Environmental impacts

The impacts of feral cats greatly depends on country or landmass. In the Northern Hemisphere most landmasses have fauna adapted to wildcat (Felis sylvestris) species and other placental mammal predators. Here it may be argued that the potential for feral cats to cause damage is little unless cat numbers are very high, or the region supports unusually vulnerable native wildlife species. A notable exception is Hawaii, where feral cats have had extremely serious impacts on native birds species; "naive" fauna on islands of all sizes, in both hemispheres, are particularly vulnerable to feral cats.

Source : http://en.wikipedia.org/wiki/Cat