Google works on a different web; ranking system inspires algorithm for predicting extinctions.

Ecologists are taking a page, and its ranking, from Google.

A new algorithm inspired by the search engine works well for predicting which species losses will trigger collapse of a food web, says Stefano Allesina of the University of Chicago.

Food webs describe the pattern of what eats what in the neighborhood. If one species disappears, creatures that fed on it would need to find another lunch. If they couldn't, or if alternative entrees went extinct too, then the loss would trigger a cascade of extinctions. In some cases, it would unravel the whole food web.

The food web algorithm works much like PageRank, Google's system for ranking the importance of Web pages. The food web version does a better job of predicting collapse than simply comparing the number of connections each species has with other species in the food web. The method also beats out analyzing the network for hub species, Allesina and Mercedes Pascual of the University of Michigan in Ann Arbor report online September 3 in PLoS Computational Biology.

"The problem of how ecosystems are likely to respond to the loss of species is quite important, particularly in light of how many different ways human activities are resulting in the local extinctions of populations," says Jennifer Dunne of the Santa Fe Institute in New Mexico.

Allesina got the idea for treating food webs like the World Wide Web while he was at the National Center for Ecological Analysis and Synthesis in Santa Barbara, Calif., and chanced upon a description of Google's ranking system. In essence, the system calculates a page's importance, or value to searchers, based on the importance of the pages that link to it. In a food web, species draw importance from the importance of the species that eat them.

With some tweaks, the researchers accounted for the fact that anything can't eat just anything. "Energ'y does not go from the grass to the lion without going through the zebra," Allesina says. The team also added a path to a "detritus pool," so all species can die and become nutrients for primary producers.