insight review articles protected areas).The latter is particularly important given that only tropics.Amat.133,240-256(1989). -15%of the total number of species estimated to be extant has been formally described taxonomically,that the distributions of most of Liversity.Seme164.947-949f1969】. these remain largely unknown (a high proportion are known from 6 Crane,P.R.Lidgard.S.Angi only a single locality),and that species whose distributions are floristic diversity.Scence 246,675-678 (1989). 7.Clarke,A.Crame,I.A.in Marine Biodiversity:Patterns and Processes(eds Ormond,R.E.G..Gage,1. well documented are strongly biased with respect to their higher tax- D.&Angel,M.V.)122-147 (Cambridge Univ.Press,Cambridge,1997). onomic affinities.But such outcomes are inevitable,because of the 8.Platnick.N.I Pattems ofbiodiversity:tropical vs temperate..Nat Hist.25,1083-1088(1991). multiple forces at work in structuring global patterns of biodiversity, 9.M Gaston.K L Spatial natterns in the species richness ofbirds in the New World and because the particular outcomes observed rest fundamentally on Ecography19,369-376(1996) 10.Rov K lablonski.D.Valentine.I W&Rosenbere G.Marine latitudinal diversity gradients testsof the balance of those forces.Indeed,even where two groups exhibit ausal hypotheses.Proc Nat Acad Sci USA95.3699-3702 (1998). similar spatial gradients in biodiversity there is substantial variation 11.Lyons,5.K.Willig.M.R.A hen ofscale depe ndence in latitudinal gradients of around those trends,and the details are seldom similar.In the species richness Ecoloyy80.2483-2491 (1999) 12.Blackburn,T.M.Gaston,K.J.A sideways look at there are so extreme,some groups exhibit patterns of biodiversity that are the Lett.3.4 3(1996) entirely contrary to the norm.For example,several major taxonomic 13.Colwell R K.Hurtt.G.C Nonbiological gr ents in species richness and a spurious Rapoport groups exhibit peaks of species richness at high or mid-latitudes(for effect..Am.Nat.14,570-595(199). example,aphids,sawflies,ichneumonids,braconids,bees,various 14.Terborgh,I.On the notior oy.Am.Na107,481-5011973) 15 Osman R W&Whitltch RB Ptternsofs cies diversity:fact or artifact?Paleobiology4,41-54(1978). groups of freshwater invertebrates,marine amphipods,and procel- 16 Ros weig.M.L Species diversity nd less than we thought.Mamm.73. lariiforms);exceptions to patterns of biodiversity tend to be 715-730(19921. observed more frequently at lower taxonomic levels than at higher 17.Rosenzweig.M.L Species Diversity in Space and Time (Cambridge Univ.Press,Cambridge,1995). levels.Which particular patterns are and are not expressed by a given 18.Blackburn,T.M.&Gaston.K.J.The graphic area and the latitudinal gradient in species rich be.11.195 204(1997) taxon rest on contingencies (for example,physiology,dispersal 19.Rosenzweig,M.L&Sandlin sity and latitudes:listening to area's signal Oikos80, ability,resource requirements and evolutionary history"). 172-176(1997). 20.MacArthur,R.H.Connell,I ogy of Popnlations (Wiley.New York.1966). 21 Gaston.K.L&Blackburn.T MAc cology.0los84.353-36819991 In conclusion 22.Lawton,I.H.Patterns 5 -147(1996) Development of a markedly improved understanding of the global 23.Rosenzweig.M.L&Abramsky.Z.in Speries Diversity in Ecological Commumities(eds Ricklefs,R.E& distribution of biodiversity is one of the most significant objectives Schluter.D.)52-65(Univ.Chi cawo.19931 for ecologists and biogeographers.Spatial heterogeneity in species 24.Currie.D.L Pa nical pat of trees.Nature 329,326-327(19871. richness,in particular,is an obvious feature of the natural world.An 25.Turner,I.R.G.,Gatehou .C M.Corey nic diversity? understanding of its determinants will impinge on applied issues of and the British climate.Oiko 548,195- 0519%7) major concern to humankind,including the role of biodiversity in 26.Turner,IR G.,Lennon,I..Lawrenson,J.A.British bird species distributions and energy theory. /re335.539-54119881 ecosystem processes,the spread of alien invasive species,the control of diseases and their vectors,and the likely effects of global environ- 27.Adams.I.M.Woodward,F.I.Patterns in tr as a test of the glacial extinction hypothesis.Nature 339,699-701 (1989) mental change on the maintenance ofbiodiversity. 28.Currie,D.J.Ene Am.a.137, A substantial proportion of regional variation in species richness 27-49(1991) 29 Wright D H Currie D L& laurer.B.A in St can be explained statistically in terms of a few environmental Ricklefs.R E.&Schluter,D.) Chicago,1993). variables'.This is,however,far from a predictive theory of species 30.Kerr,J.T.,Vincent,R.Currie,D.I. Lep ichness patterns in North America.Ecoscrence 5. richness.It is the need to identify the contingencies involved in the 。1见15310081 expression of patterns in biodiversity,and to weigh their significance, 31.Kerr.I.T.&Packer,LT rns among Epicaata(Coleoptera:Meloidse) mse8617-628(1999). that constitutes the real challenge to developing such a theory.The 32 Rutherford.s D'Hondt.S P ental co ols on the ge number of species is determined by the birth,death,immigration 753(1999) and emigration rates of species in an area.These rates in turn are 33.Lennon.ILGree 1.D.&Turner,I.R.G.Bird diversity and environmental gradients in Britain:a test ofthe thesis.Ecal (in the pr determined by the effects of abiotic and biotic factors(the latter may 34.Kerr,J.T.&Currie,D.J.Th ary ande trols on be intrinsic or extrinsic to the organisms of concern)acting at local broad-scale patterns of st ss in North America..329-337(1999) and regional scales.Although multiple factors doubtless contribute, 35.Tilman,D.Pacala,S.in iti(eds Ricklefs,R.E.&Schluter if a factor influences biodiversity on one spatial axis(for example, D.)13-25(Univ.Chicgo Pre s.Chicago,1993) 36.Gaston,K.I.Blackburn,T.M.Mapping biodiversity using surrogates for species richness:macro- latitude)then it seems reasonable to presume that all else being equal scalesand New World birds Proc R Soc Land 8262 335-341(19951 it will do so along others where the factor also varies(for example, 37.Chown,S.L&( rsity as a test of speci ergy theory in elevation).Thus,relationships between species richness and envi- marine systems.Eval.Ecol.Res.1.365-173(19991. ronmental energy have been found to be associated with latitudinal, 38.Martin,T.E.Species-are their interpret Am.Nat.118. 823 -837(1981) elevational and depth gradients?.If this were the whole story, 39.Palmer,M.W.White,p.s 。eand the s rea relationship.Am.Nat.144. patterns in richness would seem reasonably straightforward,if not 717-740(194). easy,to predict.However,it is not simply the current states of these 40.Latham.R.E Ricklefs.R.E.Global pa terns of tree species richness in moist forests:energy- diversity theory does not account for variation in species richness..325-33(1993). factors that are important but also their historical dynamics.These 41.Francis,A.P.&Cu another look have shaped variations in the distribution of different groups of k0序8,59 -6021998》 organisms,in their diversification,and hence the availability of 42.Ricklefs,R.E..Latham,R.E Qian,H.Global p species with different attributes to exploit opportunities provided by distinguishing ecological influ and historical contingency.Oikos86,369-373(1999). 13.Cousins.S.H Species richness and theenergy theory.Nature340.350-351 (1989) prevailing conditions.As such,the study of global patterns in 44 Gaston.K L&Blackburn.T M Pa ce.Oxford.in biodiversity demands insights from geneticists through to ecosystem the press ecologists.All concerned will need to remember that no single 45.Srivastava,D.S.Lawton,I.H.Why more productive sites have more species:an experimental test of mechanism need adequately explain a given pattern,that observed theory using tre nities..Am.at152,510-529(1998. 46.Gaston,K.Blackburn,T.M.&Lawton,H.Interspecific abundance-range size relationships:an patterns may vary with spatial scale,that processes at regional scales appraisal of mechanisms.1.Anim.Ecol.66,579-601(1997). influence patterns observed at local ones,and that no pattern is 47.Wright,D.H.Species- an extension of spe esatea-the 0la1.496-505119831 without variations and exceptions. 48.Hayden,B.P.Ecosystem feedb ate at the landscape scale.Phil.Trans.R.Soc Lond B353 5-18（19981. 1.Gaston,K.1.&Williams,P.H.in Biodiversity:A Biology of Numbers and Difference(ed.Gaston,K..) 49.Zheng.X.Y.Eltahir.E.A.B.Th tion in the dyn monsoons. 202-229 (Blackwell Science,Oxford,1996). 11L.11,2078-2096(1998} 2.Brown.I.H.Lomolino,M.V.Biogeography 2ndedn (Sinauer,Sunderland,MA,1998). 50.Collar,N.)..Crosby,M.&Stattersfield.A.I.Binds to Watch 2:The World List of Threatened Birds 3.Stevens,G.C.The latitudinalgradier in geographical range how so ny species co-exist in the (BirdLife International,Cambridge,1994) 51.Schlipfer,F.&Schmid,B.Ecosystem effects of biodiversity:a classification of hypotheses and 226 ש2000 Macmillan Magazines Ltd NATURE VOL 40511 MAY 2000 www.nature.comprotected areas). The latter is particularly important given that only ~15% of the total number of species estimated to be extant has been formally described taxonomically, that the distributions of most of these remain largely unknown (a high proportion are known from only a single locality72,73), and that species whose distributions are well documented are strongly biased with respect to their higher tax￾onomic affinities. But such outcomes are inevitable, because of the multiple forces at work in structuring global patterns of biodiversity, and because the particular outcomes observed rest fundamentally on the balance of those forces. Indeed, even where two groups exhibit similar spatial gradients in biodiversity there is substantial variation around those trends, and the details are seldom similar. In the extreme, some groups exhibit patterns of biodiversity that are entirely contrary to the norm. For example, several major taxonomic groups exhibit peaks of species richness at high or mid-latitudes (for example, aphids, sawflies, ichneumonids, braconids, bees, various groups of freshwater invertebrates, marine amphipods, and procel￾lariiforms1,74); exceptions to patterns of biodiversity tend to be observed more frequently at lower taxonomic levels than at higher levels. Which particular patterns are and are not expressed by a given taxon rest on contingencies (for example, physiology, dispersal ability, resource requirements and evolutionary history56). In conclusion Development of a markedly improved understanding of the global distribution of biodiversity is one of the most significant objectives for ecologists and biogeographers. Spatial heterogeneity in species richness, in particular, is an obvious feature of the natural world. An understanding of its determinants will impinge on applied issues of major concern to humankind, including the role of biodiversity in ecosystem processes, the spread of alien invasive species, the control of diseases and their vectors, and the likely effects of global environ￾mental change on the maintenance of biodiversity. A substantial proportion of regional variation in species richness can be explained statistically in terms of a few environmental variables1 . This is, however, far from a predictive theory of species richness. It is the need to identify the contingencies involved in the expression of patterns in biodiversity, and to weigh their significance, that constitutes the real challenge to developing such a theory. The number of species is determined by the birth, death, immigration and emigration rates of species in an area. These rates in turn are determined by the effects of abiotic and biotic factors (the latter may be intrinsic or extrinsic to the organisms of concern) acting at local and regional scales. Although multiple factors doubtless contribute, if a factor influences biodiversity on one spatial axis (for example, latitude) then it seems reasonable to presume that all else being equal it will do so along others where the factor also varies (for example, elevation). Thus, relationships between species richness and envi￾ronmental energy have been found to be associated with latitudinal, elevational and depth gradients75. If this were the whole story, patterns in richness would seem reasonably straightforward, if not easy, to predict. However, it is not simply the current states of these factors that are important but also their historical dynamics. These have shaped variations in the distribution of different groups of organisms, in their diversification, and hence the availability of species with different attributes to exploit opportunities provided by prevailing conditions. As such, the study of global patterns in biodiversity demands insights from geneticists through to ecosystem ecologists. All concerned will need to remember that no single mechanism need adequately explain a given pattern, that observed patterns may vary with spatial scale, that processes at regional scales influence patterns observed at local ones, and that no pattern is without variations and exceptions. ■ 1. Gaston, K. J. & Williams, P. H. in Biodiversity: A Biology of Numbers and Difference(ed. Gaston, K. J.) 202–229 (Blackwell Science, Oxford, 1996). 2. Brown, J. H. & Lomolino, M. V. Biogeography 2nd edn (Sinauer, Sunderland, MA, 1998). 3. Stevens, G. C. The latitudinal gradient in geographical range: how so many species co-exist in the tropics. Am. Nat. 133, 240–256 (1989). 4. Gaston, K. J. Biodiversity — latitudinal gradients. Prog. Phys. Geogr. 20, 466–476 (1996). 5. Stehli, F. G., Douglas, D. G. & Newell, N. D. Generation and maintenance of gradients in taxonomic diversity. Science 164, 947–949 (1969). 6. Crane, P. R. & Lidgard, S. Angiosperm diversification and paleolatitudinal gradients in Cretaceous floristic diversity. Science 246, 675–678 (1989). 7. Clarke, A. & Crame, J. A. in Marine Biodiversity: Patterns and Processes(eds Ormond, R. F. G., Gage, J. D. & Angel, M. V.) 122–147 (Cambridge Univ. Press, Cambridge, 1997). 8. Platnick, N. I. Patterns of biodiversity: tropical vs temperate. J. Nat. Hist. 25, 1083–1088 (1991). 9. Blackburn, T. M. & Gaston, K. J. Spatial patterns in the species richness of birds in the New World. Ecography 19, 369–376 (1996). 10. Roy, K., Jablonski, D., Valentine, J. W. & Rosenberg, G. Marine latitudinal diversity gradients: tests of causal hypotheses. Proc. Natl Acad. Sci. USA 95, 3699–3702 (1998). 11. Lyons, S. K. & Willig, M. R. A hemispheric assessment of scale dependence in latitudinal gradients of species richness. Ecology 80, 2483–2491 (1999). 12. Blackburn, T. M. & Gaston, K. J. A sideways look at patterns in species richness, or why there are so few species outside the tropics. Biodiv. Lett. 3, 44–53 (1996). 13. Colwell, R. K. & Hurtt, G. C. Nonbiological gradients in species richness and a spurious Rapoport effect. Am. Nat. 144, 570–595 (1994). 14. Terborgh, J. On the notion of favorableness in plant ecology. Am. Nat. 107, 481–501 (1973). 15. Osman, R. W. & Whitlatch, R. B. Patterns of species diversity: fact or artifact? Paleobiology 4, 41–54 (1978). 16. Rosenzweig, M. L. Species diversity gradients: we know more and less than we thought. J. Mamm. 73, 715–730 (1992). 17. Rosenzweig, M. L. Species Diversity in Space and Time(Cambridge Univ. Press, Cambridge, 1995). 18. Blackburn, T. M. & Gaston, K. J. The relationship between geographic area and the latitudinal gradient in species richness in New World birds. Evol. Ecol. 11, 195–204 (1997). 19. Rosenzweig, M. L. & Sandlin, E. A. Species diversity and latitudes: listening to area’s signal. Oikos 80, 172–176 (1997). 20. MacArthur, R. H. & Connell, J. H. The Biology of Populations(Wiley, New York, 1966). 21. Gaston, K. J. & Blackburn, T. M. A critique for macroecology. Oikos 84, 353–368 (1999). 22. Lawton, J. H. Patterns in ecology. Oikos 75, 145–147 (1996). 23. Rosenzweig, M. L. & Abramsky, Z. in Species Diversity in Ecological Communities(eds Ricklefs, R. E. & Schluter, D.) 52–65 (Univ. Chicago Press, Chicago, 1993). 24. Currie, D. J. & Paquin, V. Large-scale biogeographical patterns of species richness of trees. Nature 329, 326–327 (1987). 25. Turner, J. R. G., Gatehouse, C. M. & Corey, C. A. Does solar energy control organic diversity? Butterflies, moths and the British climate. Oikos 48, 195–205 (1987). 26. Turner, J. R. G., Lennon, J. J. & Lawrenson, J. A. British bird species distributions and energy theory. Nature 335, 539–541 (1988). 27. Adams, J. M. & Woodward, F. I. Patterns in tree species richness as a test of the glacial extinction hypothesis. Nature 339, 699–701 (1989). 28. Currie, D. J. Energy and large-scale patterns of animal- and plant-species richness. Am. Nat. 137, 27–49 (1991). 29. Wright, D. H., Currie, D. J. & Maurer, B. A. in Species Diversity in Ecological Communities (eds Ricklefs, R. E. & Schluter, D.) 66–74 (Univ. Chicago Press, Chicago, 1993). 30. Kerr, J. T., Vincent, R. & Currie, D. J. Lepidopteran richness patterns in North America. Ecoscience 5, 448–453 (1998). 31. Kerr, J. T. & Packer, L. The environmental basis of North American species richness patterns among Epicauta (Coleoptera: Meloidae). Biodiv. Conserv. 8, 617–628 (1999). 32. Rutherford, S., D’Hondt, S. & Prell, W. Environmental controls on the geographic distribution of zooplankton diversity. Nature 400, 749–753 (1999). 33. Lennon, J. J., Greenwood, J. J. D. & Turner, J. R. G. Bird diversity and environmental gradients in Britain: a test of the species-energy hypothesis. J. Anim. Ecol.(in the press). 34. Kerr, J. T. & Currie, D. J. The relative importance of evolutionary and environmental controls on broad-scale patterns of species richness in North America. Ecoscience 6, 329–337 (1999). 35. Tilman, D. & Pacala, S. in Species Diversity in Ecological Communities (eds Ricklefs, R. E. & Schluter, D.) 13–25 (Univ. Chicago Press, Chicago, 1993). 36. Gaston, K. J. & Blackburn, T. M. Mapping biodiversity using surrogates for species richness: macro￾scales and New World birds. Proc. R. Soc. Lond. B 262, 335–341 (1995). 37. Chown, S. L. & Gaston, K. J. Patterns in procellariiform diversity as a test of species-energy theory in marine systems. Evol. Ecol. Res. 1, 365–173 (1999). 38. Martin, T. E. Species-area slopes and coefficients: a caution on their interpretation. Am. Nat. 118, 823–837 (1981). 39. Palmer, M. W. & White, P. S. Scale dependence and the species-area relationship. Am. Nat. 144, 717–740 (1994). 40. Latham, R. E. & Ricklefs, R. E. Global patterns of tree species richness in moist forests: energy￾diversity theory does not account for variation in species richness. Oikos 67, 325–333 (1993). 41. Francis, A. P. & Currie, D. J. Global patterns of tree species richness in moist forests: another look. Oikos 81, 598–602 (1998). 42. Ricklefs, R. E., Latham, R. E. & Qian, H. Global patterns of tree species richness in moist forests: distinguishing ecological influences and historical contingency. Oikos 86, 369–373 (1999). 43. Cousins, S. H. Species richness and the energy theory. Nature 340, 350–351 (1989). 44. Gaston, K. J. & Blackburn, T. M. Pattern and Process in Macroecology (Blackwell Science, Oxford, in the press). 45. Srivastava, D. S. & Lawton, J. H. Why more productive sites have more species: an experimental test of theory using tree-hole communities. Am. Nat. 152, 510–529 (1998). 46. Gaston, K. J., Blackburn, T. M. & Lawton, J. H. Interspecific abundance-range size relationships: an appraisal of mechanisms. J. Anim. Ecol. 66, 579–601 (1997). 47. Wright, D. H. Species-energy theory: an extension of species area-theory. Oikos 41, 496–506 (1983). 48. Hayden, B. P. Ecosystem feedbacks on climate at the landscape scale. Phil. Trans. R. Soc. Lond. B 353, 5–18 (1998). 49. Zheng, X. Y. & Eltahir, E. A. B. The role of vegetation in the dynamics of West African monsoons. J. Clim. 11, 2078–2096 (1998). 50. Collar, N. J., Crosby, M. J. & Stattersfield, A. J. Birds to Watch 2: The World List of Threatened Birds (BirdLife International, Cambridge, 1994). 51. Schläpfer, F. & Schmid, B. Ecosystem effects of biodiversity: a classification of hypotheses and insight review articles 226 © 2000 Macmillan Magazines Ltd NATURE | VOL 405 | 11 MAY 2000 | www.nature.com