As earth's biodiversity continues to plummet, determining the mechanisms that underlie spatial variations in species' range size will help explain global patterns in species richness -- why the tropics are biodiversity hotspots, for example -- predict how global climate changes might affect biodiversity, and establish priorities for conservation. In 1921, Frank Lutz, of the American Museum of Natural History, found that range area for North American plant species decreased "steadily and markedly" as one moved from high to low latitudes. Nearly 70 years later, this north-to-south decline in range size was codified as Rapoport's rule -- but not without generating considerable debate about the universality of the principle. The question has remained controversial largely because it has been explored mostly at limited scales, with studies analyzing either small taxonomic groups or restricted regions within individual biogeographic realms (regions that roughly follow the divisions of the major continents). In a new study, David Orme, Kevin Gaston, and colleagues revisit this issue by studying the global distribution of a major taxonomic group -- birds -- and show that spatial patterns of range size across the globe do not follow a simple north-to-south rule. The smallest range sizes are found on islands and mountain ranges, mostly in the southern hemisphere.

The researchers collated published data on breeding ranges for over 9,500 terrestrial avian species, concentrating on sources that covered large geographical areas for a diverse set of species. Species range areas were calculated by totaling all the cells containing the species. Latitudinal extent was defined as the difference between the northern and southern limits of the vector maps for each breeding range. Species richness was calculated by adding all the species in each cell. Their analysis shows that the majority of bird species have small geographic ranges. More than a quarter of species have ranges s
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Contact: Natalie Bouaravong
press@plos.org
415-568-3445
Public Library of Science
19-Jun-2006

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