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 smaller than 86,872 square miles (equal to the area of Great Britain), with the smallest ranges found on islands, in low-altitude mountains, and throughout the southern hemisphere. The highest variation in range size is found in the northern hemisphere, particularly around the mid-latitudes. The pattern for latitudinal range size was also but, in violation of Rapoport's rule, latitudinal range decreased from low to high latitudes in both hemispheres, rather than vice versa. Similarly, overall geographic range size did not decrease toward the tropics; although the largest ranges were at high northern latitudes, range size decreased toward the high southern latitudes. Even within individual biogeographic realms, range size increased with higher latitudes in only seven out of 13 cases. There was a strong correlation between species richness and latitude, however, with the highest levels of biodiversity in the tropics, along with peaks in subtropical regions in the Andes, Himalayas, and the African Rift Valley. And there was a link, albeit weak, between species richness and range size, with high biodiversity areas harboring species with the smallest ranges.
With evidence that Rapoport's rule "does not generalize," the researchers demonstrate the risks of drawing global conclusions about spatial variations in geographic range area based on limited biogeographical data. It takes a global view, they argue, to understand the true nature of these variations and the mechanisms that create them. For example, the finding that birds inhabit small ranges not only just in islands, which is not surprising, but also in tropical and subtropical mountain ranges suggests that it's not just the availability of land area that dictates range size but the availability of land area that exists within a climate zone that meets the species' adaptive needs. Future studies can test how broadly these spatial patterns occur in other taxa -- essential information for understanding, and protecting, the current distribution of life on earth.
Citation: Orme CDL, Davies RG, Olson VA, Thomas GH, Ding TS, et al. (2006) Global patterns of geographic range size in birds. PLoS Biol 4(7): e208. DOI: 10.1371/journal.pbio.0040208.
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