New Zealand's forests evolved without browsing mammals, so its trees carry few chemical or structural defences against a folivore. Brushtails feed selectively, returning to preferred species such as rata, kamahi and pohutukawa until individual trees defoliate and die, which reshapes the canopy over decades.
They are also not the pure herbivores they were assumed to be. Video monitoring at nest sites recorded possums eating eggs and nestlings of native birds including kokako, a finding that changed how the problem was framed. Possums additionally act as a wildlife reservoir for bovine tuberculosis, tying the control programme to agricultural biosecurity as well as conservation.
Population estimates have moved downwards as methods improved, from an often-quoted 70 million towards roughly 30 million, and sustained control has reduced numbers considerably from the peak. In Australia the same species is a protected native, which is the sharpest available illustration that pest status is a fact about context rather than about the animal.