Effects of possums and rats on seedling establishment at two forest sites in New Zealand

Introduced rodents and possums in New Zealand eat flowers, fruits, seeds and seedlings, but little is known about their impact on forest regeneration. We investigated seedling establishment in exclosures with mesh of two different sizes to exclude (1) possums and (2) possums and rats, at two mainland forest sites (beech–podocarp–broadleaved and second-growth broadleaved–podocarp) near Dunedin. We recorded all new woody seedlings that established over the next 2 years. The number of seedlings with true leaves differed significantly between treatments after 1.5 years at both sites and after 2 years in beech–podocarp–broadleaved forest. This effect was broadly consistent across all species including pepper tree (Pseudowintera colorata), whose adult foliage is unpalatable to possums. Cotyledonous seedlings were relatively ephemeral, but differed significantly in abundance between treatments in second-growth broadleaved–podocarp forest after 1.5 and 2 years. In second rowth broadleaved–podocarp forest, possums were present throughout the study but rats were rare. Numbers of seedlings did not differ significantly between exclosures with different mesh sizes which admitted or excluded rats. In beech–podocarp–broadleaved forest, rats were present periodically throughout the study, but possums may have been scarce during the final 7 months as a result of pest control. At this location, 80 seedlings with true leaves occurred in exclosures that excluded possums and rats, 3.6 times as many as on control plots and 2.1 times as many as in exclosures that deterred only possums. The consequences of these pest impacts on seedling recruitment for forest regeneration must be confirmed in longer-term studies. Exclosures can be effectively used to experimentally separate the impacts of different herbivores on seedling establishment.