Food webs are woven from many plant and animal species that interact in fantastically complex ways. The intricacies of these interactions have eluded attempts to construct realistic computer models of how food webs work--until now. A mathematical model described in tomorrow's issue of Nature closely mimics the behavior of simple food webs, and the findings underscore the importance of preserving the myriad species in natural communities.
For more than 2 decades, mathematical models used by ecologists have predicted that populations in complex communities will fluctuate wildly, leading some species to go extinct. But that's not the case in the real world. "The theory had come up to a sort of quandary," says population biologist Alan Hastings of the University of California (UC), Davis.
To resolve that dilemma, Hastings and postdoc Kevin McCann devised a mathematical model for six miniature food webs, each containing three or four predator and prey species that interacted in different ways. In some of the models, two kinds of animals competed for plants; in another, omnivorous predators competed with their prey for veggies. The UC Davis team also incorporated more biologically realistic assumptions than previous models did. For example, predators could ease up on prey when those animals became rare and switch to a new prey species.
Unlike previous ones, the new model showed that predator and prey populations fluctuate less in complex food webs--those with long food chains, omnivorous predators, and competing species of plant-eaters. Animals that had several dietary options available were less likely to undergo boom-or-bust population cycles, which prevents species from getting perilously close to extinction. This in turn helps maintain community stability.
"This is the first paper that really models the way most ecologists view the world," says ecologist Gary Polis of UC Davis, who wrote a commentary in Nature. Other experts say the new findings point out the need to preserve species diversity in diverse communities, such as tropical rainforests. "If you have a lot of extinctions, you may make systems vulnerable to further extinctions by removing sources of stability," says community ecologist Robert Holt of the University of Kansas, Lawrence.