http://ucsdnews.ucsd.edu/newsrel/science/12-07ThreatenedBirds.asp
Threatened Birds May Be Rarer than Geographic Range Maps Suggest
December 10, 2007
By Kim McDonald
Geographic range maps that allow conservationists to estimate the
distribution of birds may vastly overestimate the actual population
size of threatened species and those with specific habitats, according
to a study published online this week in the journal Conservation
Biology.
“Our study found that species ranges in general tend to get
overestimated, but that this trend is particularly pronounced for birds
that are threatened, rely on specialized diets or have small
habitats,” said Walter Jetz, an assistant professor of biological
sciences at UC San Diego and the lead author of the study, which will
appear in the February issue of the printed journal. “This suggests
that many threatened species of birds may be even rarer than we
believe and are in greater danger of going extinct.”
“Our findings indicate that the ranges of most vulnerable bird species
are experiencing the highest overestimation, thereby painting a rosier
picture of their distributions than is actually the case,” said Cagan
Sekercioglu, a senior research scientist at Stanford University and a
co-author of the study. “This suggests that the conservation status of
many narrow-ranging, specialized and threatened bird species may be
worse than we think.”
Jetz, Sekercioglu and James E.M. Watson of Britain’s Oxford
University evaluated geographic range overestimation and its potential
ecological causes for 1,158 bird species across 4,040 well-studied
survey locations in Australia, North America and Southern Africa.
Comparing the range maps with actual bird surveys, such as those
conducted by the Audubon Society, the scientists found that most
species actually occur in only 40 to 70 percent of the range suggested
by their range maps. In other words, these birds are not actually
found in 30 to 60 percent of their supposed range.
The scientists also found range overestimation increases for species
with smaller ranges and with more specialized dietary and habitat
requirements. Range maps most strongly overestimate the distribution
of narrow-ranging species and ecological specialists. As a
consequence, species threatened or near threatened with extinction are
also subject to particularly high range overestimation, on average 62
percent overestimation compared to 37 percent overestimation in
non-threatened species.
The researchers’ study was limited to mostly temperate areas with
relatively high-quality data on bird distributions. They said they
expect that in the tropics, where the majority of bird species reside,
but where research and data on bird distributions are more limited,
that the problem of ounderstimating <overestimating?> bird species’
ranges may be even more extensive than the scientists found for
Australia, North America and South Africa.
“People have been treating tropical and temperate data on bird
distributions as if they are of equal quality,” said Walter Jetz. “But
the overestimation is especially large for tropical species, which
have much smaller geographic ranges, smaller population sizes, are
more specialized, and are in greater danger of extinction than those
in temperate areas.”
Jetz and Allen Hurlbert of the National Center for Ecological Analysis
and Synthesis in Santa Barbara published a related study in the August
14 issue of the Proceedings of the National Academy of Sciences , that
found that range maps of birds less than 200 kilometers in area also
“overestimate the area of occupancy of individual species and
mischaracterize spatial patterns of species richness.”
Jetz added that range maps are becoming especially important for
ecologists making projections of the impact of climate change on the
health of specific populations and that these two studies demonstrate
that conservationists need to be especially careful when making
predictions about future risks to species during rapid climate change.
“If we’re starting with a range estimate for a population that is much
larger than it truly is, then we have started with the wrong parameter
for our future projections of climate change,” he said. “If you’ve
already started with an overestimate of the range, then there’s a real
danger in understimating the risks of extinction in future projections
of climate change.”
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