"Jungle banking" from New Scientist

"Until the 1970s ecologists believed that the more complex the ecosystem, the more stable: lose one species and the rest will fill in. Then Robert May, now at the University of Oxford, and others showed that such complex systems have critical points at which a change in one species can have dramatic and non-linear effects on others. Losses can propagate, and even cause collapse. The stronger the connections and fewer differences there are among species, the greater the risk."