions among species (non-additive effects),and the temporal in an asymptotic fashion,although phosphorus uptake was and spatial variation in these properties.In addition to its effects on enhanced in a linear fashion from 1 to 14 species offungi.Microbial current functioning of ecosystems,species diversity influences the richness can lead to increased decomposition oforganic matter".In resilience and resistance ofecosystemstoenvironmental change. contrast,no consistent statistical relationship has been observed Species richness and evenness between plant species richness of litter inputs and decomposition Most theoretical and empirical work on the functional consequences rate.Thus,inexperimental communities(which typically focus on of changing biodiversity has focused on the relationship between only one or two trophic levels),there seems to be no universal species richness and ecosystem functioning.Theoretical possibilities relationship between species richness and ecosystem functioning, include positive linear and asymptotic relationships between rich- perhaps because processes differ in their sensitivity to species rich- nessandratesofecosystemprocesses,or thelackofasimplestatistical ness compared with other components ofdiversity(such as evenness, relationship'(Box 1).In experiments,species richness correlates composition or interactions).The absence of a simple relationship with rates of ecosystem processes most clearly at low numbers of between species richness and ecosystem processes is likely when one species.We know much less about the impact of species richness in or a few species have strong ecosystem effects. species-rich,natural ecosystems.Several studies using experimental Although the relationship of species richness to ecosystem func- species assemblages have shown that annual rates of primary produc- tioning has attracted considerable theoretical and experimental tivity and nutrient retention increase with increasing plant species attention because of the irreversibility of species extinction,human richness,but saturate at a rather low number of species' Arbuscular activities influence the relative abundances ofspecies more frequent- ly than the presence or absence of species.Changes in species evenness warrant increased attention,because they usually respond more rapidly to human activities than do changes in species richness and because they have important consequences to ecosystems long olo before aspecies is threatenedby extinction. Species composition Particular species can have strong effects on ecosystem processes by directly mediating energy and material fluxes or by altering abiotic conditions that regulate the rates of these processes (Fig.4)34 Species'alteration ofthe availability oflimiting resources,the distur- bance regime,and the climate can have particularly strongeffects on ecosystem processes.Such effects are most visible when introduced species alter previous patterns of ecosystem processes.For example, the introduction of the nitrogen-fixing tree Myrica faya to nitrogen- Fish nt limited ecosystems in Hawaii led to a fivefold increase in nitrogen inputs to the ecosystem,which in turn changed most ofthe function- Figure 2 Proportion of the global number of species of birds,mammals,fish and al and structural properties of native forests5.Introduction of the plants that are currently threatened with extinction" deep-rooted salt cedar (Tamarix sp.)to the Mojave and Sonoran Deserts of North America increased the water and soil solutes NATURE|VOL 405|11 MAY 2000 www.nature.com 2000 Macmillan Magazines Ltd 235evenness), the particular species present (species composition), the interactions among species (non-additive effects), and the temporal and spatial variation in these properties. In addition to its effects on current functioning of ecosystems, species diversity influences the resilience and resistance of ecosystems to environmental change. Species richness and evenness Most theoretical and empirical work on the functional consequences of changing biodiversity has focused on the relationship between species richness and ecosystem functioning. Theoretical possibilities include positive linear and asymptotic relationships between rich￾ness and rates of ecosystem processes, or the lack of a simple statistical relationship7 (Box 1). In experiments, species richness correlates with rates of ecosystem processes most clearly at low numbers of species. We know much less about the impact of species richness in species-rich, natural ecosystems. Several studies using experimental species assemblages have shown that annual rates of primary produc￾tivity and nutrient retention increase with increasing plant species richness, but saturate at a rather low number of species8,9. Arbuscular mycorrhizal species richness also seems to enhance plant production in an asymptotic fashion, although phosphorus uptake was enhanced in a linear fashion from 1 to 14 species of fungi10. Microbial richness can lead to increased decomposition of organic matter11. In contrast, no consistent statistical relationship has been observed between plant species richness of litter inputs and decomposition rate12. Thus, in experimental communities (which typically focus on only one or two trophic levels), there seems to be no universal relationship between species richness and ecosystem functioning, perhaps because processes differ in their sensitivity to species rich￾ness compared with other components of diversity (such as evenness, composition or interactions). The absence of a simple relationship between species richness and ecosystem processes is likely when one or a few species have strong ecosystem effects. Although the relationship of species richness to ecosystem func￾tioning has attracted considerable theoretical and experimental attention because of the irreversibility of species extinction, human activities influence the relative abundances of species more frequent￾ly than the presence or absence of species. Changes in species evenness warrant increased attention, because they usually respond more rapidly to human activities than do changes in species richness and because they have important consequences to ecosystems long before a species is threatened by extinction. Species composition Particular species can have strong effects on ecosystem processes by directly mediating energy and material fluxes or by altering abiotic conditions that regulate the rates of these processes (Fig. 4)13,14. Species’ alteration of the availability of limiting resources, the distur￾bance regime, and the climate can have particularly strong effects on ecosystem processes. Such effects are most visible when introduced species alter previous patterns of ecosystem processes. For example, the introduction of the nitrogen-fixing tree Myrica faya to nitrogen￾limited ecosystems in Hawaii led to a fivefold increase in nitrogen inputs to the ecosystem, which in turn changed most of the function￾al and structural properties of native forests15. Introduction of the deep-rooted salt cedar (Tamarix sp.) to the Mojave and Sonoran Deserts of North America increased the water and soil solutes insight review articles NATURE | VOL 405 | 11 MAY 2000 | www.nature.com 235 Figure 1 The role of biodiversity in global change. Human activities that are motivated by economic, cultural, intellectual, aesthetic and spiritual goals (1) are now causing environmental and ecological changes of global significance (2). By a variety of mechanisms, these global changes contribute to changing biodiversity, and changing biodiversity feeds back on susceptibility to species invasions (3, purple arrows; see text). Changes in biodiversity, through changes in species traits, can have direct consequences for ecosystem services and, as a result, human economic and social activities (4). In addition, changes in biodiversity can influence ecosystem processes (5). Altered ecosystem processes can thereby influence ecosystem services that benefit humanity (6) and feedback to further alter biodiversity (7, red arrow). Global changes may also directly affect ecosystem processes (8, blue arrows). Depending on the circumstances, the direct effects of global change may be either stronger or weaker than effects mediated by changes in diversity. We argue that the costs of loss of biotic diversity, although traditionally considered to be ‘outside the box’ of human welfare, must be recognized in our accounting of the costs and benefits of human activities. Biogeochemical cycles Land use –elevated CO2 and other greenhouse gases –nutrient loading –water consumption –type –intensity Biodiversity –richness –evenness –composition –interactions Species invasions Species traits Human activities Economic benefits Cultural, intellectual, aesthetic and spiritual benefits Ecosystem goods and services Ecosystem processes 1 2 3 7 4 6 5 8 Global changes 0 5 10 15 20 Birds Mammals Fish Plants Extinction threatened (percentage of global species) Figure 2 Proportion of the global number of species of birds, mammals, fish and plants that are currently threatened with extinction4 . © 2000 Macmillan Magazines Ltd