Once thought of as leaf eaters only, possums are now recognised as opportunistic consumers of any high-energy, high-protein foods. These include flowers, leaf buds, fruit, eggs, birds, insects and snails.
In 1993 evidence was first uncovered of the effect of possums on native wildlife. A time-lapse video camera captured possums eating the chicks and eggs of the endangered kōkako. Of 19 nests monitored over a four-year period, cameras recorded possums eating chicks and eggs in four. Possums had not been suspected in the past because they were considered vegetarian, and because it is hard to find traces of birds in possum faeces or intestines.
Since then, possums have been caught on film eating the eggs, chicks and even adults of other native bird species, including kererū, kiwi, harrier hawk, fantail, muttonbird, and tūī. This predatory behaviour has driven some species into decline.
In areas where possums are not controlled, few kōkako and kererū fledglings survive to adulthood. In a study in Pureora Forest in 1997, robins in an area treated with 1080 poison (used to kill possums) had 67% nesting success, compared with 30% in the area not treated. After a year, robin numbers had increased by 37% in the control area, compared with 16% outside.
Competing with birds
By eating fruits and flowers, possums deprive nectar-feeding birds like tūī, kākā and bellbirds of the high-energy food they need in key periods such as the breeding season. Possums also take over dens that kiwi use – sometimes their mere presence disturbs native birds in their activities.
Both of New Zealand’s endangered native bat species have been recorded as attacked by possums, including in South Canterbury where the bat population is declining by 5–9% a year. In the central North Island possums eat flowers of the threatened wood rose, a nectar source for short-tailed bats.
A study in the Orongorongo River valley near Wellington has shown that native invertebrates form an important part of the possum diet. Over half of the possum faecal pellets contained invertebrates, the most likely targets being large, slow-moving species such as giant wētā, stag beetles and weevils.
Possums also eat endangered native snails. One possum can eat 60 giant Powelliphanta snails in a single night.