There's an interesting article on feral cats and cats killing wildlife in today's online nytimes.com (science section). Some of the figures given are mindblowing.
************"Marion Island, south-east of South Africa, is a small inhospitable island (approx 20 km x 13 km [ 12 miles x 8 miles]). In 1949, a group of scientists left the island, leaving behind 5 unneutered cats. By 1975 there were 2,500 cats on the island preying on ground-nesting seabirds. Deliberate infection with feline enteritis killed around 65% of the cats but the remaining 35% developed an immunity to the disease and continued to breed. Jack Russell Terrier dogs were used to flush out the remaining cats and between 1986 and 1989 further cats were exterminated by hunting. It still required poison to eliminate those cats which had eluded the dogs. There appears to be no information on how many seabirds ate poisoned bait and died themselves.
"It took 16 years to eradicate 2,500 cats from a small island so how can eradication be successful anywhere that new cats can move in and recolonise cleared areas? The "vacuum effect" whereby cats are attracted into an unoccupied area with an under-exploited food supply can result in an area becoming more densely populated with cats than it was previously. "