insight review articles Global patterns in biodiversity Kevin J.Gaston Biodiversity and Macroecology Group,Department of Animal and Plant Sciences,University of Sheffield,Sheffield S10 2TN,UK (e-mail:k.i.gaston@sheffield.ac.uk) To a first approximation,the distribution of biodiversity across the Earth can be described in terms of a relatively small number of broad-scale spatial patterns.Although these patterns are increasingly well documented,understanding why they exist constitutes one of the most significant intellectual challenges to ecologists and biogeographers.Theory is,however,developing rapidly,improving in its internal consistency and more readily subjected to empirical challenge. iodiversity,the variety of life,is distributed ecosystems)of biological variation can be distinguished, heterogeneously across the Earth.Some areas most analyses of spatial variation concern biodiversity as teem with biological variation(for example, measured by the number ofspecies observed or estimated to some moist tropical forests and coral reefs), occur in an area(species richness).This results from wide- others are virtually devoid of life (for spread recognition ofthe significance of the species as a bio- example,some deserts and polar regions),and most fall logical unit,and from the practical issues of the ease and somewhere in between.Determining why these magnitude of data acquisition.Consideration of spatial differences occur has long been a core objective for variation in other measures of biodiversity,particularly ecologists and biogeographers.It constitutes a continuing, those concerning the difference between entities rather than an important,and to many an enthralling,challenge. simply their numbers,has been remarkably sparse(with the Indeed,the past decade has seen a veritable explosion of possible exception of patterns in body size and morpholo- studies documenting broad-scale (geographical)spatial gy).Thus,although much attention has been paid to latitu- patterns in biodiversity,seeking to explain them,and dinal variation in species richness,little is known about vari- exploring their implications.The reasons for this interest ation in the diversity of genes,individuals or populations are twofold.First,it reflects increased opportunity along latitudinal gradients. provided by improvements in available data and analytical The growth ofinterest in broad-scale spatial variation in tools,the former resulting mostly from extensive collation biodiversity has been particularly striking with regard to of existing specimen and species occurrence records,the four areas of enquiry:latitudinal gradients in species rich- establishment of dedicated distribution-mapping ness,species-energy relationships,relationships between schemes,and the use of remote-sensing technology (to local and regional richness,and taxonomic covariance in measure vegetation and other environmental variables). species richness.In this review,the progress being made in Second,it reflects concern over the future of biodiversity, each of these areas will be used to substantiate four broader and the resultant need to determine its current status,to cross-cutting observations about global patterns of biodi predict its likely response to global environmental change, versity:respectively,that no single mechanism adequately and to identify the most effective schemes for in situ explains all examples of a given pattern,that the patterns conservation and sustainable use.Many of these issues observed may vary with spatial scale,that processes operat- can be addressed satisfactorily only by resolving the ing at regional scales influence patterns observed at local historical mismatch between the fine resolution of study ones,and that the relative balance of causal mechanisms plots in ecological field work means that there will invariably be varia- (typically a few square metres)and, tions in and exceptions to any given pat- by comparison,the poor resolution tern. of land-use planning and models of environmental change. Latitudinal gradients in species A host of global patterns of spatial richness variation in biodiversity has been High proportions ofterrestrial and fresh- explored (Fig.1).This includes water species occur in the tropics patterns in hotspots and coldspots Moving from high to low latitudes the (highs and lows)of diversity (includ- average species richness within a sam- ing comparisons between biological pling area of a given size increases,as realms and between biogeographical has been documented for a wide regions),variation with spatial scale spectrum of taxonomic groups (for example,species-area relation- (including groups as different as ships and relationships between local protists,trees,ants,woodpeckers and regional richness)and along and primates)for data across a gradients across space or environmen- range of spatial resolutions.Such tal conditions(for example,latitude, gradients in species richness may be longitude,altitude,depth,peninsulas, steep (for a given area,tropical bays,isolation,productivity/energy assemblages are often several times and aridity).Although several differ- more speciose than temperate ones),and ent levels of organization (genes to have been a persistent feature of the 220 2000 Macmillan Magazines Ltd NATURE|VOL 40511 MAY 2000 www.nature.cominsight review articles 220 NATURE | VOL 405 | 11 MAY 2000 | www.nature.com Biodiversity, the variety of life, is distributed heterogeneously across the Earth. Some areas teem with biological variation (for example, some moist tropical forests and coral reefs), others are virtually devoid of life (for example, some deserts and polar regions), and most fall somewhere in between. Determining why these differences occur has long been a core objective for ecologists and biogeographers. It constitutes a continuing, an important, and to many an enthralling, challenge. Indeed, the past decade has seen a veritable explosion of studies documenting broad-scale (geographical) spatial patterns in biodiversity, seeking to explain them, and exploring their implications. The reasons for this interest are twofold. First, it reflects increased opportunity provided by improvements in available data and analytical tools, the former resulting mostly from extensive collation of existing specimen and species occurrence records, the establishment of dedicated distribution-mapping schemes, and the use of remote-sensing technology (to measure vegetation and other environmental variables). Second, it reflects concern over the future of biodiversity, and the resultant need to determine its current status, to predict its likely response to global environmental change, and to identify the most effective schemes for in situ conservation and sustainable use. Many of these issues can be addressed satisfactorily only by resolving the historical mismatch between the fine resolution of study plots in ecological field work (typically a few square metres) and, by comparison, the poor resolution of land-use planning and models of environmental change. A host of global patterns of spatial variation in biodiversity has been explored (Fig. 1). This includes patterns in hotspots and coldspots (highs and lows) of diversity (includ￾ing comparisons between biological realms and between biogeographical regions), variation with spatial scale (for example, species–area relation￾ships and relationships between local and regional richness) and along gradients across space or environmen￾tal conditions (for example, latitude, longitude, altitude, depth, peninsulas, bays, isolation, productivity/energy and aridity1,2). Although several differ￾ent levels of organization (genes to ecosystems) of biological variation can be distinguished, most analyses of spatial variation concern biodiversity as measured by the number of species observed or estimated to occur in an area (species richness). This results from wide￾spread recognition of the significance of the species as a bio￾logical unit, and from the practical issues of the ease and magnitude of data acquisition. Consideration of spatial variation in other measures of biodiversity, particularly those concerning the difference between entities rather than simply their numbers, has been remarkably sparse (with the possible exception of patterns in body size and morpholo￾gy). Thus, although much attention has been paid to latitu￾dinal variation in species richness, little is known about vari￾ation in the diversity of genes, individuals or populations along latitudinal gradients. The growth of interest in broad-scale spatial variation in biodiversity has been particularly striking with regard to four areas of enquiry: latitudinal gradients in species rich￾ness, species–energy relationships, relationships between local and regional richness, and taxonomic covariance in species richness. In this review, the progress being made in each of these areas will be used to substantiate four broader cross-cutting observations about global patterns of biodi￾versity: respectively, that no single mechanism adequately explains all examples of a given pattern, that the patterns observed may vary with spatial scale, that processes operat￾ing at regional scales influence patterns observed at local ones, and that the relative balance of causal mechanisms means that there will invariably be varia￾tions in and exceptions to any given pat￾tern. Latitudinal gradients in species richness High proportions of terrestrial and fresh￾water species occur in the tropics. Moving from high to low latitudes the average species richness within a sam￾pling area of a given size increases, as has been documented for a wide spectrum of taxonomic groups (including groups as different as protists, trees, ants, woodpeckers and primates) for data across a range of spatial resolutions3,4. Such gradients in species richness may be steep (for a given area, tropical assemblages are often several times more speciose than temperate ones), and have been a persistent feature of the Global patterns in biodiversity Kevin J. Gaston Biodiversity and Macroecology Group, Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK (e-mail: k.j.gaston@sheffield.ac.uk) To a first approximation, the distribution of biodiversity across the Earth can be described in terms of a relatively small number of broad-scale spatial patterns. Although these patterns are increasingly well documented, understanding why they exist constitutes one of the most significant intellectual challenges to ecologists and biogeographers. Theory is, however, developing rapidly, improving in its internal consistency, and more readily subjected to empirical challenge. CONSERVATION INTERNATIONAL © 2000 Macmillan Magazines Ltd