insight overview Causes,consequences and ethics of biodiversity David Tilman Department of Ecology,Evolution and Behavior,University of Minnesota,St Paul,Minnesota 55108,USA (e-mail:tilman@lter.umn.edu) The existence of so great a diversity of species on Earth remains a mystery,the solution to which may also explain why and how biodiversity influences the functioning of ecosystems.The answer may lie in quantifying the trade-offs that organisms face in dealing with the constraints of their environment.Societal responses to the loss of biodiversity also involve trade-offs,and the elaboration of these will be essential in developing wiser environmental ethics and policy. he most striking feature of Earth is the existence Pressey(pages 243-253)discuss strategies for the preserva- of life,and the most striking feature of life is its tion of biodiversity. diversity.This biological diversity, or biodiversity,has long been a source of The effects of biodiversity on ecosystems wonderment and scientific curiosity,but is In broad summary,these reviews show that,on average, increasingly a source of concern.Human domination of greater diversity leads to greater productivity in plant Earth's ecosystems'is markedly reducing the diversity of communities,greater nutrient retention in ecosystems and species within many habitats worldwide,and is greater ecosystem stability.For instance,grassland field accelerating extinction.One of the more pragmatic experiments both in North America(Fig.1)36 and across questions raised by these threats to biodiversity is the eight different European sites,ranging from Greece in the extent to which this loss of biodiversity matters;that is,are south and east to Portugal and Ireland in the west and stability,productivity and other aspects of the functioning Sweden in the north',have shown that each halving of the of both managed and natural ecosystems dependent on number ofplant species withinaplot leadstoa 10-20%loss biodiversity? of productivity.An average plot containing one plant There are strong reasons to hypothesize,as did Darwin2 species is less than half as productive as an average plot and Elton',that biodiversity might impact ecosystem containing 24-32 species.Lower plant diversity also leads processes.But ecology is no longer a discipline in which nat- to greater rates of loss of limiting soil nutrients through ural history observations and simple verbal logic hold sway. leaching,which ultimately should decrease soil fertility, Therekindledinterest in the potentialeffectsofbiodiversity further loweringplant productivity. on ecosystem processes,which followed the publication in Both laboratory and field studies have shown that 1993 of a book edited by Schulze and Mooney is occurring ecosystem processes are more variable (less stable or in a discipline for which hypotheses are now tested against reliable)at lower diversity (see review by McCann,pages the results of field experiments,mechanistic theory 228-233,and refs 8-10).The greater stability of more and quantitative field observations.Anything less than the diverse ecosystems seems to result from three processes concordance ofall three lines ofevidence leads to the modi- The first is comparable to the economic process that causes a fication or rejection of hypotheses.Given that this topic more diverse investment portfolio to be less volatile. became a principal focus of scientific inquiry only about Because species,like corporations,differ from each other, seven years ago,it is not surprising that it remains they tend to respond somewhat independently to contentious.Indeed,the greatest surprise may be environmental variability.The more species the rapidity,breadth and depth of work that that such variability is averaged across, already has occurred,and the generalities the less variable is their total".Second, that are emerging from it. species within a given trophic level Five papers in this issue summarize often compete with each other, this work.Purvis and Hector (pages which causes their abundances to 212-219),McCann(pages228-233), negatively covary.When one and Chapin and collaborators(pages speciesdeclines,another is freed 234-242)review and synthesize from competition and recent experimental,theoretical and increases. This negative observational studies that have covariance reduces the vari- demonstrated links between biodi- ability of the community as a versityand thestability,productiv- whole Finally,measures ity and nutrient dynamics of of temporal stability compute ecosystems. Gaston (pages variability relative to mean abun- 220-227)summarizes global dance,such as by using the ratio of patterns of biodiversity and community abundance to its some possible explanations temporal standard deviation. for these patterns.Margulesand The tendency for community 208 2000 Macmillan Magazines Ltd NATURE|VOL 40511 MAY 2000 www.nature.com
insight overview 208 NATURE | VOL 405 | 11 MAY 2000 | www.nature.com T he most striking feature of Earth is the existence of life, and the most striking feature of life is its diversity. This biological diversity, or biodiversity, has long been a source of wonderment and scientific curiosity, but is increasingly a source of concern. Human domination of Earth’s ecosystems1 is markedly reducing the diversity of species within many habitats worldwide, and is accelerating extinction. One of the more pragmatic questions raised by these threats to biodiversity is the extent to which this loss of biodiversity matters; that is, are stability, productivity and other aspects of the functioning of both managed and natural ecosystems dependent on biodiversity? There are strong reasons to hypothesize, as did Darwin2 and Elton3 , that biodiversity might impact ecosystem processes. But ecology is no longer a discipline in which natural history observations and simple verbal logic hold sway. The rekindled interest in the potential effects of biodiversity on ecosystem processes, which followed the publication in 1993 of a book edited by Schulze and Mooney4 , is occurring in a discipline for which hypotheses are now tested against the results of field experiments, mechanistic theory and quantitative field observations. Anything less than the concordance of all three lines of evidence leads to the modification or rejection of hypotheses. Given that this topic became a principal focus of scientific inquiry only about seven years ago, it is not surprising that it remains contentious. Indeed, the greatest surprise may be the rapidity, breadth and depth of work that already has occurred, and the generalities that are emerging from it. Five papers in this issue summarize this work. Purvis and Hector (pages 212–219), McCann (pages 228–233), and Chapin and collaborators (pages 234–242) review and synthesize recent experimental, theoretical and observational studies that have demonstrated links between biodiversity and the stability, productivity and nutrient dynamics of ecosystems. Gaston (pages 220–227) summarizes global patterns of biodiversity and some possible explanations for these patterns. Margules and Pressey (pages 243–253) discuss strategies for the preservation of biodiversity. The effects of biodiversity on ecosystems In broad summary, these reviews show that, on average, greater diversity leads to greater productivity in plant communities, greater nutrient retention in ecosystems and greater ecosystem stability. For instance, grassland field experiments both in North America (Fig. 1)5,6 and across eight different European sites, ranging from Greece in the south and east to Portugal and Ireland in the west and Sweden in the north7 , have shown that each halving of the number of plant species within a plot leads to a 10–20% loss of productivity. An average plot containing one plant species is less than half as productive as an average plot containing 24–32 species5–7. Lower plant diversity also leads to greater rates of loss of limiting soil nutrients through leaching, which ultimately should decrease soil fertility, further lowering plant productivity. Both laboratory and field studies have shown that ecosystem processes are more variable (less stable or reliable) at lower diversity (see review by McCann, pages 228–233, and refs 8–10). The greater stability of more diverse ecosystems seems to result from three processes11–14. The first is comparable to the economic process that causes a more diverse investment portfolio to be less volatile. Because species, like corporations, differ from each other, they tend to respond somewhat independently to environmental variability. The more species that such variability is averaged across, the less variable is their total11. Second, species within a given trophic level often compete with each other, which causes their abundances to negatively covary. When one species declines, another is freed from competition and increases. This negative covariance reduces the variability of the community as a whole13,14. Finally, measures of temporal stability compute variability relative to mean abundance, such as by using the ratio of community abundance to its temporal standard deviation. The tendency for community Causes, consequences and ethics of biodiversity David Tilman Department of Ecology, Evolution and Behavior, University of Minnesota, St Paul, Minnesota 55108, USA (e-mail: tilman@lter.umn.edu) The existence of so great a diversity of species on Earth remains a mystery, the solution to which may also explain why and how biodiversity influences the functioning of ecosystems. The answer may lie in quantifying the trade-offs that organisms face in dealing with the constraints of their environment. Societal responses to the loss of biodiversity also involve trade-offs, and the elaboration of these will be essential in developing wiser environmental ethics and policy. CONSERVATION INTERNATIONAL © 2000 Macmillan Magazines Ltd
insight overview Figure 1 Biodiversity experiments,such as this one in Minnesota or the other experiments reviewed by Chapin et al.(pages 234-242)and by Purvis and Hector (pages 212-219),have shown that a greater number of plant species leads to greater community productivity.In the experiment shown,245 plots,each 9 m x 9 m,were assigned randomly to have from 1 to 16 prairie plant species,with the species composition of each plot being separately chosen at random. Species composition and plant diversity were both strong determinants of ecosystem functioning. abundance to increase as diversity increases thus causes this ratio, local scales within which individuals of one species interact which is a measure of stability,to increase as diversity increases'4. with individuals of other species.It is from such interactions among In total,biodiversity,which ten years ago was considered unim- individuals of different species that diversity is expected to impact portant by most ecosystem ecologists,has now been shown to impact ecosystem processes. significantly upon many aspects ofecosystem functioning.Diversity What are these mechanisms ofcoexistence?At present there are an must nowbe added to the list offactors-including species composi- abundance of alternative hypotheses but no clear demonstrations of tion,disturbance regime,soil type and climate-that influence the actual processes that maintain the diversity of species-rich ecosystem functioning.The recent rediscovery of the importance of ecosystems.In a general sense,coexistence requires the existence of biodiversity highlights an under-appreciated truth-although soci- evolutionarily persistent interspecific trade-offs in the abilities of ety is dependent on natural and managed ecosystems for goods and species to deal with the factors that constrain their fitness and abun- services that are essential for human survival,we know all too little dance.However,there are many potential constraints and trade-offs. about how ecosystems work. Species may coexist because of interspecific trade-offs (1)between Two sets ofunanswered scientific questions come to the forefront. their competitive abilities and their dispersal abilities;(2)between First,why is the world so diverse;that is,what forcesand processes led their competitive abilities and their susceptibility to disease, to the evolution and persistence of so many species?This is not mere- herbivory or predation;(3)between their abilities to live off ly an academic question.The processes that allow interacting species average conditions and their abilities to exploit resource pulses;or to coexist in an ecosystem simultaneously influence the productivity, (4)between their abilities to compete for alternative resources in a nutrient dynamics and stability of that ecosystem.Second,what are heterogeneous landscape-18 the mechanisms by which the loss of diversity impacts the function- The effects on ecosystem functioning of many such mechanisms ing of ecosystems,how general are these mechanisms,and how of coexistence have yet to be determined theoretically.However,it is important is biodiversity relative to other factors that influence already clear that the underlying mechanisms of coexistence can ecosystem functioning?In addition,the realization that human greatly influence how diversity affects ecosystem processes9.20 actions are harming,perhaps irreversibly,the ecosystems upon Consider,for instance,plant species that coexist in a spatially hetero- which humans depend raises a third,philosophical question:what geneous habitat because of differences in both the soil pH and the should be the role of scientists and science in the development of temperature(which varies seasonally)at which each grows optimally ethics and policy? (Fig.2a).Such niche differentiationcauses the predicted productiv ity ofplant communities to be an increasing function ofplant diversi- Coexistence and ecosystem functioning ty (Fig.2b).Moreover,the pattern of this increase is such that there Both our understanding of the effects of biodiversity on ecosystem are some species combinations at a given level of diversity that are processes,and the effectiveness of alternative strategies for the more productive than any possible combination of fewer species preservation of biodiversity,are limited by our knowledge of the (Fig.2b).The greater productivity of higher diversity communities mechanisms that maintain diversity.The mechanisms most relevant occurs because,in such heterogeneous habitats,each species is a to ecosystem functioning are those that maintain diversity on the superior performer in only a portion of sites.Clearly,the magnitude NATURE|VOL 40511 MAY 2000www.nature.com ☆©20o0 Macmillan Magazines Ltd 209
abundance to increase as diversity increases thus causes this ratio, which is a measure of stability, to increase as diversity increases14. In total, biodiversity, which ten years ago was considered unimportant by most ecosystem ecologists, has now been shown to impact significantly upon many aspects of ecosystem functioning. Diversity must now be added to the list of factors — including species composition, disturbance regime, soil type and climate — that influence ecosystem functioning. The recent rediscovery of the importance of biodiversity highlights an under-appreciated truth — although society is dependent on natural and managed ecosystems for goods and services that are essential for human survival, we know all too little about how ecosystems work. Two sets of unanswered scientific questions come to the forefront. First, why is the world so diverse; that is, what forces and processes led to the evolution and persistence of so many species? This is not merely an academic question. The processes that allow interacting species to coexist in an ecosystem simultaneously influence the productivity, nutrient dynamics and stability of that ecosystem. Second, what are the mechanisms by which the loss of diversity impacts the functioning of ecosystems, how general are these mechanisms, and how important is biodiversity relative to other factors that influence ecosystem functioning? In addition, the realization that human actions are harming, perhaps irreversibly, the ecosystems upon which humans depend raises a third, philosophical question: what should be the role of scientists and science in the development of ethics and policy? Coexistence and ecosystem functioning Both our understanding of the effects of biodiversity on ecosystem processes, and the effectiveness of alternative strategies for the preservation of biodiversity, are limited by our knowledge of the mechanisms that maintain diversity. The mechanisms most relevant to ecosystem functioning are those that maintain diversity on the local scales within which individuals of one species interact with individuals of other species. It is from such interactions among individuals of different species that diversity is expected to impact ecosystem processes. What are these mechanisms of coexistence? At present there are an abundance of alternative hypotheses but no clear demonstrations of the actual processes that maintain the diversity of species-rich ecosystems. In a general sense, coexistence requires the existence of evolutionarily persistent interspecific trade-offs in the abilities of species to deal with the factors that constrain their fitness and abundance. However, there are many potential constraints and trade-offs. Species may coexist because of interspecific trade-offs (1) between their competitive abilities and their dispersal abilities; (2) between their competitive abilities and their susceptibility to disease, herbivory or predation; (3) between their abilities to live off average conditions and their abilities to exploit resource pulses; or (4) between their abilities to compete for alternative resources in a heterogeneous landscape15–18. The effects on ecosystem functioning of many such mechanisms of coexistence have yet to be determined theoretically. However, it is already clear that the underlying mechanisms of coexistence can greatly influence how diversity affects ecosystem processes19,20. Consider, for instance, plant species that coexist in a spatially heterogeneous habitat because of differences in both the soil pH and the temperature (which varies seasonally) at which each grows optimally (Fig. 2a). Such niche differentiation20 causes the predicted productivity of plant communities to be an increasing function of plant diversity (Fig. 2b). Moreover, the pattern of this increase is such that there are some species combinations at a given level of diversity that are more productive than any possible combination of fewer species (Fig. 2b). The greater productivity of higher diversity communities occurs because, in such heterogeneous habitats, each species is a superior performer in only a portion of sites. Clearly, the magnitude insight overview NATURE | VOL 405 | 11 MAY 2000 | www.nature.com 209 Figure 1 Biodiversity experiments, such as this one in Minnesota6 or the other experiments reviewed by Chapin et al. (pages 234–242) and by Purvis and Hector (pages 212–219), have shown that a greater number of plant species leads to greater community productivity. In the experiment shown, 245 plots, each 9 m 2 9 m, were assigned randomly to have from 1 to 16 prairie plant species, with the species composition of each plot being separately chosen at random6 . Species composition and plant diversity were both strong determinants of ecosystem functioning. © 2000 Macmillan Magazines Ltd
insight overview for the types of communities studied so far.Expressed another way, much of nature may have a free-market economy,structured by the efficiencies of open competition among species,rather than an economy structured by pre-emption and other monopolistic practices.Such speculations may be premature,especially because complex systems containing many trophic levels (for example, plants,decomposers,herbivores and predators)are,as yet,poorly 工a= studied.However,they highlight the conceptual links between economics and ecology-disciplinary links that must be strength- ened ifecological knowledge is to be used to help create a sustainable human economy. Societal trade-offs and ethics The progress made during the past seven years in understanding Temperature for optimal growth these issues underscores the potential implications ofhabitat simpli- fication and loss of diversity for the ecosystem goods and services? upon which humans depend.The species presently inhabiting Earth b 1.0 are the result of over 3 billion years of natural selection that likely favoured efficiency,productivity and specialization.These organ- isms are the catalysts that capture and transform energy and materi- als,producing,among other things,food,fuel,fibre and medicines. These species recycle wastes,create pure drinking water,drive global biogeochemical cycles that created and maintain an aerobic atmos- phere,regulate global climate through effects on greenhouse gases and local climate through effects on evapotranspiration,generate soil fertility,and provide other ecosystem goods and services2.In 2 addition,the Earth's biodiversity is the source of all crops and all pollinators of crops,of all livestock,and of many pharmaceuticals 0.0 and pesticides.Just three crops-corn,rice and wheat-provide 10 1520 15 30 about 60%of the human food supply.The viability of these crops Plant species richness depends on the maintenance of high genetic diversity,which can allow,among other things,development of strains that are resistant to emerging and evolving diseases and pests".In the long term,food Figure 2 Niche differentiation and productivity.a.A simple model-the 'snowballs on stability will require development of new crops from what are now the bam'model-of niche differentiation and coexistence2.The range of conditions wild plants,because disease or pesticide-resistant pests will cause the in which each species can exist is shown with a circle,the position of which is defined loss of current crops,just as disease caused the loss of chestnut,elm by its centre.By randomly choosing locations for various numbers of circles(species),it and other tree species from North American forests. is possible to calculate the effect of diversity on the'coverage'of the heterogeneous Humans,like all other organisms,experience trade-offs.The loss habitat.The amount of such coverage is proportional to community biomass.b,Results of biodiversity will diminish the capacity of ecosystems to provide of simulations (triangles)and of an analytical solution (solid curve)to the effects of society with a stable and sustainable supply of essential goods and diversityoncommnity productiviy for the snoballs on the bam mode services,but many ofthe very actions that harm biodiversity simulta- neously provide valuable societal benefits.There exists a trade-off defining the net benefits that society receives from the various ways that humans could use and impact nature,but,as yet,this is poorly of this effect increases as heterogeneity or diversity increase. defined.This trade-offitselfis likely to shift through time in response Increased diversity leads,on average,to increased 'coverage'of the to the remaining amounts and states of various resources,including habitat conditions,that is,to increased efficiency of resource capture biodiversity.The amounts and states of biotic resources have and use,because diversity increases the chance that the species that changed rapidly during the past century,as global population are better able to handle particular conditions are present.Assuming increased 3.7-fold and per capita gross domestic product,a reason- that species are chosen at random,diversity is a simple way to mea- able proxy for consumption,increased 4.6-fold2.It seems likely that sure the range and coverage ofspecies traits in a community. environmental policy that is optimal from a societal perspective In contrast,consider a case in which interspecific interactions are would be markedly different now from that of 250 years ago.Howev- based on direct antagonismand not onefficiency ofresource use.For er,we still use environmental and land-use ethics,codified inlaw,that a simple formulation,let there be an interspecific trade-off between were articulated during the era when the human population,at competitive ability and productivity.Species that achieve greater one-tenth its present size,tamed wilderness with axe and ox. productivity in monoculture would be poorer competitors,attaining Science has much to contribute to dialogues on policy and ethics. lower abundances when competing.Because greater diversity Although academic institutions seem to value such contributions increases thechance that a competitively superior butlower-yielding less than contributions to peer-reviewed journals,this is short- species would be present,productivity would,on average,be a sighted.Ultimately,society invests in science because advances in decreasing function of diversity.This is a simple variant on the scientific knowledge benefit society.The ethics of science cannot sampling-effect model22,here modified to have better competitors eschew involvement in public discourse.Science must contribute,in beless,rather than more,productive. an open,unbiased manner,to relevant issues. What,then,is implied by available experimental results,which Because ofthe emergence ofhuman domination ofglobal ecosys- have shown that productivity is an increasing function of plant tems,society faces new,tough trade-offs.These include trade-offs species diversity?They indicate that coexistence through niche dif- between the current benefits and the future costs of environmental ferentiation and related processes may be more prevalent in nature damage,and between benefits to a few and costs to many.Research is than coexistence through antagonism and related processes,at least needed to quantify these trade-offs,and the work done so far on 210 ☆©20o0 Macmillan Magazines Ltd NATURE VOL 40511 MAY 2000 www.nature.com
of this effect increases as heterogeneity or diversity increase. Increased diversity leads, on average, to increased ‘coverage’ of the habitat conditions, that is, to increased efficiency of resource capture and use, because diversity increases the chance that the species that are better able to handle particular conditions are present. Assuming that species are chosen at random, diversity is a simple way to measure the range and coverage of species traits in a community. In contrast, consider a case in which interspecific interactions are based on direct antagonism and not on efficiency of resource use. For a simple formulation, let there be an interspecific trade-off between competitive ability and productivity. Species that achieve greater productivity in monoculture would be poorer competitors, attaining lower abundances when competing. Because greater diversity increases the chance that a competitively superior but lower-yielding species would be present, productivity would, on average, be a decreasing function of diversity. This is a simple variant on the sampling-effect model20–22, here modified to have better competitors be less, rather than more, productive. What, then, is implied by available experimental results, which have shown that productivity is an increasing function of plant species diversity? They indicate that coexistence through niche differentiation and related processes may be more prevalent in nature than coexistence through antagonism and related processes, at least for the types of communities studied so far. Expressed another way, much of nature may have a free-market economy, structured by the efficiencies of open competition among species, rather than an economy structured by pre-emption and other monopolistic practices. Such speculations may be premature, especially because complex systems containing many trophic levels (for example, plants, decomposers, herbivores and predators) are, as yet, poorly studied. However, they highlight the conceptual links between economics and ecology — disciplinary links that must be strengthened if ecological knowledge is to be used to help create a sustainable human economy. Societal trade-offs and ethics The progress made during the past seven years in understanding these issues underscores the potential implications of habitat simplification and loss of diversity for the ecosystem goods and services23 upon which humans depend. The species presently inhabiting Earth are the result of over 3 billion years of natural selection that likely favoured efficiency, productivity and specialization. These organisms are the catalysts that capture and transform energy and materials, producing, among other things, food, fuel, fibre and medicines. These species recycle wastes, create pure drinking water, drive global biogeochemical cycles that created and maintain an aerobic atmosphere, regulate global climate through effects on greenhouse gases and local climate through effects on evapotranspiration, generate soil fertility, and provide other ecosystem goods and services23. In addition, the Earth’s biodiversity is the source of all crops and all pollinators of crops, of all livestock, and of many pharmaceuticals and pesticides. Just three crops — corn, rice and wheat — provide about 60% of the human food supply. The viability of these crops depends on the maintenance of high genetic diversity24, which can allow, among other things, development of strains that are resistant to emerging and evolving diseases and pests25. In the long term, food stability will require development of new crops from what are now wild plants, because disease or pesticide-resistant pests will cause the loss of current crops, just as disease caused the loss of chestnut, elm and other tree species from North American forests. Humans, like all other organisms, experience trade-offs. The loss of biodiversity will diminish the capacity of ecosystems to provide society with a stable and sustainable supply of essential goods and services, but many of the very actions that harm biodiversity simultaneously provide valuable societal benefits. There exists a trade-off defining the net benefits that society receives from the various ways that humans could use and impact nature, but, as yet, this is poorly defined. This trade-off itself is likely to shift through time in response to the remaining amounts and states of various resources, including biodiversity. The amounts and states of biotic resources have changed rapidly during the past century, as global population increased 3.7-fold and per capita gross domestic product, a reasonable proxy for consumption, increased 4.6-fold26. It seems likely that environmental policy that is optimal from a societal perspective would be markedly different now from that of 250 years ago. However, we still use environmental and land-use ethics, codified in law, that were articulated during the era when the human population, at one-tenth its present size, tamed wilderness with axe and ox. Science has much to contribute to dialogues on policy and ethics. Although academic institutions seem to value such contributions less than contributions to peer-reviewed journals, this is shortsighted. Ultimately, society invests in science because advances in scientific knowledge benefit society. The ethics of science cannot eschew involvement in public discourse. Science must contribute, in an open, unbiased manner, to relevant issues. Because of the emergence of human domination of global ecosystems, society faces new, tough trade-offs. These include trade-offs between the current benefits and the future costs of environmental damage, and between benefits to a few and costs to many. Research is needed to quantify these trade-offs, and the work done so far on insight overview 210 NATURE | VOL 405 | 11 MAY 2000 | www.nature.com Temperature for optimal growth Plant species richness 0 5 10 0.0 0.2 0.4 0.6 0.8 1.0 15 20 15 30 Soil pH Community biomass a b Figure 2 Niche differentiation and productivity. a, A simple model — the ‘snowballs on the barn’ model — of niche differentiation and coexistence20. The range of conditions in which each species can exist is shown with a circle, the position of which is defined by its centre. By randomly choosing locations for various numbers of circles (species), it is possible to calculate the effect of diversity on the ‘coverage’ of the heterogeneous habitat. The amount of such coverage is proportional to community biomass. b, Results of simulations (triangles) and of an analytical solution (solid curve) to the effects of diversity on community productivity for the snowballs on the barn model20. © 2000 Macmillan Magazines Ltd
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