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REVIEW: Biodiversity and Functioning: Current Knowledge and Future Challenges M. Loreau,1* S. Naeem,2 P. Inchausti,1 J. Bengtsson,3 J. P. Grime,4 A. Hector,5 D. U. Hooper,6 M. A. Huston,7 D. Raffaelli,8 B. Schmid,9 D. Tilman,10 D. A. Wardle4

in The ecological consequences of have aroused considerable interest (20–23). Because , as primary produc- and controversy during the past decade. Major advances have been made in describing ers, represent the basal component of most the relationship between diversity and ecosystem processes, in identifying ecosystems, they represented the logical functionally important species, and in revealing underlying mechanisms. There is, place to begin detailed studies. Several, al- however, uncertainty as to how results obtained in recent experiments scale up to though not all, experiments using randomly and regional levels and generalize across ecosystem types and processes. assembled communities found that primary Larger numbers of species are probably needed to reduce temporal variability in production exhibits a positive relationship ecosystem processes in changing environments. A major future challenge is to with species and functional-group di- determine how biodiversity dynamics, ecosystem processes, and abiotic factors versity (Fig. 1). interact. These results attracted a great deal of interest, not only because they were novel, but also because they seemed counter to pat- he relationship between biodiversity the current debate in which dis- terns often observed in , where the and ecosystem functioning has emerged agree about the relative importance of func- most productive ecosystems are typically T as a central issue in ecological and tional substitutions and declining species characterized by low (26, environmental sciences during the last de- richness as determinants of changes in eco- 27). The controversy over the interpretation cade. Increasing domination of ecosystems system functioning. Comparative studies of these results started with the realization by is steadily transforming them into have begun to reveal the extent to which that they can be generated by different mech- depauperate systems (1, 2). Because ecosys- functional substitutions alter ecosystem anisms. The mechanisms discussed so far tems collectively determine the biogeochemi- properties such as , decompo- may be grouped into two main classes. First cal processes that regulate the system, sition rates, cycling, and resistance are local deterministic processes, such as the potential ecological consequences of and resilience to perturbations (17, 18). On and facilitation, which biodiversity loss have aroused considerable the other hand, a new wave of experimental increase the performance of communities interest (3–9). studies has manipulated by above that expected from the performance of Recent experimental and theoretical using synthesized model ecosystems in individual species grown alone. We will sub- work in this area has also led to animated both terrestrial and aquatic environments sume them here under the term “complemen- debates and controversies (10–14). (19–25). Both approaches suggest that a tarity” for convenience’s sake. Second are impacts on the environment from local to large pool of species is required to sustain local and regional processes in- global scales cause not only a general de- the assembly and functioning of ecosys- volved in assembly, which are in diversity, but also predictable func- tems in subject to increasingly mimicked in experiments by random sam- tional shifts as sets of species with partic- intensive use. It is not yet clear, how- pling from a species pool. Random sampling ular traits are replaced by other sets with ever, whether this dependence on diversity coupled with local of highly pro- different traits (15, 16). This has resulted in arises from the need for of a ductive species can also lead to increased few key species from within the regional average primary production with increasing 1Laboratoire d’Ecologie, UMR 7625, Ecole Normale species pool or is due to the need for a rich diversity, because plots that include many Supe´rieure, 46 rue d’Ulm, F–75230 Paris Cedex 05, assortment of complementary species with- species have a higher probability of contain- France. 2Department of , University of Wash- ington, 24 Kincaid Hall, Box 351800, Seattle, WA in particular ecosystems. ing highly productive species (10, 11, 28). 98195–1800, USA. 3Department of Ecology and Crop In this article, we seek to set a common Two issues are involved in this controversy: Production , Swedish University of Agricultural framework to understand these issues, to Are stochastic community assembly process- Sciences, Box 7043, SE–750 07 Uppsala, Sweden. move beyond past differences of opinion, and es relevant? And what is the relative impor- 4Department of and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, UK. 5NERC Centre for to define new perspectives, after a recent tance of the two classes of mechanisms? , Imperial College at Silwood Park, conference held in Paris. We do not attempt There are diverging views on the rele- Ascot, Berks, SL5 7PY, UK. 6Department of Biology, to comprehensively review these issues, ele- vance of the sampling component of biodi- Western Washington University, 516 High Street, ments of which can be found elsewhere (3– versity effects. As sampling processes were 7 Bellingham, WA 98225–9160, USA. Environmental 9). Rather, we focus on major regions where not an explicit part of the initial hypotheses, Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831–6400, USA. 8Environment De- recent advances have been made. they have been viewed by some as “hidden partment, University of York, York, YO10 5DD, UK. treatments” (10), whereas others have viewed 9Institut fu¨r Umweltwissenschaften, Universita¨t Zu¨- Experimentally Altered Diversity them as the simplest possible mechanism rich, Winterthurerstrasse 190, CH–8057 Zu¨rich, Swit- Although the first study that experimentally linking diversity and ecosystem functioning zerland. 10Department of Ecology, and Be- havior, , St. Paul, MN 55108, manipulated diversity did so across several (28). This debate should be resolved through USA. trophic levels (19), later studies focused increasing knowledge about the patterns and *To whom correspondence should be addressed. E- mainly on effects of plant taxonomic diversi- processes of biodiversity loss in nature, mail: [email protected] ty and plant functional-group diversity on which are still poorly known overall. If dom-

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Fig. 1. Responses of total (A) or aboveground (B and C) plant (in ported elsewhere (23). Filled squares and line 1, ; filled circles grams per meter squared) to experimental manipulations of plant species and line 2, ; filled triangles and line 3, Switzerland; solid dia- richness (A and B) or functional-group richness (C) in in monds and line 4, Greece; open squares and line 5, Ireland; open circles Minnesota (A) (31), across (B) (23), and in California (C) (22). and line 6, Sweden; open diamonds and line 7, Sheffield (UK); open Points in (A) and (B) are data for individual plots. In (B) different diamonds and line 8, Silwood Park (UK). Symbols in (C) correspond to regression slopes are shown for the eight sites to focus on between- functional groups and their combinations: B, bare ground; E, early-season location differences rather then the general log-linear relationship re- annuals; L, late-season annuals; P, perennial bunchgrasses; N, N fixers. inant species control ecosystem processes Fig. 2. Hypothesized mecha- and mostly go extinct, the vagar- nisms involved in biodiversity ies of community assembly or disassembly experiments using synthetic may have little relevance. But environmental communities. Sampling effects are involved in community as- changes and landscape fragmentation could sembly, such that communi- prevent recruitment of appropriate dominants ties that have more species (29). Also, change could lead to grad- have a greater probability of ual losses of species as abiotic conditions containing a higher pheno- begin to exceed species’ tolerance limits. typic trait diversity. Pheno- Such losses could be random with respect to typic diversity then maps onto ecosystem processes species effects on any given ecosystem pro- through two main mecha- cess, leading to patterns of process response nisms: dominance of species to changes in diversity similar to those ob- with particular traits, and served in randomly assembled communities. complementarity among spe- It should be emphasized that recent experi- cies with different traits. In- ments were not intended to reproduce any termediate scenarios involve complementarity among par- particular sequence of species loss; they re- ticular species or functional flect potential patterns, unaffected by corre- groups or, equivalently, dom- lations between diversity loss and composi- inance of particular subsets tional changes, rather than actual predictions of complementary species. of functional consequences of biodiversity loss under specific scenarios. Recent experiments were also not de- signed to investigate detailed underlying mechanisms. Assessing the relative impor- ent traits are two ways by which this pheno- is a minimum subset of complementary spe- tance of complementarity and sampling ef- typic diversity maps onto ecosystem process- cies that is sufficient to explain diversity fects has been done so far indirectly, by using es (6). These two mechanisms, however, may effects will often be difficult because it would comparisons between the performances of be viewed as two poles on a continuum from ideally require testing, with replication, the mixtures and (14, 23, 30, 31). pure dominance to pure complementarity. In- performance of all species combinations at all Furthermore, it is becoming clear that termediate scenarios involve complementar- diversity levels. Re-analysis of data from pre- complementarity and sampling are not mutu- ity among particular sets of species or func- viously published experiments suggests sig- ally exclusive mechanisms as previously tional groups, or dominance of particular sub- nificant effects of species richness on plant thought. Communities with more species sets of complementary species (Fig. 2). Any biomass even after controlling for the strong have a greater probability of containing a bias in community assembly that leads to effects of certain species, such as legumes higher phenotypic trait diversity. Dominance correlations between diversity and communi- (30, 31). Although these new results present- that is brought about by ecological “selec- ty composition may involve both dominance ed at the Paris conference will need to be tion” of species with particular traits and and complementarity. critically evaluated, they suggest that comple- complementarity among species with differ- Rigorously testing the hypothesis that there mentarity does occur among at least several

www.sciencemag.org SCIENCE VOL 294 26 OCTOBER 2001 805 S CIENCE’ S C OMPASS species belonging to different functional on comparisons with monocultures, to sepa- cies responses to intrinsic or extrinsic en- groups in these experiments. No clear evi- rate dominance and complementarity effects. vironmental fluctuations (34–40). What re- dence, however, has been provided so far for There is also a great need for other approach- mains unclear, however, is whether this complementarity among a large number of es based on “natural” ecosystems, such as stabilizing effect saturates at low or high species, although complementarity among removal experiments (32) and comparative diversity, which depends on model condi- rare species would be difficult to detect. With approaches that control for variation of fac- tions (5, 9, 34, 38, 39). our knowledge now, we cannot reject the tors other than diversity (33). Whereas experimental work has played hypothesis that a few dominant species suf- a leading role regarding short-term effects fice to provide the functional diversity that is Biodiversity as Insurance of biodiversity on ecosystem functioning, necessary to explain the level of primary Even when high diversity is not critical for theory has been prominent in the diversity- production observed in grassland ecosystems maintaining ecosystem processes under con- stability debate, both historically and re- at the small spatial and temporal scales con- stant or benign environmental conditions, it cently. A number of empirical and experi- sidered in recent experiments. might nevertheless be important for maintain- mental studies have shown decreased vari- Future experiments should strive to over- ing them under changing conditions. The in- ability of ecosystem processes as diversity come the limitations that led to the recent surance hypothesis (34) and related hypoth- increases (Fig. 3). These studies, however, controversy. Greater attention should be paid eses (35–40) propose that biodiversity pro- have been based either on diversity gradi- to what individual species do in these exper- vides an “insurance” or a buffer, against en- ents established naturally or after other iments. One option for assembly experiments vironmental fluctuations, because different treatments (36, 38), or on microcosm ex- is to have carefully balanced designs to allow species respond differently to these fluctua- periments in which variability among rep- contrasts between plots with and without par- tions, leading to more predictable aggregate licates was also considered (24, 25), which ticular species or subsets of species. Another community or ecosystem properties. In this does not fully preclude alternative interpre- option is to include manipulations of even- hypothesis, species that are functionally re- tations (10, 13). Experiments in which both ness within a level of species richness, which dundant for an ecosystem process at a given diversity and environmental fluctuations could provide an alternative to methods based are no longer redundant through time. are controlled are now needed to perform In a way, this is the old stability-versus- rigorous tests of the insurance hypothesis. complexity debate resurfacing in a new Theory too should evolve to provide bet- form (7). Several problems, however, have ter guidance for experiments. Most of the confused this historical controversy: (i) The classical equilibrium approaches may be in- general concept of “stability” actually cov- adequate to understand stability properties ers a wide array of different properties (41); such as resilience and resistance at the eco- (ii) the relationship between these proper- system level. New approaches should be de- ties and diversity may change across eco- veloped that take into account the dynamics logical levels of organization such that of diversity and the potential for large variability at the population level may through , evolutionary not imply large variability of ecosystem changes, and species replacement. processes (38, 41); and (iii) stability has been approached mainly within a determin- From Experiments to Patterns istic, equilibrium theoretical framework. The relationship between productivity and Recent theoretical work has attempted to diversity has long been studied from an remove these obstacles and has provided angle different from that in recent experi- support for the insurance hypothesis. As mental studies. It is often, although not diversity increases, the variability of indi- always, described by a hump-shaped curve, vidual may increase as a result in which diversity is considered a of the destabilizing influence of strong spe- of productivity (Fig. 4A) (26, 27). These cies interactions internal to the system, but curves have typically been obtained by us- the variability of aggregate ecosystem ing correlations across different sites or properties often decreases because of the nutrient addition treatments. Some compar- stabilizing influence of asynchronous spe- ative approaches have also suggested neg- Fig. 3. Observed decreases in variability of eco- system processes as species richness increases. Interpretation of these patterns, however, is Fig. 4. Hypothesized relation- AB complicated by the correlation of additional between (A) diversity- Favorable factors with species richness. (A) Adjusted co- productivity patterns driven and climate

efficient of temporal variation of annual total by environmental conditions Diversity plant biomass (in grams per meter squared) across sites, and (B) the local over 11 years for plots differing in number of effect of species diversity on Productivity species in experimental and natural grasslands productivity. (A) Compara- in Minnesota (38). The correlation of variations tive data often indicate a uni- in soil nitrogen with species richness in these modal relationship between plots precludes the interpretation of increased diversity and productivity stability as a pure diversity effect (10), al- driven by changes in environ- Unfavorable though the diversity effect remained significant mental conditions. (B) Exper- soil and climate even after controlling for potentially confound- imental variation in species Productivity Diversity ing variables (36). (B) Standard deviation of richness under a specific set Soil and climate effects CO2 flux (in microliters per 18 hours) from of environmental conditions microbial microcosms (24). In these data, tem- produces a pattern of decreasing between-replicate variance and increasing mean response with poral variability in response to diversity is con- increasing diversity, as indicated by the thin, curved regression lines through the scatter of response founded with between-replicate variability. values (shaded areas).

806 26 OCTOBER 2001 VOL 294 SCIENCE www.sciencemag.org S CIENCE’ S C OMPASS ative relationships between plant species feedbacks among biodiversity changes, eco- studied despite their importance in the mainte- evenness and rates of various ecosystem system functioning, and environmental fac- nance of ecosystem processes, as shown by one processes (42). The differences between tors. Relationships between local, landscape, experiment on mycorrhizal fungal diversity these large-scale, observational approaches and regional scales also require particular (54). Although there is a clear case for incor- and the small-scale, experimental ap- attention. porating multiple trophic levels into studies of proaches have also generated debate (12). biodiversity-ecosystem functioning relation- How can these results be reconciled? Generalizing Across Ecosystems ships, logistical constraints, such as the high The two approaches examine different caus- Most of the recent experiments that found mobility of and and the al relationships under different sets of condi- significant effects of species diversity have difficulty of taxonomic identification of decom- tions. The classical approach attempts to iden- concerned effects of plant diversity on prima- posers, partly explains why so few studies have tify the causes of spatial variation in diversity ry production and nutrient retention in tem- done so as yet. Of particular importance are the across environmental gradients. Variation in di- perate grasslands, both of which are under vast areas of biodiversity that involve small versity is often correlated with productivity, but direct plant control. These and other experi- such as , , , also with many other factors that influence pro- ments have often failed to detect significant , and microarthropods, which drive the ductivity, such as , climate, distur- effects on below-ground pro- bulk of ecosystem processes. For example, bance regime, or herbivory. The recent experi- cesses (19, 45), perhaps because these pro- chemical transformations in the mental approach examines whether diversity cesses are under microbial control. This ques- are predominantly driven by prokaryotic - alone has a local effect on productivity within tions whether results obtained on primary isms, as is decomposition of organic . each site, when all these other factors are held production in grasslands can be generalized Modern molecular tools are beginning to make constant. The two approaches can be reconciled to other processes and ecosystems. possible the integration of microbial diversity by considering that spatial patterns reveal cor- Plants can affect soil processes either di- into studies of ecosystem processes. relations between diversity and productivity rectly, by stimulating or inhibiting decompo- There is also a need to extend our current driven by environmental factors, whereas sition rates, or indirectly, through increased knowledge to ecosystem types other than small-scale experiments reveal the effects of primary production, by enhancing decompo- temperate grasslands, such as , freshwa- species properties and diversity on productivity sition fluxes. Although some experiments ter, and marine (55) ecosystems. Top-down that are detected after the effects of other envi- found positive effects of plant diversity on control is often thought to be more common ronmental factors have been removed (Fig. 4B) soil microbial processes (46), experiments in freshwater than in terrestrial ecosystems (6). using litter addition, cotton strips, or litter (56); significant differences might then be Whether biodiversity loss will affect mixing often showed variable and weak ef- expected between ecosystem types just as large-scale patterns of productivity hinges on fects of plant diversity on decomposition between trophic levels. Generally speaking, the shape and steepness of the local depen- rates (45). Current evidence suggests that differences in coexistence mechanisms may dence of productivity on diversity. Generally properties of individual plant species are lead to differences in biodiversity effects on speaking, the relative effects of individual more important than plant diversity in gov- ecosystem functioning. For example, in dis- species and species richness may be expected erning soil process rates. This conclusion is turbance-driven systems, the colonization to be greatest at small-to-intermediate spatial echoed by theoretical work predicting that ability and growth rate of individual species, scales, but these biological factors should be plant chemical quality diversity should de- rather than niche complementarity, might less important as predictors of ecosystem pro- crease or not affect long-term nutrient recy- drive ecosystem processes. cesses at regional scales, where environmen- cling efficiency and productivity (47). In tal heterogeneity is greater. Whereas diversi- contrast, increased primary production gener- Conclusions ty was manipulated as the independent vari- ated by higher plant diversity is expected to Significant advances in science occur when able in recent experiments, at large scales stimulate secondary productivity. More gen- observational, experimental, and theoreti- species diversity itself is a dynamical variable erally, diversity changes at one cal studies coincide. Recent work has done and adjusts to changes in environmental con- may lead to a variety of potential responses this to some extent for studies of the effects ditions. Abiotic factors then tend to be the for processes at higher trophic levels (48). of diversity on productivity and temporal main drivers of variations in ecosystem pro- Species diversity in trophic levels stability at local scales, although much ad- cesses across environmental gradients (43). can also have complex effects on production at ditional work is still needed, in particular to Diversity loss at regional scales and dis- these and lower levels. Complementarity and apply results to larger spatial scales. There persal limitations due to landscape fragmen- sampling effects should tend to improve re- is consensus that at least some minimum tation, however, will very likely feed back source exploitation just as in plants. This should number of species is essential for ecosys- and reduce the pool of potential colonists at lead to higher secondary productivity if bottom- tem functioning under constant conditions local scales and hence the potential for local up control prevails, as in plant- in- and that a larger number of species is prob- compositional adjustments to environmental teractions (47). Enhanced exploita- ably essential for maintaining the stability changes. Species-area relations imply that the tion, however, can lead to , and of ecosystem processes in changing envi- long-term maintenance of a given level of thus decreased productivity, if top-down con- ronments. Determining which species have diversity at local scales requires a much high- trol is important, as might be the case with a significant impact on which processes in er diversity at regional scales (44). One of the herbivores and predators. There have been few which ecosystems, however, remains an most potent effects of declining diversity experiments to test these hypotheses. Recent open empirical question. could be the decline in the rate at which microcosm experiments found significant ef- There are many reasons—including aes- appropriate potential dominants are recruited fects of bacterial diversity on bacterial and algal thetic, cultural, and economic—why we may during ecosystem assembly (29). biomasses (49) and of diversity of leaf-eating wish to conserve biodiversity. From a strictly To understand and predict changes in on decomposition rates (50), but others functional point of view, species matter so far biodiversity and ecosystem processes at large suggested that individual species and functional as their individual traits and interactions con- scales, therefore, we need to move beyond composition were the most important factors tribute to maintain the functioning and stabil- unidirectional causality approaches in which (51–53). The functional role of diversity in ity of ecosystems and biogeochemical cycles. diversity is either cause or effect, and address mutualistic interactions has also been poorly Although species richness is easier to mea-

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sure, a more predictive science might be 4. F. Schla¨pfer, B. Schmid, Ecol. Appl. 9, 893 (1999). 35. S. J. McNaughton, Am. Nat. 111, 515 (1977). achieved if appropriate functional classifica- 5. M. W. Schwartz et al., Oecologia 122, 297 (2000). 36. D. Tilman, Ecology 77, 350 (1996). tions were devised (26, 27, 57, 58). Specific 6. M. Loreau, Oikos 91, 3 (2000). 37. D. F. Doak et al., Am. Nat. 151, 264 (1998). 7. K. S. McCann, Nature 405, 228 (2000). 38. D. Tilman, Ecology 80, 1455 (1999). knowledge of functional types may be critical 8. F. S. Chapin III et al., Nature 405, 234 (2000). 39. C. L. Lehman, D. Tilman, Am. Nat. 156, 534 (2000). to predict ecosystem responses under differ- 9. K. L. Cottingham, B. L. Brown, J. T. Lennon, Ecol. Lett. 40. A. R. Ives, J. L. Klug, K. Gross, Ecol. Lett. 3, 399 (2000). ent global change scenarios, or where man- 4, 72 (2001). 41. S. L. Pimm, Nature 307, 321 (1984). 10. M. A. Huston, Oecologia 110, 449 (1997). agement priorities seek to manipulate species 42. D. A. Wardle, O. Zackrisson, G. Ho¨rnberg, C. Gallet, 11. L. W. Aarssen, D. Tilman, Oikos 80, 183 (1997). Science 277, 1296 (1997). composition directly, for example, in com- 12. D. Tilman et al., Science 278, 1865 (1997). 43. M. Loreau, Proc. Natl. Acad. Sci. U.S.A. 95, 5632 plex agroecosystems, , or ecosystem 13. D. A. Wardle, S. Naeem, S. Li, Nature 394, 30 (1998). (1998). 14. M. A. Huston et al., Science 289, 1255a (2000). 44. D. Tilman, Science 286, 1099 (1999). restoration with particular functional goals in 15. D. McCollin, L. Moore, T. Sparks, Biol. Conserv. 92, 45. D. A. Wardle, K. I. Bonner, K. S. Nicholson, Oikos 79, mind. 249 (2000). 247 (1997). 16. J. P. Grime et al., Science 289, 762 (2000). The traditional approach in community 46. A. Stephan, A. H. Meyer, B. Schmid, J. Ecol. 88, 988 17. C. W. MacGillivray et al., Funct. Ecol. 9, 640 (1995). ecology has considered species diversity as a (2000). 18. J. P. Grime et al., Oikos 79, 259 (1997). dependent variable controlled by abiotic con- 19. S. Naeem, L. J. Thompson, S. P. Lawler, J. H. Lawton, 47. M. Loreau, Proc. R. Soc. London Ser. B 268, 303 ditions and ecosystem-level constraints. The R. M. Woodfin, Nature 368, 734 (1994). (2001). traditional approach in 20. D. Tilman, D. Wedin, J. Knops, Nature 379, 718 48. N. M. Haddad, D. Tilman, J. Haarstad, M. Ritchie, (1996). J. M. H. Knops, Am. Nat. 158, 17 (2001). has primarily focused on dominant species as 21. D. Tilman et al., Science 277, 1300 (1997). 49. S. Naeem, D. R. Hahn, G. Schuurman, Nature 403, biotic controllers of ecosystem processes. Re- 22. D. U. Hooper, P. M. Vitousek, Science 277, 1302 762 (2000). cent approaches have broadened the perspec- (1997). 50. M. Jonsson, B. Malmqvist, Oikos 89, 519 (2000). tives of both subdisciplines by assessing the 23. A. Hector et al., Science 286, 1123 (1999). 51. J. Mikola, H. Seta¨la¨, Oikos 83, 180 (1998). 24. J. McGrady-Steed, P. M. Harris, P. J. Morin, Nature 52. J. Laakso, H. Seta¨la¨, Oikos 87, 57 (1999). role of biodiversity as a potential modulator 390, 162 (1997). 53. J. Norberg, Oecologia 122: 264 (2000). of processes. In reality, there are mutual in- 25. S. Naeem, S. Li, Nature 390, 507 (1997). 54. M. van der Heijden et al., Nature 396, 69 (1998). teractions among biodiversity changes, eco- 26. M. A. Huston, Biological Diversity (Cambridge Univ. 55. M. C. Emmerson, M. Solan, D. M. Paterson, D. Raf- system functioning, and abiotic factors. Inte- Press, Cambridge, 1994). faelli, Nature 411, 73 (2001). 27. J. P. Grime, Plant Strategies, Vegetation Processes and 56. D. R. Strong, Ecology 73, 747 (1992). grating these interactions into a single, uni- Ecosystem Properties ( Wiley, New York, 2nd ed., 57. S. Lavorel, S. McIntyre, J. Landsberg, T. D. A. Forbes, fied picture, both theoretically and experi- 2001). Ecol. Evol. 12, 474 (1997). mentally, and across ecosystem types and 28. D. Tilman, C. Lehman, K. Thompson, Proc. Natl. Acad. 58. F. D. Hulot, G. Lacroix, F. Lescher-Moutoue´, M. Loreau, Sci. U.S.A. 94, 1857 (1997). processes, is a major challenge which may Nature 405, 340 (2000). 29. J. P. Grime, J. Ecol. 86, 902 (1998). 59. The conference “Biodiversity and ecosystem func- help bring about a true synthesis of commu- 30. M. Loreau, A. Hector, Nature 412, 72. tioning: Synthesis and perspectives” was held from 6 nity and ecosystem ecology. 31. D. Tilman, in Ecology: Achievement and Challenge, to 9 December 2000 as a contribution to the Inter- M. C. Press, N. J. Huntly, S. A. Levin, Eds. (British national Geosphere- Programme–Global Ecological Society Symp. Vol. Ser. 41, Blackwell Sci- Change and Terrestrial Ecosystems (IGBP-GCTE) Fo- References and Notes ence, Oxford, 2001), pp. 183–207. cus 4 and Diversitas Core Programme Element 1. We 1. P. M. Vitousek, H. A. Mooney, J. Lubchenco, J. M. 32. D. A. Wardle et al., Ecol. Monogr. 69, 535 (1999). gratefully acknowledge financial support by the Eu- Melillo, Science 277, 494 (1997). 33. A. Y. Troumbis, D. Memtsas, Oecologia 125, 101 ropean Science Foundation, the Centre National de la 2. O. E. Sala et al., Science 287, 1770 (2000). (2000). Recherche Scientifique (France), and the U.S. Nation- 3. Biodiversity and Ecosystem Function, E.-D. Schulze, 34. S. Yachi, M. Loreau, Proc. Natl. Acad. Sci. U.S.A. 96, al Science Foundation. We thank all other conference H. A. Mooney, Eds. (Springer Verlag, Berlin, 1993). 1463 (1999). participants for stimulating discussions.

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