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Do rats and possums reduce the reproductive capacity of large-fruited broadleaved species in Waikato hill-country forests?

Although brushtail possum (Trichosurus vulpecula) browsing can have conspicuous impacts by killing trees outright, there is also evidence of more insidious threats to regeneration of some species through depression of seed production. It is not known how widespread these more cryptic effects are. The elimination of all introduced mammals except mice (Mus musculus) from Sanctuary Mountain Maungatautari (herein referred to as Maungatautari) fourteen years ago, provided a valuable reference system for gauging the impacts of possums and rats (Rattus spp.) on seed production in very similar forest at Maungakawa where these invasive mammals are common. This study compares phenology, fruit development and seed fall of three large-fruited species in the two forests: tawa (Beilschmiedia tawa) and mangeao (Litsea calicaris) are dominant canopy trees, and pigeonwood (Hedycarya arborea, porokaiwhiri) is an important subcanopy species. The reproductive cycles of tawa, mangeao and pigeonwood were extended (18 to 27 months), but the extended stage was different in each species: mangeao inflorescence buds developed over a nine-month period, pollinated tawa flowers were dormant over autumn before fruit emerged during the winter and ripened during the following summer. In pigeonwood part of the fruit crop remained on the trees until anthesis the following year. Despite almost concurrent tawa and mangeao anthesis, the resulting fruit crops matured in consecutive autumns: mangeao in 2020 and tawa in 2021. In the absence of rats and possums, tawa seed fall varied significant in consecutive years. Tawa seed fall in the first season (2020) was negligible at both forests. At Maungatautari seed fall was significantly more abundant in the second season (2021). Despite the successful pest control campaign significantly reducing possum abundance at Maungakawa in June 2020, seed fall did not increase significantly in 2021. There was evidence of green tawa fruit consumption by possums. They also consumed the flesh of ripe fruit in the trees and on the ground but discarded the seeds. Mangeao seed production was higher at Maungatautari than in the forest inhabited by rats and possums, although this effect fell short of statistical significance. Seed production at Maungatautari was abundant in 2020 with more than 4000 seeds captured in seed traps but in 2021 seed production was negligible. Seed production was reduced at Maungakawa in both years. In 2021 only two seeds were captured in each forest. The timing of mangeao’s reproductive cycle leaves the swollen inflorescence buds in winter especially vulnerable to damage by possums and possibly rats when food resources are low. Elimination of the possums rather than control at low abundance may be required to restore tawa seed production to historical levels. The underlying cause of low mangeao seed production is likely to be related to loss of inflorescence buds, but the role of arboreal invasive mammals requires further investigation. There was no evidence rats and possums reduce pigeonwood fruit production or destroy the seeds. However, by discarding seeds where they feed, rats and possums may limit seed dispersal across the spatial landscape. Further investigations are required to fully understand the extent to which the reproductive cycles of tawa, mangeao and pigeonwood are depressed and the consequences for the large-fruited species within Waikato hill-country forests. It is reassuring that abundant tawa and mangeao seed production has rebounded at Maungatautari within 15 years of eradication of invasive species. This suggests that loss of seed fall from vulnerable large-fruited species can be overcome by removal of invasive mammals.
Type of thesis
The University of Waikato
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