When users seek information from Google, the search engine relies on a proprietary algorithm called PageRank™ to determine the order of the sites that show up in search results. Now, two researchers say a similar algorithm can be used to determine which species are critical to the preservation of ecosystems, allowing scientists to focus conservation efforts on species that will most benefit the entire system.

The research, by Stefano Allesina with the National Center for Ecological Analysis and Synthesis at the University of California, Santa Barbara, and Mercedes Pascual of the University of Michigan at Ann Arbor, was published today in the journal PLoS Computational Biology.

Google's PageRank algorithm ranks Web pages in importance based on the number of other Web sites that link to them. Allesina and Pascual have taken this approach into the wild and determined that PageRank could be adapted to apply to the study of food webs—the complex networks describing who eats whom in an ecosystem. Basically, according to Allesina and Pascual, the species that the greatest number of other species rely on for food are the ones that are most essential to the health of an ecosystem. Or as the authors put it, "a species is important if important species rely on it for their survival."

This approach contrasts with other ways of looking at ecosystems, which use a "hub" approach to rank species based on the number of other species that are directly linked to it through the food web. According to the authors, this technique, which emphasizes the number of connections, does not take into account the position of a species in the food web and the cascading effects its removal would create. They say the extinction of one species could cause the elimination of another, which in turn would cause the loss of a third species. The "PageRank" way of looking at ecosystems makes the species that goes extinct first the most important because it would result in further extinctions down the line.

Coming up with a mathematical equation to determine the top-ranking, or most important, species in an ecosystem wasn't easy. Allesina and Pascual actually reverse engineered their algorithm and used it to determine which species' extinction would create the most ecological harm. As the authors wrote: "We study how we can make biodiversity collapse in the most efficient way in order to investigate which species cause the most damage if removed."

So why is this advanced mathematics even necessary when looking at nature? Writing in the paper's abstract, the authors warned that "because of their mutual dependence, the loss of a single species can cascade in multiple co-extinctions." But food webs are so complex, it would take forever to go through all possible extinction scenarios without an algorithm like this.

What comes next? The authors say they hope their method could be applied beyond ecology to solve problems in other network-related biological fields, such as protein interaction and gene regulation.


Image: Google's Earth Day, 2008 home page logo, via http://www.google.com/holidaylogos08.html