The Rotoiti Nature Recovery Project area comprises a range of Nothofagus beech forest types from tall red beech adjacent to Lake Rotoiti at 620m above sea level through silver beech then mountain beech to tussock tops at 1600m on top of the St Arnaud Range.
Historic accounts by early naturalists tell of species that are now extinct in the beech forests of the South Island or those that now have very much contracted ranges. Mohua/yellowhead were once described as the second most common bush bird in this area but have not been seen in the national park for more than thirty years. Roa/great spotted kiwi inhabited the Rotoiti forests up to around the 1930s. Other species such as the large forest parrot South Island kaka and kakariki/parakeet once recorded in large numbers have been very much affected by the introduction of a range of pests.
Putting a transmitter on a kaka
A range of animals introduced either accidentally or deliberately to New Zealand are now considered pests and many of these are present and problematic in the South Island beech forests. The most significant of those are outlined below:
Ferrets, stoats and weasels
These pests belong to the mustelid family and are widespread on mainland New Zealand. They were introduced in the 1880s to control rabbits which themselves had built to pest numbers in the dryer areas of the South Island after being introduced for sport hunting. Stoats are best adapted to forests with the larger ferrets and smaller weasels preferring the more open country. Stoats are expert tree climbers and fierce predators which prey on nesting kaka, chicks and eggs. Kiwi are also known to be killed by stoats, with eggs, chicks and smaller adults being vulnerable. Stoats also prey on rodents and can quickly build their populations to take advantage of increased rodent numbers in a beech mast (i.e. years with high seed fall).
First introduced from Australia for the fur trade in 1858, possums are now widespread in New Zealand, with an estimated national population of 70 million. They are omnivorous, eating bird eggs, chicks and insects as well as a wide range of forest plants. In the beech forests of Rotoiti they are known to browse native mistletoe (Peraxilla spp.), broadleaf (Grisellenia spp.) and fuschia, and prey on kaka eggs and chicks.
Wasp feeding on honeydew
In the Rotoiti forest common wasps (Vespula vulgaris), introduced accidentally in the 1970s, have largely displaced the earlier introduced German wasps (V. germanica), introduced in the 1940s. Common wasps live in colonies and construct nests generally underground and about the size of a football, though occasionally these can be much larger. Wasps are voracious feeders on honeydew, a sugary waste product from scale insects burrowed into the beech trees. The wasps can take as much as 70% of this high energy food depriving native birds and lizards such as bellbirds, tui and geckos. Wasps also prey on other insects (including caterpillars, ants, bees and flies) and spiders.
Feral cats are a major predator of small birds and lizards in the dry eastern South Island. They are not common at Rotoiti, but range far and are accomplished hunters. In an attempt to gauge how far feral cats roam, a male and female cat were caught and released with tracking devices attached. After six months the cats were recaptured. The female was found to have a range of around 300 ha, predominantly in open grasslands at the southern end of Lake Rotoiti. The male cat circled the 7 km long lake three times, and climbed up to 700 m above the lake.
Two species of rats (ship rat and Norway rat) and mice are found in the Rotoiti forests. Of the rat species, the smaller ship rats are most prevalent and like stoats are excellent tree climbers. Rats prey on on many small forest birds; robin and kakariki are particularly vulnerable and decline significantly when rat numbers increase in a beech mast year.
Ecosystem boost and burden
Honeydew and the beech mast are two distinctive features of the Rotoiti forests which provide very significant energy inputs into the local ecosystem.
A scale insect feeds on the sap of many of the beech trees, exuding a sweet honeydew which provides food for invertebrates and nectivorous birds and reptiles. Introduced wasps also feed on the honeydew, competing with other consumers, but also building up into high numbers and preying directly on native fauna. Excess honeydew falls to the ground where it increases the abundance of nitrogen-fixing bacteria that benefit the trees.
There are several species of beech trees, their distribution influenced by altitude and fertility. Beech seeding is a mast (periodic) event which normally occurs every three to five years, prompting the breeding of several native bird species including parakeets and kaka. As more seed falls it becomes food for rodents, and peaks of mice and rats follow. The rodent irruption, in turn, sees an increase in stoats, that quickly build their numbers to take advantage of the abundance of rodents. The stoats prey on the rodents until the seed germinates and rodent numbers decline in the absence of this food. Stoat impact on native birdlife may increase at this time.
Mistletoe (Peraxilla spp.) are hemi-parasitic plants that grow on beech, and for which there is an interesting relationship with nectar feeding birds. Bellbirds had developed a technique to twist the unopened flower buds to collect the nectar, and in doing so fertilised the flower. Mistletoe is a favourite food of possums. With most mistletoe browsed-out, bellbirds were deprived of a valuable nectar source. When some plants did recover, the birds had lost the ability to open the buds.