Rodent detection and monitoring for conservation on islands: gnawed seeds provide reliable indicator of rodent presence

Invasive rodents pose one of the biggest threats to island ecosystems globally. Reliable methods for detecting and monitoring rodent presence are essential for the effective conservation management of islands, but many detection devices fail to attract rodents when natural resources are abundant. Using a toolbox of detection methods is therefore key to detecting rodents as individual rodents vary in their susceptibility to detection devices. Rodents are well-established seed predators, and the distinct gnaw marks they create and leave on woody seedcases potentially add another method to the rat detection toolbox on islands where seeds are sufficiently large to preserve teeth marks. We tested the reliability of rodent-gnawed miro (Prumnopitys ferruginea) seeds as an indicator of rodent presence on fifteen islands and one mainland site in southern New Zealand. Seeds were collected from beneath one miro canopy at each site and examined for characteristic gnaw marks. Presence or absence of gnaw marks was compared with records of rodent presence obtained using traditional methods including tracking tunnels, kill traps, and/or rodent detection dogs. Measurements of bite marks on seeds suggested that mice create smaller bite marks on seeds than rats, allowing discrimination between the taxa. Gnawed seeds had a slightly lower probability of detecting rats than traditional methods (rats detected at 5/6 sites where they had been previously recorded using other methods), and a higher probability of detecting mice where they existed (mice detected at 7/8 islands where they existed, vs 5 for other methods). These results suggest that gnawed seeds can complement other rodent detection devices to increase the probability of detection. The method could be used to detect rodent presence on other island groups globally, e.g. using opened coconuts in the field as a kind of natural waxtag or ‘cocotag’.