To effectively keep an area at a minimum number of feral cats, an aggressive daily trapping regimen would need to be followed. This is neither time nor cost effective. The problem of feral cat overpopulation cannot be controlled through ignoring the problem. In a perfect world, residents would own their cats, all felines would be sterilized, and no animal would ever be abandoned. Yet, not living in a perfect world, efficient and effective solutions must be drafted that minimize the impact of feral cats.
Cats are opportunistic killers.
Given the choice between scavenging or being predators, they will choose to be scavengers - but will continue to kill wildlife. So the question of impact is moot. Cats do have an impact on wildlife. The real question is whether this impact is sustainable. Research has shown that in non-island environments feral cat colony impact on bird life is believed to be sustainable.