Reliable detection of low-density Pacific rats by using camera trapping

Abstract

Context Long-term monitoring is essential for control and eradication of invasive mammalian predators. Relative abundance indices are increasingly used when assessing population changes. However, indexing assumes constant detectability, when, in fact, it varies depending on numerous factors, including the type and spacing of monitoring devices, seasons, vegetation types, and inter- and intra-specific interactions. Aims We studied a population of Pacific rat (Rattus exulans) and examined the influence of vegetation types, seasons and inter-specific interactions on their detection. Methods We deployed tracking-tunnels, live-traps, chew cards, and trail cameras in three vegetation types during summer and winter. Detection rates of Pacific rats, mice (Mus musculus), stoats (Mustela erminea) and weka (Gallirallus australis) were calculated and compared among vegetation types, seasons and devices. Key results Pacific rats were not detected by any monitoring devices in the farmland, despite their presence in this habitat. In the forest and shrubland, cameras had the highest detection rate among all of the monitoring devices, whereas live-trap and chew cards failed to detect rats. Tracking tunnels detected Pacific rats only in the shrubland. Camera detections of Pacific rats were lower in winter than in summer, and detections were dominated by weka and stoats for both seasons. The seasonal effects may have reflected the population cycle of Pacific rats. Weka and stoats caused substantial physical interference to the tracking tunnels, live-traps and chew cards, which is likely to have deterred Pacific rat interactions through imposing high predation risks. Conclusions Cameras were the most effective device at detecting Pacific rats in low density and under high predator disturbances. Tracking tunnels and chew cards that are successful at detecting other Rattus spp. might not be reliable for detecting Pacific rats. Implications We recommend using camera monitoring for rodents where population density is low, or predator disturbance is high, and species are identifiable from pictures. Studies that draw inferences from relative abundance indices on rodents should exercise caution when rodent detectability is unknown.