Species Diversity--Scale Matters

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Science  15 Feb 2002:
Vol. 295, Issue 5558, pp. 1245-1248
DOI: 10.1126/science.1067335

As predictions of the loss of global biodiversity [HN1] grow increasingly pessimistic, identifying the factors that determine species richness has become a hot topic. The best-known pattern in species diversity is the gradient ranging from low at the poles to high at the equator. This pattern is so general across so many taxa that it suggests the existence of an equally general explanation. Much attention, therefore, has been given to finding the mechanism that explains patterns of species richness, with the underlying assumption that whatever scale the relationship is measured at can be scaled up or down in simple fashion. If this were so, a model that successfully accounts for local patterns in richness could be scaled up to account for variations seen at a coarser regional or even global scale. This reasoning resembles that used to explain evolutionary change, where variations manifest over decades or centuries can be scaled up to describe the more dramatic patterns of macroevolutionary change over geological time. But, as some evolutionists have argued (1), biological and environmental systems are more complex than this. It is becoming increasingly apparent that the factors best accounting for patterns of biodiversity seem to be delimited by scale. This finding needs to be taken into account when assessing present or predicting future worldwide patterns of species richness [HN12].

Rahbek and Graves [HN3] (2) have examined the geographic range of 2869 species of birds breeding in South America [HN4]. By analyzing the same species data set over 10 spatial scales, ranging from 12,300 to 1,225,000 km2, the authors were able to make direct comparisons of patterns of numbers of species as a function of scale. The number of species was statistically analyzed against a suite of environmental variables including climate, ecosystem diversity, topography, and latitude. The investigators found that the order in which the explanatory variables entered the statistical models varied with scale: Whereas precipitation was the most influential factor at finer spatial scales, cloud cover and area (2) were more important predictors of species richness at coarser spatial scales.

Two recent studies of British plants (3) and birds (4), have produced similar insights [HN5]. The first study analyzed species-area curves [HN6] for vascular plants, using sampling units of 0.01 m2up to 100 km2 in size. The investigators demonstrated that the slope of the log-species/log-area plot is not constant, but varies systematically with spatial scale (3). The authors conclude that different processes are likely to determine plant diversity at different spatial scales. The second study, which analyzed the distributions of British bird species, found that the location of richness hotspots varies radically depending on the scale of observation. The authors discovered that the most species-rich areas underwent a shift northwards as the scale coarsened (4). The richness pattern at the finest scale (10-km resolution) was statistically unrelated to the pattern at the coarsest scale (90-km resolution). They point out the serious implications of this finding for conservation policy: The location of a reserve selected according to maximum species richness may change considerably depending on the size of the reserve or the scale of the analysis.

This complexity in the relation between species richness and scale is not confined to the terrestrial realm. The distributions of bryozoans (marine invertebrates) in the North Atlantic (5) indicate that different factors account for species richness at different scales [HN7]. At the finest scale (a single bryozoan assemblage) there was no apparent variation in species richness with latitude, whereas at a regional scale, there was a strong latitudinal trend indicating a peak in species richness between 15° and 30°N.

Similarly, factors driving variations in diversity over time may manifest themselves at different spatial scales. At the broadest spatial scale, for example, the distributions of mammal families between continents, fundamental differences in biogeographical realms have been driven by historical processes acting over tens to hundreds of millions of years. These processes include environmental variables such as tectonic plate movements and variations in sea level (6). In comparison, at regional spatial scales (variations within continents), environmental factors acting over time scales of 1 to 10 million years have had the strongest influence on species richness of particular lineages. For example, in the South African Cape, Richardson et al. [HN8] (7) found from their molecular analysis of the species-rich plant genus Phylica [HN9], that the diversity of this genus resulted from a series of rapid speciation events that began about 7 to 8 million years ago (see the figure). Similarly, rapid speciation events have been recognized in the molecular record of the Neotropical rainforest genus Inga over the last 3 million years [HN10] (8). Both studies concluded that these events contributed significantly to the biotic character and biodiversity of these regions and were driven by environmental factors operating over time scales of 1 to 10 million years. The South African speciation event appears to have been associated with an extensive aridification episode caused by changes in ocean currents. Similarly, the diversification of Inga in forests of the Neotropics can best be explained by climate changes associated with the late Tertiary uplift of the Andes and Quaternary cycles of glaciation (8). Finally, on a local scale, numerous studies suggest that environmental events—such as fires, storms, hurricanes, and landslides—operating over periods of 1 to 1000 years, markedly influence species richness patterns at a local-to-landscape scale (9).

The beauty of diversity.

Phylica pubescens, a member of the species-rich plant genus Phylica (Kirstenbosch Botanic Garden, Cape Town, South Africa).


Emerging from these findings is the realization that a variety of factors over different spatial and temporal scales contribute to an understanding of biodiversity. A hierarchical approach, as argued for evolutionary change [HN11] (1), may be more appropriate for accurately modeling the distribution of species richness (9, 10). In a hierarchical model, processes can be nested according to both spatial and temporal scales (see the table). There will be some transmission of signal from one scale to the next, but different variables should emerge at different scales. Thus, variables that best account for species richness on a local spatial scale or recent time scale may not be the same as those accounting for richness at regional spatial scales or longer time scales. We should take this into account during future attempts to model and assess species diversity for conservation purposes.

View this table:

HyperNotes Related Resources on the World Wide Web

General Hypernotes

Dictionaries and Glossaries

The University of California Museum of Paleontology offers a glossary of natural history and biological terms.

A glossary of biodiversity terms is provided by the World Resources Institute.

A glossary of ecology terms is provided by the companion web site for Essentials of Ecology by C. Townsend, J. Harper, and M. Begon.

A glossary for biogeography is made available by S. Woodward, Department of Geography, Radford University, VA, for a biogeography course.

Web Collections, References, and Resource Lists

Yahoo! provides collections of Internet resources for ecology and biodiversity.

The Ecology WWW page is maintained by A. Brach, Harvard University Herbaria.

The National Biological Information Infrastructure provides a collection of links to biodiversity Internet resources.

The Nearctica Web site provides introductions to biodiversity and biogeography and links to Internet resources.

The Natural Resources Research Information Pages are maintained by Y.-F. Leung, Department of Parks, Recreation and Tourism Management, North Carolina State University.

Conservation and Biodiversity: World Wide Web Resources is maintained by K. Holsinger, Department of Ecology and Evolutionary Biology, University of Connecticut.

Online Texts and Lecture Notes

M. Pidwirny, Department of Geography, Okanagan University College, Kelowna, BC, Canada, provides a unit on biogeography and ecology in his Web textbook Fundamentals of Physical Geography.

The E-Study Center for the fifth edition of R. Ricklefs' The Economy of Nature provides chapter summaries, study aids, and Web resources.

H. Grissino-Mayer, Department of Geography, University of Tennessee, Knoxville, makes available lecture notes for a Valdosta State University biogeography course.

M. J. Fouquette, Department of Biology, Arizona State University, offers lecture outlines for a biogeography course.

J. Blair, Division of Biology, Kansas State University, Manhattan, provides lecture notes for an ecology course.

R. Myers, Department of Biology, Dalhousie University, offers lecture notes for an ecology course.

J. Wiens, Department of Biology, Colorado State University, provides lecture notes (in Adobe Acrobat format) for an ecology course. Lecture notes on scale are included in a section on underlying themes of ecology.

K. Holsinger, Department of Ecology and Evolutionary Biology, University of Connecticut, provides lecture notes for a course on ecology and evolutionary biology.

J. Danoff-Burg, Center for Environmental Research and Conservation, Columbia University, offers lecture outlines for a course on population and community ecology.

General Reports and Articles

The National Academy Press makes available the 1988 book Biodiversity edited by E. O. Wilson and the 1996 book Biodiversity II edited by M. Reaka-Kudla, D. Wilson, and E. O. Wilson.

The Convention on Biological Diversity Web site makes available (in Adobe Acrobat format) the 2001 report Global Biodiversity Outlook.

The 11 May 2000 issue of Nature had a special section on biodiversity. A review by K. Gaston titled “Global patterns in biodiversity” is included.

An essay by J. Brown titled “An ecological perspective on the challenge of complexity” is made available by the National Center for Ecological Analysis and Synthesis, University of California, Santa Barbara.

The 10 March 2000 issue of Science had a review article by O. Sala et al. titled “Global biodiversity scenarios for the year 2100.” The 26 October 2001 issue had a review article by M. Loreau et al. titled “Biodiversity and ecosystem functioning: Current knowledge and future challenges.”

Numbered Hypernotes

1. Global biodiversity. The World Resources Institute offers a resource page on biodiversity. The IUCN (World Conservation Union) offers a presentation on biodiversity. Biodiversity and WORLDMAP is a presentation of the Natural History Museum, London. The Redpath Museum of McGill University offers a presentation on the theories of biodiversity as part of the Quebec Biodiversity Project Web site; a presentation on the patterns of biodiversity in space and time is included. Biodiversity and Conservation, a hypertext book by P. Bryant, Department of Developmental and Cell Biology, University of California, Irvine, includes a chapter on global patterns of biodiversity. J. Allen, School of Natural Resources and Environment, University of Michigan, offers lectures notes for a course on conserving biological diversity.

2. Patterns of species diversity and richness. The companion Web site for the fourth edition of P. Stiling's Ecology offers a lecture outline and other resources for a chapter on global patterns in species richness. J. Blair provides lecture notes on species diversity and species richness for an ecology course. A seminar presentation on biodiversity patterns and latitudinal gradients by M. Putze and S. Rinkevich is available on the Seminar in Ecological Complexity Web site of the Department of Biology, University of New Mexico. R. Myers offers lecture notes on patterns of species richness for an ecology course. C. Toft, Division of Biological Sciences, University of California, Davis, offers lecture notes on spatial patterns of biodiversity for a course on evolution and ecology. E. Thomas, Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT, makes available a presentation on biodiversity and species richness. G. Fox, Department of Biology, University of South Florida, offers lecture notes (in Adobe Acrobat format) on patterns of species diversity for an ecology course.

3. The article by C. Rahbek and G. Graves (“Multiscale assessment of patterns of avian species richness”) appeared in the 10 April 2001 issue of the Proceedings of the National Academy of Sciences (2) C. Rahbek is at the Zoological Museum, University of Copenhagen, Denmark; a presentation on his research projects is provided. G. Graves is in the Division of Birds, Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution.

4. Birds of South America. The Centres of Plant Diversity Web site, provided by the Department of Systematic Biology, Smithsonian Institution, offers an overview of South America. InfoNatura is a database from NatureServe (formerly the Association for Biodiversity Information) with information about the birds and mammals of Latin America and the Caribbean. A Neotropical Companion by J. Kricher includes a chapter on neotropical birds, which is provided on the Web as a sample chapter by the publisher Princeton University Press. BIRDNET, a service of the Ornithological Council, provides information resources and Web links related to the scientific study of birds. OWL (Ornithological Web Library) provides classified lists of Internet avian resources.

5. Two recent studies of British plants and birds. The study of British plants (“Scale dependence in plant biodiversity”) by M. J. Crawley and J. E. Harral appeared in the 2 February 2001 issue of Science (3). M. Crawley is at the Department of Biological Sciences and the NERC Centre for Population Biology, Imperial College, Silwood Park, UK. “The geographical structure of British bird distributions: Diversity, spatial turnover and scale” by J. Lennon, P. Koleff, J. Greenwood, and K. Gaston appeared in the November 2001 issue of the Journal of Animal Ecology (4). J. Lennon is at the Centre for Biodiversity and Conservation, School of Biology, University of Leeds, UK. P. Koleff and K. Gaston are in the Biodiversity and Macroecology Research Group, Department of Animal and Plant Sciences, University of Sheffield, UK. J. Greenwood is at the British Trust for Ornithology. The UK Department for Environment, Food and Rural Affairs offers a resource page on wildlife and countryside issues; the 2000 report Accounting for Nature: Assessing Habitats in the UK Countryside is made available. The English Nature Web site provides information about the natural areas of England. The UK Biodiversity Web site provides information about British birds and vascular plants that are endangered.

6. Species-area relationship is defined in xrefer's Dictionary of Geography. D. McShaffrey, Biology Department, Marietta College, OH, provides an introduction to species-area curves in a study guide for an ecology course. J. Allen offers lectures notes on species-area relationship for a course on conserving biological diversity. J. Ryan, Department of Biology, Hobart and William Smith Colleges, Geneva, NY, provides a tutorial by K. Mitchell on the species-area relation. L. Gross, Department of Ecology and Evolutionary Biology, University of Tennessee, offers a presentation on species-area relationships, which is made available on the Mathematical Life Sciences Page for Education Web site. The 4 April 1999 issue of Science had an Enhanced Perspective by M. Rosenzweig titled “Heeding the warning in biodiversity's basic law.”

7. A study of bryozoan distribution in the North Atlantic. The article by A. Clarke (Biological Sciences Division, British Antarctic Survey, Cambridge) and S. Lidgard (Geology Department, Field Museum of Natural History, Chicago) titled “Spatial patterns of diversity in the sea: Bryozoan species richness in the North Atlantic” appeared in the September 2000 issue of the Journal of Animal Ecology (5). Bryozoa is defined in xrefer's Dictionary of Biology and in xrefer's Dictionary of Earth Sciences. The University of California Museum of Paleontology provides an introduction to the Bryozoa. P. Bock's Recent and Fossil Bryozoa Web page provides an introduction to Bryozoa. Information about Bryozoa is provided by the International Bryozoology Association.

8. The article by J. E. Richardson et al. (“Rapid and recent origin of species richness in the Cape flora of South Africa”) appeared in the 12 July 2001 issue of Nature (7).

9. A presentation on Phylica is provided by the Agulhas Fynbos Web site. The South African Museum provides an image of Phylica. South African Botany on the Internet is maintained by G. Maneveldt, Botany Department, University of the Western Cape. C. Puff, Institute of Botany, University of Vienna, offers a presentation on the flora of the Cape Region for a course on the flora and vegetation of the paleotropics.

10. Rapid speciation of Inga. An introduction to the genus Inga is provided by the Botanical Research and Herbarium Management System Web site. The Rainforest Conservation Fund provides a data sheet about Inga. The 21 September 2001 issue of Science had an Enhanced Perspective by E. Bermingham and C. Dick titled “The Inga—Newcomer or museum antiquity?” about the report in that issue by J Richardson et al. titled “Rapid diversification of a species-rich genus of neotropical rain forest trees” (8).

11. Hierarchical approach. “Gulliver's further travels: The necessity and difficulty of a hierarchical theory of selection” is an article (1) by S. J. Gould that appeared in the February 1998 issue (a special issue on the evolution of diversity) of Philosophical Transactions: Biological Sciences of the Royal Society; CatchWord provides the full-text of the article in Adobe Acrobat format. An information page about Gould and his views is provided by the Stanford Presidential Lectures and Symposia in the Humanities and Arts Web site. D. Rand, Department of Ecology and Evolutionary Biology, Brown University, offers lecture notes for a course in evolutionary biology; lecture notes on the tempo and mode of macroevolution (parts one and two) are included.

12. K. J. Willis and R. J. Whittaker are in the School of Geography and the Environment, University of Oxford.

References and Notes

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