Unexpected effects of predators upon their prey: the case of the American alligator

Indirect trophic effects play important roles in eco-system dynamics and can at times oppose and dominate the action of direct feeding linkages. Each predator directly exerts a negative effect upon its prey, but predators may also provide indirect ben-efits to their prey. In ecosystems, such benefits are effected via indirect trophic pathways that can provide a more than compensating positive influ-ence. The ecosystem of the Big Cypress National Preserve (southwest Florida) appears to contain an unusually high number of such predators—most notably, the American alligator, Alligator mississippi-ensis. The trophic exchanges of carbon among the 68 principal taxa comprising the cypress wetland eco-system have been quantified during both wet and dry seasons. The network analysis program IMPACTS identified predators that potentially have a positive influence on some of their prey. A total of 64 of these instances were recorded for the wet season and 44 for the dry. Taxa that, on balance, have positive effects upon their prey include fishes, turtles, snakes, birds, and, most significantly, alliga-tors. The feeding habits of alligators benefit a con-spicuous number (11) of their prey (invertebrates, frogs, mice, and rats). Further trophic analysis re-veals that the predation by alligators on snakes and turtles accounts for most of the trophic benefits bestowed. The actions of alligators in modifying their physical environment has been cited else-where as contributing to the maintenance of biotic diversity. It appears that the trophic influence of this species adds further evidence to the important role it plays in the functional ecology of the cypress wetland. Key words: cypress swamps; ecosystem; indirect interactions; network analysis; predator–prey inter-action; Alligator mississippiensis