Possums are just the beginning
The natural values of the forest are constantly under threat from a wide range of pest species, from small rodents to feral goats through to invasive plant pests.
Many of our native species, which evolved over the last 60 million years in the absence of ground based mammalian predators, are poorly equipped to deal with these efficient killing machines and vigorous weeds.
Rodents (such as the norway rat, ship or black rat, and the common mouse) eat bird eggs, young chicks, native insects and small lizards.
The seeds and fruits of native trees are also eaten, preventing the growth of new seedlings.
We have been hugely successful at controlling their numbers since 2006. Our rat population monitoring shows us that we have consistently reduced these pests to an index average of less than 3% annually.
Possums can, in high densities, kill large trees by heavy browsing, cause local extinction of threatened plants (such as mistletoe and Dactylanthus), and limit regrowth.
Possums also prey on bird eggs, chicks, and native insects. Possums have been controlled in the park since 1996. Numbers are currently low to moderate.
Stoats, weasels, ferrets and cats
Stoats can kill birds up to twice their size and climb trees to reach nests.
Stoats and weasels also eat native insects. Ferrets prefer ground-dwelling birds like pukeko and fernbirds. Cats are also efficient killers of birds and insects.
Work to eradicate Pirongia’s mustelid predators began in 2019. DOC200 and DOC250 traps have been deployed to reduce pest numbers and protect our nesting native birds.
Goats, deer and pigs
Feral goats and deer browse on plants and in high numbers can remove the entire forest understory. Animal tracks can cause erosion, especially in wet weather.
Wild pigs eat native plants, causing more erosion through rooting, and will kill and eat ground-dwelling birds. The Department of Conservation has undertaken goat control in the park since the late 1990s and numbers are now low.
Deer and pig populations are also small.
Magpies and mynahs are known to kill native birds and are aggressive and territorial.
Introduced parrots such as Eastern Rosellas, lorikeets and budgies compete with native birds for the same seed sources, as well as carrying potentially damaging diseases.
Plant Pests are found throughout the forest especially on the fringes.
Climbing vines such as Japanese honeysuckle grow quickly, smothering tall trees. Other weeds such as tradescantia form dense mats cutting out all light and stopping the growth of native seedlings.
Some introduced trees, like pines and privet, grow vigorously and can change the forest makeup. Gorse, pampas and buddleia quickly invade eroded areas and slips, preventing native species from restablishing.
Other threats to forest health include fire and livestock encroachment from surrounding land.
MONITORING AND CONTROL
Plant and animal pests can be monitored to determine population levels and the need for further control.
Forest ecosystems can be checked to determine what impact pests are having. Bird species can be monitored by individually banding birds, videoing nest sites, or listening for native birdcalls.
Even insect populations can be assessed using pitfall traps that capture live insects for counting then release.
Pests can be controlled in a number of ways. For animal pests methods such as poisoning, shooting, trapping and fencing are used. Plant pests can be controlled by manual removal such as digging, cutting or felling, or by chemical removal using herbicides.
New Zealand leads the way in the development of monitoring methods and control techniques ensuring our native ecosystems are maintained for all future generations.