Efficacy of chew-track-card indices of rat and possum abundance across widely varying pest densities

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). The CTC indices were strongly correlated with the WTI and RTR and were no more sensitive than these measures, and they showed little indication of saturation at high pest abundances. We found no evidence that rat interference altered possum bite rates, as had been observed for longer deployment periods. CTCs, deployed for two nights, are a promising tool for use over a wide range of pest abundances. Further research is required to examine whether rat interference is ever sufficiently high to obscure possum sign, and to confirm that the index can reflect meaningful variation in population density.