On Natividad Island we witnessed nightly killings over the past three years, and it was quite clear that cats were the single greatest threat to the health and persistence of black-vented shearwaters. They were killing hundreds of birds a week as well as preying on other species native to the island. Over 75 percent of all recorded animal extinctions have been island endemics, the majority as a direct result of introduced species. Predators, such as feral cats and rats, prey on native fauna and extirpate nesting seabird colonies. Introduced herbivores, such as European rabbits, sheep, goats, and donkeys, can devastate island plant communities and threaten many endemic plants with extinction.
Feral pigs are an island catastrophe, devouring bird eggs one hour and native plants the next.
Only habitat destruction is a greater threat to global biodiversity than introduced animals and plants. As we study these feral cats and how they threaten native island species, island endemics all over the world continue to flicker out of existence while common weeds flicker in. Even on such a remote, arid island as Natividad, annual non-native grasses, the crystalline iceplant from South Africa, and many European weeds are becoming major problems. These invasive aliens will be even harder to control than non-native mammals, and their ecological effects are even less understood. Despite all these problems, islands offer excellent conservation prospects.