by Tom Cogill
Gittleman (right foreground), with students (left to right)
Sam Ross, Kate Jones, Samantha Price and Mike Habib, study
global mammalian diversity.
Carnivore numbers: they are
what they eat
By Fariss Samarrai
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
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
and the studys 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 or endangered.
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
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
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
Ross, a U.Va. undergraduate student funded by a Harrison Award,
is extending this research to other species and writing a computer
program that will model how size, density and food availability
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. Its 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
online journals and databases. Were looking for global patterns
by making comparisons across all of the mammals, for example.
are using findings from such studies for informed management decisions.
The National Science Foundation, which funds some of Gittlemans
work, is increasingly interested in large-scale, synthetic studies
that make use of existing data.
need to step back and start putting together the pieces of the
global picture, Gittleman said. I tell my students,
youre entering a new age for broad-based research. You dont
necessarily have to go out and do work in a specialized area.
You can pull together existing data and discover patterns that
nobody has seen before. Its all out there, but somebody
has to synthesize it. You can be published next year.
1984 doctoral dissertation on the behavioral ecology of carnivores
took four years of data crunching research, months of meticulous
typing (on a typewriter) and days of hand-drawing graphs. I
could accomplish the entire project in a few months now
with a computer and access to the Web, he said. And
the data would be a lot better. Im fortunate to have come
of age in the computer age. Its the center of my work.
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
humans are the curators of the planet, Gittleman said. We
need to make our conservation decisions based on clear, broad