Ecosystems are such intricate webs of connections that few studies have been able to explore exactly what happens when a species dies out. Now, a Cornell study using computer simulations has teased out how the disappearance of a freshwater fish can affect the availability of certain nutrients that other species rely on. Algae, at the base of the food chain, for example, rely on fish to cycle back into the water such nutrients as nitrogen and phosphorus, which are otherwise locked up in animal or plant cells. Fish excrete dissolved nutrients back into the water, making them available to algae, which need them to grow. The study, published in the Feb. 27 issue of the Proceedings of the National Academy of Sciences, finds that overfishing could threaten the overall health of an ecosystem because it targets important fish species that play major roles in recycling nutrients. In fact, 20 percent of fish species accounted for more than half of all the recycled nutrients in the ecosystems studied, the computer simulations found. "The loss of the most heavily fished species led to the fastest declines in nutrient recycling," said lead author Peter McIntyre, a postdoctoral researcher at Wright State University who was a graduate student in Cornell's Department of Ecology and Evolutionary Biology when he conducted the study. "Fishermen are targeting relatively large and abundant species that happen to play a major role in nutrient recycling." The simulations, which relied on data from Rio Las Marias, a Venezeulan river, and Lake Tanganyika, a massive lake bordering Tanzania, Zaire, Zambia and Burundi, also shed light on the roles that surviving species might play in replacing the lost nutrients. In both ecosystems studied, when surviving species successfully picked up the slack in nutrient recycling left by an extinct species, nitrogen and phosphorus were maintained at 80 percent of their starting values until over half the total number of species were lost. Studies of complex ecosystems, especially those involving large, highly mobile fish, are almost impossible to carry out in the wild, but new methods are helping researchers better understand these systems. "Computer simulations provide a means to assess patterns of species loss in a system in which we just cannot do complex experiments," said co-author Alex Flecker, Cornell associate professor of ecology and evolutionary biology, who served as McIntyre's adviser. "But we have to be aware that there is a whole set of assumptions that goes into simulating species loss." For example, it is unknown whether surviving species can truly compensate for extinctions. In a study of two species of fish in the Venezuelan river that eat mud from the river bottom, Flecker found that the rarer of the two species was unable to make up for the loss of the more common one. Thus, it appears that human overfishing of the common species, coporo (Prochilodus mariae), may have large effects on the ecosystem, in part because of its large contribution to nitrogen recycling. The current study also revealed that species that heavily recycle nitrogen are not always the same ones that recycle the most phosphorus. These differences would make it difficult for conservationists to prioritize species to protect.