If grains of sand fall onto a tabletop, a conical pile forms until a critical slope is reached. While a sand pile and an ecosystem may seem quite different, Bak and others suggest that they are analogous. Adding grains of sand can be compared to introducing new species or increasing the biodiversity, while the avalanches correspond to extinctions of varying numbers of species. What Kaufman and Melroy found, however, was that self-organized criticality may provide an incomplete picture of patterns of extinction. Consonant with Bak, their model ecosystems would evolve to a point of critical biodiversity, at which they became highly susceptible to extinctions.
Systems appear to be able to evolve through that point, after which they become remarkably stable.
Neither Kaufman nor Melroy is a biologist. Kaufman is a condensed matter physicist and Melroy is an electrochemist at IBM's Almaden Research Center. As Melroy tells it, the work grew out of casual conversations between the two researchers and began in earnest after Melroy, on a flight from San Jose to New York, read The Diversity of Life, by the Harvard University biologist E. O. Wilson. People have performed studies of islands, looking at the amount of diversity as a function of the size of the island or its distance from the mainland. After Melroy returned to Almaden, he and Kaufman set about creating a simulation of an evolving ecosystem that made the minimum of assumptions about the interactions between species.