3.4 Possum impacts on native animals
Predation on birds
Effects on birds range from competition for vital foods at critical times of the year (described in Chapter 3.3) to direct predation. It is only in the last decade that researchers have confirmed that possums are significant predators of the eggs, nestlings, and even adults, of many native birds. Their diet also includes a wide range of invertebrates. As a consequence, researchers have changed from thinking of the possum as primarily a folivore, to recognising that possums are generalist and opportunistic feeders on foods that may be high energy, high protein or contain high levels of other nutrients, whenever such foods are available. These foods include flowers, leaf buds, fruit, eggs, birds and insects. As described above, one group of researchers coined the description of possums as "reluctant folivores", who will eat leaves to survive, but prefer other food. Use of these high-energy foods is seasonal, depending on availability. Obtaining direct evidence of possum predation is difficult, but time-lapse video cameras have shown possums eating adult birds, chicks and eggs, all in the nest.
The implications of possums as predators of native birds are very serious, given the large number of possums in native forests. Even if less than half of the possums in these forests were to eat one egg or chick during the year, their combined impact would be enormous. Research already shows that possums, along with ship rats, are major predators of kukupa/ kereru (native pigeon) and the endangered kokako. As a result, very few eggs and fledglings survive to adulthood without active control of these predators. What role possums play in the 5% per year decline in kiwi numbers is unknown, but in one study 14 adult kiwi were radio-tagged, and one of them was killed by a possum. Similarly, in another study, possums were identified as the culprits when kiwi eggs were destroyed before hatching. Chapter 5.1 documents the benefits for native bird populations of controlling predators with 1080.
Predation on bats
New Zealand's two species of bats, the short-tailed and the long-tailed, are both classified as nationally endangered, meaning they face extinction if conservation management is not successful at reversing their declines. The major threats to their populations are introduced pests - rats, stoats, cats and possums as well. In South Canterbury, where the bat population is declining by 5-9% per year, possums seek out young bats for prey at the roosts. Video cameras have recorded possums reaching into the roost cavities on half of the nights that the roosts were monitored.
Predation on invertebrates
Possums have seriously reduced the numbers of at least six species of the unique land snails (Powelliphanta species) of the southern North Island and Nelson/north Westland regions. Originating over 235 million years ago, these large and beautiful snails can live up to 20 years when undisturbed by predators such as possums, rodents and pigs. Possum and rodent control has led to increases in snail numbers at a number of sites6.
Large, sluggish, nocturnal species such as giant weta, large stag beetles and large weevils, may be the insects most at risk from possums. One study of a forest near Wellington showed that over half of the possum faecal pellets analysed contained invertebrates. Consumption was highest in the summer and autumn. Usually invertebrates are a small part of a possum's diet but, at times, insect larvae can constitute up to 28% of the contents of a possum's stomach. It is difficult to convert these figures into total amounts, but just as the amount of vegetation possums eat per night is probably in the tens of thousands of tonnes, the weight of invertebrates eaten would probably also average out to a substantial tonnage on a yearly basis.
In addition to direct predation on invertebrates, there are the indirect effects on invertebrates from possums eating the flowers, fruits and leaves that are an important part of invertebrate diets.
Possums, Kapiti Island and bird numbers
Once the stronghold of Ngati Toa chief, Te Rauparaha, Kapiti Island is one of New Zealand's most important sanctuaries for rare and endangered bird species such as the kokako, little spotted kiwi, stitchbird, saddleback (tieke) and others.
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