Most species more closely linked and dependent than previously known

SAN FRANCISCO, October 10, 2002--A team of scientists led by researchers at San Francisco State University's Romberg Tiburon Center for Environmental Studies has demonstrated that ecological systems are more closely related than was thought. Their research, published in the current issue of the Proceedings of the National Academy of Sciences (vol.99, no20:12913-12922), suggests that natural habitats are more connected, and thus more fragile and easily influenced by change, than was previously believed.

Led by SFSU's Neo Martinez, Ph.D., assistant professor of biology, Richard Williams, Ph.D., adjunct professor of biology, and Jennifer Dunne, Ph.D., postdoctoral fellow in collaboration with the Santa Fe Institute for complexity studies, the scientists combined computer network models with ecological data to analyze food webs--the prey/predator relationships--in a variety of land and water ecosystems. They demonstrated that species within large communities are on average two links apart, with greater than 95 percent of species typically within three links of each other. Prior to this research, ecologists believed that many, if not most, species were four or more links away from each other and much less likely to impact one another in the event of extinction, invasion by predatory species or changes in population.

"Our findings show that invasions by other species, loss of biodiversity and other changes in populations have the potential to affect many more of the species in the same habitat than was previously believed," said Martinez. "These ongoing analyses are a powerful tool for exploring how robust or fragile ecosystems are, and can help us determine what aspects of a system contribute to robustness."

Food webs are one of several self-organizing networks that are gaining interest in the scientific community. Network analysis has popularized the "six degrees of separation" theory that humans in the United Sstates are connected by six or fewer circumstances or acquaintances, and has led to analysis of such "small worlds" as Internet e-mail, screen actors and neural networks. In a second paper published in the same issue of PNAS, "Food-web structure and network theory: The role of connectance and size" Dunne, Williams and Martinez demonstrated that, while food webs have some patterns consistent with small-world networks, in general they are not as cliquish or "clustered" as most small-world networks, rather food web connections are more widespread and interdependent.

Co-authors with Martinez, Williams and Dunne on the first paper, "Two degrees of separation in complex food webs," are Eric L. Berlow of the White Mountain Research Station, University of California, San Diego and Albert-László Barabási of the University of Notre Dame Department of Physics.

The Romberg Tiburon Center is SFSU's marine field station located 30 minutes north of San Francisco on the Tiburon Peninsula. The center is the only academic research facility situated on San Francisco Bay, the largest estuary on the West Coast of the United States. The center's mission is to perform basic scientific research and educate and train the next generation of scientists. RTC scientists pursue their research in their laboratories at the center, at field sites around the world, and through collaborations with colleagues at other universities and institutions. As an affiliate of SFSU, the center offers graduate and undergraduate level courses and practical research experience in the laboratories of RTC scientists.