Photo by Tom Cogill

John 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

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.

The 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.

“What’s 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 study’s co-principal investigator. “Understanding this relationship provides a basis for identifying declining carnivore species that may require immediate conservation measures.”

Gittleman points out that many carnivore populations and species are declining worldwide, with at least 90 of 235 carnivore species listed as threatened or endangered.

“Population density is a predictive factor influencing extinction risk, and likewise prey density is critical to the future of stable carnivore populations,” he said.

Gittleman 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.

“We 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.”

Gittleman 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.

Sam 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 interact.

Gittleman 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 online journals and databases. We’re looking for global patterns by making comparisons across all of the mammals, for example.”

Policymakers 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.

“We need to step back and start putting together the pieces of the global picture,” Gittleman said. “I tell my students, you’re entering a new age for broad-based research. You don’t 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. It’s all out there, but somebody has to synthesize it. You can be published next year.”

Gittleman’s 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. I’m fortunate to have come of age in the computer age. It’s the center of my work.”

Gittleman 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.

“We humans are the curators of the planet,” Gittleman said. “We need to make our conservation decisions based on clear, broad rigorous science.”