Evaluation of two relative-abundance indices to monitor brushtail possums in New Zealand

Population monitoring of brushtail possums (Trichosurus vulpecula) is an essential part of their management in New Zealand, with a trap-catch removal method being used most commonly. An alternative monitoring method (bait-interference), using bite marks on wax blocks, has been promoted as a more cost-effective alternative to using traps. However, neither of these methods has been validated. We assessed the utility of these two methods regarding their accuracy ( unbiasedness and precision) in detecting changes in possum abundance by comparing the estimates of relative change in possum density following control obtained from both methods with the kill rate among radio-collared possums in the same study areas. In each of seven control operations, 48 - 50 possums were collared with mortality-sensing radio-transmitters, and trap-catch and wax-block lines were assessed before and after control. The correlation between trap-catch and radio-transmitter kill estimates (R-2 = 0.91) suggests that trap catch, as currently used to monitor relative possum abundance, appears to be sufficiently accurate to manage these pests, with any bias being small. The kill estimates based on the wax-block monitoring were correlated less strongly with the radiotransmitter estimates than the trap-catch estimates (R-2 = 0.66), although still significant. Until the extent of the potential bias in the wax-block estimates is known, we recommend that traps continue to be used as the main method to monitor possum abundance.