The biodiversity of remaining natural habitat remnants on a floodplain in northern New Zealand was assessed to prioritize opportunities for restoration. The floodplain has been extensively developed, mainly for dairy farming, and very little of the original forest and wetland ecosystems remains. Two ecosystems, lowland forest, and raised peat bog, were identified as being under serious threat, with ecological decline driven by weed invasion (forest) and loss of keystone species (bog). Regeneration of native forest remnants was being compromised by dense layers of the alien herb Tradescantia fluminensis, which carpeted the ground, especially in farm remnants fenced from stock. Peat formation in the bogs was being compromised by the absence of Sporadanthus ferrugineus, and only local occurrence of Empodisma robustum, both key peat-forming species in restiad (dominated by Restionaceae) raised bogs. We analysed peat cores to confirm historical presence of Sporadanthus, now locally extinct from northern New Zealand. To enhance recovery of these ecosystems, two restoration projects were set up: 1) weed control of forest remnants using biocontrol beetles; and 2) re-establishing/expanding peat-forming species in the largest bog remnant.
Results/Conclusions
A forest restoration experiment was established in March 2016 to compare two different approaches to management of Tradescantia, namely three biocontrol beetle species from Brazil (released as a trio), which target different parts of the plant, and hand clearing. We compare the impact of biocontrol beetles and manual clearing on Tradescantia growth, native invertebrate communities, and native plant regeneration, as well as providing source populations of the biocontrol beetles for wider control of the plant. Monitoring of browse damage indicates establishment of the beetles within 12 months. In the bog project, analysis of peat macrofossils revealed Sporadanthus originally occurred in restiad raised bogs on the floodplain, but had become locally extinct, probably during the land clearing activities in the early 1900s. Analyses of nutrients and peat physical characteristics indicate that environmental conditions in the bog are still suitable for the re-introduction of Sporadanthus and expansion of Empodisma. Restoration of hydrological regimes including blocking drains is currently underway, followed by a restoration experiment on translocating key peat forming species. These projects provide early promise of slowing or reversing biodiversity decline in floodplain habitat remnants.