such as "Introduction", "Conclusion"..etc
A new report finds little empirical evidence to support a widely held
ecological assumption that species are most abundant near the centers
of their geographic ranges and decline in abundance near the ranges'
"When we reviewed data from published studies that looked at species
abundance at multiple sites across a range, we found almost no evidence
that supported the so-called 'abundant-center hypothesis' and strong
evidence that contradicted it," said Raphael D. Sagarin, associate
director for oceans and coastal policy at Duke University's Nicholas
Institute for Environmental Policy Solutions.
"This is troubling," Sagarin said, "because a lot of current
thinking on ecological and evolutionary issues -- including how species
will respond to climate change, how to identify probable locations of
pest outbreaks, how genetic diversity is distributed among populations
and where to locate habitat preserves -- has been based on the
The validity of these ideas now needs to be re-examined using empirical studies, he said.
Sagarin is one of the principal authors of the report, which
appeared in the September 2006 issue of the journal Trends in Ecology
and Evolution. Other authors are Steven D. Gaines of the University of
California-Santa Barbara's Marine Science Institute and Department of
Ecology, Evolution and Marine Biology; and Brian Gaylord of the
University of California-Davis's Bodega Marine Laboratory and Section
of Evolution and Ecology.
For their analysis, the authors reviewed not only published studies
but also some new sets of data that they had compiled from field
observations in a number of coastal locations of such invertebrate
species as sea urchins, sea anemones and snails. They found that most
of the studies showed that patterns of abundance were affected by a
complex interplay of environmental, physical, biological, genetic and
geographical factors that the abundant-center hypothesis failed to take
Population clusters and high abundance sometimes occurred right at the geographic edges of the species' ranges, they found.
"Ecologists need to go back into the field and sample populations,
taking advantage of new technologies that allow us to see what
populations are actually like on scales not previously possible,"
Sagarin said. "In some way, it's a return to old-school ecology, but
armed with high-tech tools we didn't have 30 years ago."
Advances in remote sensing, biophysical monitoring, ecological
physiology, molecular genetics and genomics are rapidly enhancing
scientists' ability to identify population and individual patterns
across large spatial scales, he said. Scientists can collect data on
such factors as growth rates, genetics, climate, human-caused impacts
and species interactions in different parts of a population's range,
and then look at the overlay of these variables and see the larger
story, rather than making a simplifying assumption based on one
"Theory and experimentation have their place," Sagarin said. "They
can play important roles in helping us predict, in general, future
changes in species' ranges due to climate change. But you need
empirical field-based data to know, more specifically, how this is
going to look on the ground. When a range shifts, is it going to look
like the gradual arrival of a new species, or like an actual invasion?
Theory alone can't tell us that."
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