Possums are voracious eaters – in New Zealand they consume an estimated 21,000 tonnes of vegetation a night. Most noticeably, they destroy spectacular flowering trees such as pōhutukawa and rātā. They also change the overall structure and composition of native forests.
Although the amount they consume seems astonishing, it is just a small percentage of the amount of foliage produced each day in native forests – around 300,000 tonnes.
Favoured food species are tall canopy species such as tawa, northern and southern rātā, kohekohe, kāmahi and Hall’s tōtara.
Possums can cause catastrophic dieback, the complete collapse of a forest canopy – especially tree species that possums prefer, such as rātā and kāmahi. In the southern rātā–kāmahi forests of Westland, many valleys lost more than 50% of canopy trees within 15–20 years of possums arriving. The forest trees are then replaced by shrubs that are unpalatable to possums, and the area changes from tall forest to low open forest and shrublands.
New Zealand’s eight surviving species of native mistletoe are threatened by possums. These shrubs live on larger trees, and have fleshy leaves and juicy stems which are readily eaten. Possum control can lead to dramatic increases in mistletoe growth and flowering.
In more diverse (conifer–broadleaf) forests, possums selectively remove their preferred species over a number of decades, changing the composition of the forest. Possums often select one tree and systematically strip it. They feed on new shoots, making it harder for trees to recover from weather and insect damage, and slowing their growth.
By eating flowers, possums stop seeds forming. Many plants fail to regenerate under possum assault.