By Judith Burns
Science and Environment Producer BBC News
The large blue butterfly faced extinction when the ant it feeds on began dying off
Google's algorithm for ranking web pages can be adapted to determine which species are critical for sustaining ecosystems, say researchers.
According to a paper in PLoS Computational Biology, "PageRank" can be applied to the study of food webs.
These are the complex networks of who eats whom in an ecosystem.
The scientists say their version of PageRank could be a simple way of working out which extinctions would lead to ecosystem collapse.
Every species is embedded in a complex network of relationships with others. So a single extinction can cascade into the loss of seemingly unrelated species.
Investigating when this might happen using more conventional methods is complicated as even in simple ecosystems, the number of combinations exceeds the number of atoms in the universe. So it would be impossible to try them all.
Co-author Dr Stefano Allesina realised he could apply PageRank to the problem when he stumbled across an article in a journal of applied mathematics describing the Google algorithm.
The researchers say they had to make minor changes to it to adapt it for ecology.
Dr Allesina, of the University of Chicago's department of ecology and Evolution, told BBC News: "First of all we had to reverse the definition of the algorithm.
"In PageRank, a web page is important if important pages point to it. In our approach a species is important if it points to important species."
They also had to design in a cyclical element into the food web system in order to make it applicable to the algorithm.
They did this by including what Dr Allesina terms the "detritus pool". He said: "When an organism dies it goes into the detritus pool and in turn gets cycled back into the food web through the primary producers, the plants.
"Each species points to the detritus and the detritus points only to the plants. This makes the web circular and therefore leads to the application of the algorithm."
Dr Allesina and co-author Dr Mercedes Pascual of University of Michigan have tested their method against published food webs, using it to rank species according to the damage they would cause if they were removed from the ecosystem.
They also tested algorithms already in use in computational biology to find a solution to the same problem.
They found that PageRank gave them exactly the same solution as these much more complicated algorithms.
Dr Glyn Davies, director of programmes at WWF-UK, welcomed the work. He said: "As the rate of species extinction increases, conservation organisations strive to build political support for maintaining healthy and productive ecosystems which hold a full complement of species.
"Any research that strengthens our understanding of the complex web of ecological processes that bind us all is welcome."