insight review articles number ofways in which ranges can be distributed changes systemat- local climatic variability at high latitudes effectively increases the area ically between the bounds.Thus,whereas species with latitudinal of ecoclimatic zones that species can actually occupy,because it midpoints midway between the bounds can extend a little or a long requires that individuals have broad environmental tolerances way before those bounds are encountered,those with midpoints The observation that area alone is insufficient as a determinant of close to the bounds can extend only a little way before this occurs.A latitudinal gradients in species richness could equally be made about null model has been wanting from discussions of latitudinal almost any other factor that has been proposed as being important gradients in species richness.The 'mid-domain'model is thus likely (although critical tests are typically lacking).This highlights an issue to stimulate much interest.It is also likely to be most applicable for that has been central to much ofthe debate about the cause ofthis and groups whose distributions are genuinely limited by a physical other global patterns in biodiversity,namely the assumption that boundary(for example,those of large islands such as Madagascar), where a pattern is common to many taxa it must result from the same although its extension to two spatial dimensions is problematic, single mechanism-"wherever there is a widespread pattern,there given the longitudinal variation in land and ocean area.The is likely to be a general explanation which applies to the whole application of the model to other kinds of constraints is more ques- pattern To argue for a single primary cause may be to expect from tionable,as the position of those constraints that are recognized will ecological interactions a simplicity for which there is little evidence. be dependent on the inclusiveness ofthe set ofspecies considered. There is no necessary reason why latitudinal gradients exhibited by The second attempt to explain latitudinal gradients in species taxa as distinct as protozoa and mammals,and in environments as richness based on the physical structure ofthe Earth concerns the role structurally different as the deep sea and tropical forests,need be gen- of area (its importance has long been entertained 4s and recently erated in the same way,whatever the attractions of Occam's razor. brought to prominence).The tropics have a larger climatically Increasingly it seems that patterns in biodiversity are likely to be similar total surface area than any other ecoclimatic zone.This is generated by several contributory mechanisms221.The strongest because:(1)the surface area of latitudinal bands decreases towards and most general may be those where all the different mechanisms the poles;(2)the temperature gradient between the Equator and the pull in the same direction2.It is instructive that although numerous poles is nonlinear (the mean being relatively constant between mechanisms for latitudinal gradients in species richness have been approximately 20Nand 20S);and(3)the regions ofsimilar climate identified,and rather few processes that would oppose such a trend, immediately north and south of the Equator abut.It has been no single mechanism has ofitself proven sufficient. contended that,for a given species richness,larger mean geographi- cal-range sizes of species in the tropics result from the large area Species-energy relationships (which is not to be confused with any observed pattern in mean One factor thought to be important in modulating any effect of the range sizes at differentlevels ofrichness),and that these translate into physical structure ofthe Earth in determining latitudinalgradients in higher speciation rates (presuming larger ranges have higher species richness is the relationship between the number of species in probabilities of speciation)and lower extinction probabilities an area and ambient available ('usable')environmental energy. (presuming larger ranges have lower probabilities ofextinction)67. (This energy is usually estimated from models or indirectly from As a consequence,tropical regions have greater numbers of species other variables,and often used interchangeably with net primary than extratropical ones. productivity.)The form and cause ofthis relationship are some ofthe Area is almost certainly an important contributor to latitudinal most hotly debated topics in the study of global patterns in biodiver- gradients in species richness(indeed,area effects have a pervasive sity,with many fundamental issues as yet unresolved.Much of the influence on patterns of biodiversity).However,tests of the 'area discussion centres on the influence of spatial scale on observed model'have been limited (and often tangential),and have seldom relationships. sought the signal of the influence of area on latitudinal gradients At a relatively local scale (spatial resolution and extent),there is a when other factors have been controlled for.Moreover,as a sole marked tendency for a general hump-shaped relationship between explanation the area model is insufficient.To account fully for a species richness and available energy,with species richness increas- latitudinal gradient in species richness(rather than simply for the ing from low to moderate levels of energy and then declining again greater richness of the tropics)the model requires that ecoclimatic towards high levels of energy when a sufficient range of energy zones decline systematically in area moving from the Equator values is sampled At least across temperate to polar areas,at towards the poles.However,they do not do so (ecoclimatic zones at geographical scales there is substantial evidence for a broadly posi- high latitudes tend to belarge).Three possible explanations have tive monotonic relationship between species richness and energy been advanced for why a latitudinal gradient in species richness availabilitytobecommon(Fig.2).The bestcorrelates for plants might nonetheless be expressed:(1)low productivity/energy tend to be measures ofboth heat and water(such as actual evapotran- availability at high latitudes reduces the species richness they would spiration and net primary productivity),whereas for terrestrial,and gain as a result of area alonel.7 perhaps marine,animals the best correlates are measures of heat (2)zonal bleeding of tropical (such as mean annual temperature and potential evapotranspira- species into extratropical tion)For example,whereas the species richness of trees in regions smoothes out temperate Europe,eastern North America and East Asia increases species-richness gra- with primary productivity2,the richness of butterflies and birds in dients19;and areas of Britain increases with the temperature during the appropri- (3) high ate season2,and the species richness ofamphibians,reptiles,birds and mammals in areas of North America increases with annual potential evapotranspiration (estimated as a measure of the net atmospheric energy balance,independent of water availability2). The form taken by species-energy relationships at geographical scales,when extended to include subtropical and tropical areas,or at least to include the fullest range of variation in available energy (which may not be the same thing),remains unclear.There is evidence to suggest that they remain broadly positive and monoton- omem the answer may depend critically on the measure ofenergy used and the taxon concerned. 222 2000 Macmillan Magazines Ltd NATURE VOL 405|11 MAY 2000 www.nature.comnumber of ways in which ranges can be distributed changes systemat￾ically between the bounds. Thus, whereas species with latitudinal midpoints midway between the bounds can extend a little or a long way before those bounds are encountered, those with midpoints close to the bounds can extend only a little way before this occurs. A null model has been wanting from discussions of latitudinal gradients in species richness. The ‘mid-domain’ model is thus likely to stimulate much interest. It is also likely to be most applicable for groups whose distributions are genuinely limited by a physical boundary (for example, those of large islands such as Madagascar), although its extension to two spatial dimensions is problematic, given the longitudinal variation in land and ocean area. The application of the model to other kinds of constraints is more ques￾tionable, as the position of those constraints that are recognized will be dependent on the inclusiveness of the set of species considered. The second attempt to explain latitudinal gradients in species richness based on the physical structure of the Earth concerns the role of area (its importance has long been entertained14,15 and recently brought to prominence16,17). The tropics have a larger climatically similar total surface area than any other ecoclimatic zone. This is because: (1) the surface area of latitudinal bands decreases towards the poles; (2) the temperature gradient between the Equator and the poles is nonlinear (the mean being relatively constant between approximately 207N and 207 S); and (3) the regions of similar climate immediately north and south of the Equator abut. It has been contended that, for a given species richness, larger mean geographi￾cal-range sizes of species in the tropics result from the large area (which is not to be confused with any observed pattern in mean range sizes at different levels of richness), and that these translate into higher speciation rates (presuming larger ranges have higher probabilities of speciation) and lower extinction probabilities (presuming larger ranges have lower probabilities of extinction)16,17. As a consequence, tropical regions have greater numbers of species than extratropical ones. Area is almost certainly an important contributor to latitudinal gradients in species richness (indeed, area effects have a pervasive influence on patterns of biodiversity). However, tests of the ‘area model’ have been limited (and often tangential), and have seldom sought the signal of the influence of area on latitudinal gradients when other factors have been controlled for. Moreover, as a sole explanation the area model is insufficient. To account fully for a latitudinal gradient in species richness (rather than simply for the greater richness of the tropics) the model requires that ecoclimatic zones decline systematically in area moving from the Equator towards the poles. However, they do not do so (ecoclimatic zones at high latitudes tend to be large10,17). Three possible explanations have been advanced for why a latitudinal gradient in species richness might nonetheless be expressed: (1) low productivity/energy availability at high latitudes reduces the species richness they would gain as a result of area alone10,17; (2) zonal bleeding of tropical species into extratropical regions smoothes out species-richness gra￾dients18,19; and (3) high insight review articles 222 NATURE | VOL 405 | 11 MAY 2000 | www.nature.com local climatic variability at high latitudes effectively increases the area of ecoclimatic zones that species can actually occupy, because it requires that individuals have broad environmental tolerances3 . The observation that area alone is insufficient as a determinant of latitudinal gradients in species richness could equally be made about almost any other factor that has been proposed as being important (although critical tests are typically lacking). This highlights an issue that has been central to much of the debate about the cause of this and other global patterns in biodiversity, namely the assumption that where a pattern is common to many taxa it must result from the same single mechanism — “wherever there is a widespread pattern, there is likely to be a general explanation which applies to the whole pattern”20. To argue for a single primary cause may be to expect from ecological interactions a simplicity for which there is little evidence. There is no necessary reason why latitudinal gradients exhibited by taxa as distinct as protozoa and mammals, and in environments as structurally different as the deep sea and tropical forests, need be gen￾erated in the same way, whatever the attractions of Occam’s razor. Increasingly it seems that patterns in biodiversity are likely to be generated by several contributory mechanisms12,21. The strongest and most general may be those where all the different mechanisms pull in the same direction22. It is instructive that although numerous mechanisms for latitudinal gradients in species richness have been identified, and rather few processes that would oppose such a trend, no single mechanism has of itself proven sufficient. Species–energy relationships One factor thought to be important in modulating any effect of the physical structure of the Earth in determining latitudinal gradients in species richness is the relationship between the number of species in an area and ambient available (‘usable’) environmental energy. (This energy is usually estimated from models or indirectly from other variables, and often used interchangeably with ‘net primary productivity’.) The form and cause of this relationship are some of the most hotly debated topics in the study of global patterns in biodiver￾sity, with many fundamental issues as yet unresolved. Much of the discussion centres on the influence of spatial scale on observed relationships. At a relatively local scale (spatial resolution and extent), there is a marked tendency for a general hump-shaped relationship between species richness and available energy, with species richness increas￾ing from low to moderate levels of energy and then declining again towards high levels of energy when a sufficient range of energy values is sampled16,17,23. At least across temperate to polar areas, at geographical scales there is substantial evidence for a broadly posi￾tive monotonic relationship between species richness and energy availability to be common10,24–33 (Fig. 2). The best correlates for plants tend to be measures of both heat and water (such as actual evapotran￾spiration and net primary productivity), whereas for terrestrial, and perhaps marine, animals the best correlates are measures of heat (such as mean annual temperature and potential evapotranspira￾tion)28,29,34. For example, whereas the species richness of trees in temperate Europe, eastern North America and East Asia increases with primary productivity27, the richness of butterflies and birds in areas of Britain increases with the temperature during the appropri￾ate season25,26, and the species richness of amphibians, reptiles, birds and mammals in areas of North America increases with annual potential evapotranspiration (estimated as a measure of the net atmospheric energy balance, independent of water availability28). The form taken by species–energy relationships at geographical scales, when extended to include subtropical and tropical areas, or at least to include the fullest range of variation in available energy (which may not be the same thing), remains unclear. There is evidence to suggest that they remain broadly positive and monoton￾ic, that they become mildly or strongly hump-shaped, and that they begin to break down altogether10,32,35–37; the answer may depend critically on the measure of energy used and the taxon concerned. CONSERVATION INTERNATIONAL © 2000 Macmillan Magazines Ltd