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 209abundance 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 unim￾portant 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 composi￾tion, disturbance regime, soil type and climate — that influence ecosystem functioning. The recent rediscovery of the importance of biodiversity highlights an under-appreciated truth — although soci￾ety 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 mere￾ly 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 function￾ing 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 abun￾dance. 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 hetero￾geneous 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 productiv￾ity of plant communities to be an increasing function of plant diversi￾ty (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