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

Abstract

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'.

Eradication confirmation of mice from Antipodes Island and subsequent terrestrial bird recovery

“…Chew-cards and wax-tags are regularly used for rodent abundance index surveys (Sweetapple et al 2006;Wilmshurst & Carpenter 2020), but tracking tunnels were the only detection device used on Antipodes Island to minimise identification ambiguity. There is a high level of confidence identifying mice footprints, but bite marks can be hard to distinguish (Thomas 1999;Olivera et al 2010).…”

“…2012) and on Adele Island, New Zealand wētā created marks on wax tags that confounded results of the survey for mice (Livingstone et al 2022). Given individual rodents vary in their susceptibility to detection devices (Wilmshurst & Carpenter 2020) the use of detection trained dogs and human observations were an important contribution to the confidence that non-detection of mouse sign equated to complete absence of mice. The detection dogs were all small bodied shortlegged breeds and they struggled in areas of tall Poa litorosa vegetation and Polystichum fern, which also hindered humans.…”

Biotic Threats to Cycas micronesica Continue to Expand to Complicate Conservation Decisions

“…A parasitoid survey should be conducted on Rota, and the parasitoids from Guam should be released on Rota if needed. More research on the frequent Rota C. micronesica seed gnawing by R. rattus may be useful for informing global island conservation efforts [ 89 ].…”

The Contribution of Kurī (Polynesian Dog) to the Ecological Impacts of the Human Settlement of Aotearoa New Zealand

“…However, potential feral populations may be cryptic in the recent fossil record given hypothesised low population densities (cf. sizeable human-dependant populations), especially in isolated areas where taphonomic settings are not conducive to preservation (e.g., high rainfall Fiordland), compared to widespread kiore-gnawed seeds in sediment cores and kiore bones in natural subfossil deposits indicative of high abundance populations (e.g., Holdaway and Worthy, 1996;Wilmshurst and Higham, 2004;Wilmshurst et al, 2008;Wilmshurst and Carpenter, 2020). It may be that in these isolated, rugged areas where human population density was lower (e.g., Jacomb et al, 2010;Waters et al, 2017) that the combined effects of kurī, potentially exhibiting pack hunting behaviour, and kiore, had a greater impact on native species than human hunting ever did.…”