<P> Rabbits, introduced as a food source by sailors in the 1800s, have become a severe nuisance to farmers, notably in the South Island . The myxomatosis virus was illegally imported and illegally released, but it had little lasting effect upon the rabbit population other than to make it more resistant to the virus . </P> <P> Cats, brought later by Europeans, have had a devastating effect upon the native birdlife, particularly as many New Zealand birds are flightless . Feral cats and dogs which were originally brought as pets are also known to kill large numbers of birds . A recent (2006) study in the South Island has shown that even domestic cats with a ready supply of food from their owners may kill hundreds of birds in a year, including natives . </P> <P> Sparrows, which were brought to control insects upon the introduced grain crops, have displaced native birds as have rainbow lorikeets and cockatoos (both from Australia) which fly free around areas west of Auckland City such as the Waitakere Ranges . </P> <P> In much of New Zealand, the Australian black swan has effectively eliminated the existence of the previously introduced mute swan . </P>

Which of the following is not a negative factor of nonnative species