Twenty–five years of monitoring reveals that uninterrupted rodent control is the fundamental driver of breeding success in the Galapagos Petrel <i>Pterodroma phaeopygia</i>

The long–standing misconception that the Galápagos petrel ( Pterodroma phaeopygia ) and the Hawaiian petrel ( Pterodroma sandwichensis ) were conspecific masked the severe vulnerability of the Galápagos population. By the time its distinct status was recognized, the Galápagos petrel was already in marked decline, primarily due to invasive predators. Consequently, sustained rodent control programs have been implemented on Santa Cruz Island. An unintentional one–year failure in rodent control provided a rare quasi–experimental opportunity to quantify the demographic consequences of the invasive black rat predator. During this year, hatching success declined by ≈35% and breeding success by ≈40% relative to long–term means (66% and 62%, respectively), representing a substantial reproductive collapse. Fledging success exhibited a comparatively modest decline (from a long–term mean of 94% to 86% in 2017), suggesting stage–specific vulnerability. These results support the hypothesis that invasive black rats primarily affect early reproductive stages through egg predation and predation on small chicks, while older chicks surpass a critical size threshold that reduces susceptibility. Across the remaining managed years, reproductive metrics exhibited great stability, demonstrating the petrel's resilience against other environmental or climatic stressors. Our findings provide robust empirical evidence that invasive rodent control is the dominant driver of reproductive success in this endangered seabird. The quasi–experimental failure underscored both the effectiveness and the necessity of continuous predator management, highlighting the severe and immediate consequences of even short–term lapses.