How removal of cats and rats from an island allowed a native predator to threaten a native bird

Abstract

The question of how to program the removal of two invasive mammals, typically cats and rats, from a marine island without increasing risk to native prey species has received two general answers based on ecological theory: removal of cats must be accompanied by control of their mesopredator prey, and risk is minimized by removing both invaders simultaneously. Nonetheless, a 31-year study showed that in a 82-ha tropical marine bird sanctuary, predation on a native prey, the blue-footed booby, by a native predator, the Atlantic Central American milk snake, apparently diminished after removal of cats then increased 11-fold after the additional removal of black rats. These novel effects are explained in terms of a hypothetical three-link trophic web in which cat removal released rats to increasingly compete with or prey on the snakes that feed on hatchling boobies, and subsequent rat removal released snakes from all remaining predation. The upshot is a disturbing scenario in which approximately 200 milk snakes currently aggregate annually in roughly 1 hectare of booby colony and predate roughly forty percent of the hatchlings. Where the lowest link of an insular trophic web is a native mesopredator that feeds on native prey, the predictions of the classic mesopredator release scenario can be inverted, and removal of invasive mammals may endanger native prey species.