Abstract

Protected natural forests are critical for biodiversity conservation, but they are threatened by pervasive drivers of change that may not be mitigated by legal protection. Selective herbivory by non-native or hyper-abundant browsing mammals can affect forest composition and structure by suppressing palatable species and facilitating avoided species. Impacts can be severe, especially on islands, like Aotearoa New Zealand (NZ), which historically lacked herbivorous mammals. We used a national network of permanent plots with a 12-year record of stem census data to determine how species-level dietary preference and site-level abundance of non-native herbivorous mammals (possums and ungulates) were associated with mortality rates and recruitment rates into a 2.5 cm diameter class among common tree species in forests with different levels of legal protection. National parks (NPs) had higher legal protection and lower herbivorous mammal indices than non-national parks (NNPs) but across both there were 1702 live stems of species palatable to possums and ungulates recorded when plots were first measured but 1502 in the latest measurements. Mortality of these species exceeded their recruitment in both NPs and NNPs. The total tree population was stable, but while most species palatable to ungulates and possums were in decline, some species avoided by the herbivores had growing populations, with recruitment rates exceeding mortality. At current levels of abundance, non-native herbivorous mammals are driving significant compositional shifts in tree populations of both NPs and NNPs. Long-term (decadal) suppression of their abundance is needed to preserve the ecological integrity of NZ's protected forests.