Numbers: They Are What They Eat
21, 2002-- A new study will help conservation
biologists rethink factors that promote healthy carnivore populations.
Biologists long have believed that the size of carnivores determines
their population in a given area. Bigger animals, such as bears,
are fewer in number than smaller species, such as mice, according
to a theory known as scaling.
new study, however, indicates that the availability of food is more
important in determining population density, although the two are
related. The findings, which will appear in the March 22 issue of
the journal Science, may help predict which species are in decline
or might face extinction
important is that the relationship between prey availability and
predator density is extremely tight and predictable when examined
across carnivore populations and species around the world," said
John Gittleman, a professor of biology at the University of Virginia
and the study's co-principal investigator. "Understanding this relationship
provides a basis for identifying declining carnivore species that
may require immediate conservation measures."
points out that many carnivore populations and species are declining
worldwide, with at least 90 of 235 carnivore species listed as threatened
density is a predictive factor influencing extinction risk, and
likewise prey density is critical to the future of stable carnivore
populations," he said.
and his colleague, Chris Carbone, a biologist with the Zoological
Society of London, developed a model to predict carnivore density
in relation to prey abundance and productivity. They tested the
model with detailed population density data on 25 species of carnivores
and their common prey. Gittleman and Carbone found common direct
relationships between prey availability and predator populations,
across species. They were able to demonstrate that the body size
of carnivores may be less of a factor in determining their density
than the availability of prey. This suggests that researchers studying
carnivore populations should understand and assess prey populations
as a separate, though closely related, issue to body size.
found, for example, that the European badger, which weighs the same
as the coyote, has a population density 20 times that of the coyote,"
Gittleman said. "The reason is, badgers feed primarily on earthworms,
which are very common and easy to obtain, while coyotes feed primarily
on small mammals, which are far less dense in population numbers
and much more difficult to catch. We found these sorts of comparisons
for all 25 species surveyed."
selected carnivore species with a range of body sizes, habitats
and feeding patterns, including the red fox, African lion, bobcat,
gray wolf and polar bear. His study also explains that some differences
in predator density are due to competition for prey from other species.
is one of the lead players internationally in the emerging research
field of biodiversity and conservation biology. The overarching
focus of the research is a quantitative, geographic and global view
of species patterns. "It's computer-driven macro-ecology and evolution,"
Gittleman said. "We look at the big picture by analyzing and synthesizing
hundreds of smaller scale field studies. It involves heavy number
crunching and the use of 100 years worth of data that are available
to anyone through on-line journals and databases. We're looking
for global patterns by making comparisons across all of the mammals,
are using findings from such studies for informed management decisions.
The National Science Foundation, which funds some of Gittleman's
work, is increasingly interested in large-scale, synthetic studies
that make use of existing data.
continues to focus his investigations on large carnivores -- wolves,
bears, tigers and lions -- "the charismatic animals." But he also
is working with his students to assemble valuable data on other
animals that receive less attention, such as bats and rats. He has
published more than 100 journal articles and three books providing
new insight about patterns and changes of global species diversity,
including four papers in two years in Science.
humans are the curators of the planet," Gittleman said. "We need
to make our conservation decisions based on clear, broad rigorous
Fariss Samarrai, (434) 924-3778