I’m sorry, that’s totally fair, I don’t mean to sound dickish. They can’t hit zero. Australia is a model for how bad invasive cats can get, and the cat is already out of the bag, so to speak. They either target aggressive goals, which are primarily culling mixed with chemical and some limited physical spay/neutering programs. But when you are talking bang for buck, it’s really easy to look to culling primarily.

Spay nueter programs are much more expensive and usually donor funded. You gotta do a lot to an animal to even modestly safely remove it’s sex organs, especially females. I wouldn’t be surprised if they pop up in NZ, privately funded, in addition to the other programs. Desexing is still is the best way to deal with an active colony with a food source, you are right.

That’s when you are trying to control the population. These are invasive species, worse than that, invasive predators. Eradication and control are two different things.

Neither of those solves the problem entirely, but an eradication aims to keep the population of invasive species much lower than control. Any amount of invasive predators, especially as effective as feral cats, needs to be controlled.

Feral cats and pet cats are just two different things, like feral pigs and wild pigs. Even pet cats need to be tightly controlled, every bird a pet cat kills is multiplied in aggregate, these things we love are absolutely brutal.