Mostly we get rid of introduced predators by, well – killing predators. It works, up to a point. But if you can’t get rid of every single rat or stoat then the few survivors suddenly find themselves with ample food supplies and very little competition. They breed and they breed very successfully. Numbers climb rapidly again, particularly in the case of rats.
So what’s the solution? The answer may be in what they eat. In many, if not most cases, predator numbers are thought to be controlled by food availability. Prey is the limiting factor. It’s what’s known as a bottom-up system. Predators don’t control prey. It’s the other way around. Prey controls predator numbers. Feral cats, ferrets and stoats all prey on native wildlife and we certainly don’t want to start limiting the number of native wildlife we have.
But for each of those predators, their main and ‘natural’ prey is actually another introduced mammal. In open grassland such as Central Otago, feral cats and ferrets prey mainly on rabbits and are limited by the rabbit population. Stoats are limited by rat availability. Rats and mice in turn, thrive where there is plenty of seed.