relative abundance

Invasive ship rats (Rattus rattus) pose a threat to the biota of Goat Island (9.3 ha), New Zealand. In June 2016 we installed 10 Goodnature A24 CO2 powered self-resetting rat and stoat traps equipped with digital strike counters (Goodnature Ltd., Wellington, NZ) to control rat numbers on the island. The self-resetting traps were monitored with motion-activated cameras to develop a measure of rat abundance from camera traps. All devices were checked on 10 occasions from August 2016 to October 2017. The videos revealed high rat activity on the island, which reduced over time.

Abstract: The Department of Conservation has implemented a Biodiversity and Monitoring Reporting System (BMRS) that estimates occupancy rates and relative abundances of introduced brushtail possums (Trichosurus vulpecula) at a representative sample of sites on public conservation land. Leg-hold traps have been used to monitor possums in the BMRS, but wax tags and chew cards have logistical and financial advantages over traps.

Chew-track-cards (CTCs) are potentially a cost-effective way to estimate the relative abundance of invasive rats and possums in New Zealand, but previous research suggested that their high sensitivity may limit use to low-density populations. Using a short two-night deployment period, we compared CTC indices of rat and possum abundance with a footprint tracking rate (RTR) index of rat abundance and a wax tag bite rate index (WTI) of possum abundance in 11 forest remnants that varied widely in rat and possum abundance (RTR and WTI of 0–100% over two nights).