[redtrummy]

But its self regulating, Magpies reduce numbers of song birds so then not enough food to feed their chicks so then the magpie chicks starve and their numbers reduce. - different of course to a non native species such as the rat finding itself on an island that's a sea bird colony without any natural predators.
Which brings to mind 'Sea room' a book by Adam Nicholson after he inherited the Shiant Isles off the Outer Hebrides - there a successful culling of the brown rat has restored the status quo.