http://www.eurekalert.org/pub_releases/2006-06/plos-gpo061306.php
Another perspective of the PLOS article
Public release date: 19-Jun-2006
Public Library of Science
Global patterns of geographic range sizes: A bird's eye view
A global analysis of variation in the range sizes of birds shows that
range area does not follow a simple latitudinal pattern.
Click here for more information.
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 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.
===============================
www.birding-aus.org
birding-aus.blogspot.com
To unsubscribe from this mailing list,
send the message:
unsubscribe
(in the body of the message, with no Subject line)
to:
===============================
|
