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Image: GARY ROEMER

From zebra mussels to brown tree snakes, introduced species can seriously impact the ecosystems they invade. The case of the feral pigs on California's Channel Islandsnotably Santa Cruzis no exception. Indeed, according to research published today in the early online edition of the Proceedings of the National Academy of Sciences, the pigs have thoroughly scrambled food-web dynamics there, turning the once-dominant predator into prey.

Gary Roemer, now at New Mexico State University, and colleagues documented the reorganization of the food web, focusing on four key creatures: the pigs, the golden eagle, the island fox and the island spotted skunk. Using data collected starting in 1993, the researchers tracked shifts in the population sizes of these animals over time. Importantly, they found a sharp decrease in the island fox population accompanied by increased numbers of its prey, the skunk. Widespread predation of the foxes by golden eagles, the team determined, sparked this change. For their part, the eaglesthemselves relative newcomers to Santa Cruzappear to have colonized the island upon finding the pigs there.

Though the eagles initially preyed on piglets, they soon turned their attention to the foxes, cat-size creatures that, until the arrival of the eagles, had been the resident top predators. (The eagles may prefer the foxes because, unlike pigs, foxes remain viable prey items into adulthood. They also are active during the day.) "In sum," the authors write, "golden eagles impacted the pig population little, drove the foxes to near extinction through hyperpredation, and indirectly caused an increase in skunks."

On the surface it might appear that the pigs are outcompeting the foxes, Roemer remarks, but in fact they are not competing directly with each other. "This phenomenon is called apparent competition," he explains. "The two prey species share a predator that has an asymmetric impact, causing one species to decline." If the team's findings are any indication, apparent competition, as Roemer puts it, "may be an important mechanism in the global decline of biodiversity."