Cute Cat

To date, there are few scientific data available to assess the impact of cat predation on prey populations. Cat numbers in the UK are growing annually and their abundance is far above the ‘natural’ carrying capacity, because their population sizes are independent of their prey’s dynamics: i.e. cats are ‘recreational’ hunters. Population densities can be as high as 2000 individuals per km2 and the current trend is an increase of 0.5 million cats annually.
Even well-fed domestic cats may hunt and kill, mainly catching small mammals, but also birds, amphibians, reptiles, fish and invertebrates. Hunting by domestic cats may be contributing to the decline in the numbers of birds in urban areas, although the importance of this effect remains controversial. In the wild, the introduction of feral cats during human settlement can threaten native species with extinction. In many cases controlling or eliminating the populations of non-native cats can produce a rapid recovery in native animals. However, the ecological role of introduced cats can be more complicated: for example, cats can control the numbers of rats, which also prey on birds' eggs and young, so in some cases eliminating a cat population can actually accelerate the decline of an endangered bird species in the presence of a mesopredator, controlled by cats.
In the Southern Hemisphere - cats are a particular problem in landmasses such as Australasia, where cat species have never been native and there were few equivalent native medium-sized mammalian predators. Native species such as the New Zealand Kakapo and the Australian Bettong, for example, tend to be more ecologically vulnerable and behaviorally "naive" to predation by feral cats. Feral cats have had a major impact on these native species and have played a leading role in the endangerment and extinction of many animals.