3.3 Possum damage to native forests
Over the past 50 years, possums have emerged as one of the major threats to the health and wellbeing of forests throughout New Zealand. Many of these impacts are subtle and indirectly affect native birds and insects. Possums cause damage to native forests from the ground level to the canopy where, by concentrating on individual plants of their preferred species, they can kill trees by defoliation over several years. Possums preferentially feed on some of the tall canopy species - such as tawa, northern rata, kohekohe, southern rata, kamahi, pohutukawa and 20 Hall's totara - while ignoring others. They also prefer some of the smaller trees, such as tree fuchsia and wineberry, along with mistletoe, forest herbs, some ferns, and a number of endangered shrubs.
It is difficult to imagine that possums, which are about the size of a large cat, can kill individual trees that have dominated forest landscapes for centuries before possums were released here. But when the number of possums is combined with the total amount each one eats, their impact on their preferred species is easier to appreciate. The amount of food consumed by an adult possum each night is about 160 gm of digestible dry matter. There are probably tens of millions of possums living in native forests. In total, possums are consuming thousands of tonnes of vegetation each night.
Possum populations have now modified many New Zealand forests. The rate and extent of these changes vary widely between different types of forests. Beech forests are the least affected, but in the vulnerable southern rata-kamahi forests of Westland many valleys have lost between 20% to 50% or more, of their canopy trees. In severe situations, possums have caused the complete collapse of the canopy within 15-20 years of their arrival. Tall forest is then replaced by shrublands.
While the impact of possums is most visible and dramatic when it involves canopy trees, their most pervasive impacts are often less visible. Possums have recently been described as "reluctant folivores". This means that possums prefer to eat other forest foods than the leaves of trees. Flowers, fruit, leaf buds, fungi and insects are all highly favoured. The consumption of these foods has the largest impact on the healthy functioning of forests and the animals that rely on them. The consequences of possums concentrating on these foods are:
Loss of flowers:
- preventing the formation of seeds
- removing nectar sources for birds and bats
- reducing the food supply for many invertebrates
- nectar loss reducing food supplies for chicks, e.g. kaka, tui.
Loss of fruits:
- reducing food supplies for birds and invertebrates
- affecting bird breeding condition and nesting success, e.g. kakapo, kereru
- reducing or eliminating seed dispersal
- reducing the regenerative capacity of native plants.
Loss of new shoots:
- reducing the ability of plants to overcome leaf loss from weather and seasonal patterns
- reducing numbers of new leaves, jeopardising plant health.
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