As New Zealand attempts to become predator free by 2050, transitioning aerial 1080 (sodium fluoroacetate) operations from predator control to complete predator removal has become an important research objective. Aerial 1080 operations may not remove every last target animal, but they may be able to remove a very high proportion (> 0.99). We trialled a modified [dual] aerial 1080 operation for the removal of brushtail possums (Trichosurus vulpecula), ship rats (Rattus rattus), and stoats (Mustela erminea) at large spatial scale. Our trial involved two toxin applications (TA#1 & TA#2), each preceded by two applications of non-toxic pre-feed bait. Different types of cereal baits and lures were used for each toxin application. Camera traps (n = 142; 200-1431 m asl) were used to detect the target species and to incidentally observe house mouse (Mus musculus), birds, and ungulates across the study area. The operation had no significant impact on the number of cameras detecting most birds and ungulate species; however, house mouse, blackbird (Turdus merula), and red deer (Cervus elaphus) were detected on significantly fewer cameras after TA#1 than before. Numbers of cameras that detected target species were significantly reduced for all three target species after TA#1, and significantly reduced again for possums after TA#2, when no stoats were detected on cameras and a small number of cameras detected rats. Mean relative abundance estimates based on camera trap indices (proportion of camera trap days that target species were detected) showed the operation appears to have removed over 99% of ship rats, stoats, and possums.

ContextEradication of invasive species is necessary to protect and assist the recovery of native species and ecosystems. Knowing when to declare an eradication has been successful after ongoing non-detections is a challenge. AimsThe rapid eradication assessment (REA) model is a powerful simulation framework to determine, given model parameters and a fixed level of monitoring effort, the level of confidence in declaring the success of pest eradication. The aim of the present study was to extend the current functionality of the REA model for broader applicability. MethodsThe REA model was advanced so that it was able to account for (1) usage of multiple static device types with different probabilities of detection, (2) incursion detection at a known location and (3) usage of mobile detection devices, which are increasingly being used in conservation. Key resultsAn invasive rat incursion response on Great Mercury Island in New Zealand is used as a comprehensive example to demonstrate the distribution of estimated probability of pest absence among the cases using the current REA model and the extensions presented here. ConclusionsAlthough Great Mercury Island already had a sparse but extensive island-wide network of static biosecurity surveillance devices, and deployed additional static devices around the area of incursion, the greatest improvement in the estimated probability of pest absence following a rat incursion was from additionally using a trained rodent-detection dog. ImplicationsThe added functionality in the REA model and demonstration of its use on a real-world scenario will allow more realistic application by wildlife managers.

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Unfenced sites on mainland New Zealand have long been considered impossible to defend from reinvasion by possums, and are thus unsuitable for eradication. In July 2019, we began eliminating possums from 11,642 ha (including approximately 8700 ha of suitable possum habitat) in South Westland, using alpine rivers and high alpine ranges to minimise reinvasion. Two aerial 1080 (sodium fluoroacetate) applications, each with two pre-feeds, were used. Here, we detail the effort to mop up existing possums and subsequent invaders in the 13 months following the aerial operation. Possums were detected and caught using a motion-activated camera network, traps equipped with automated reporting and a possum search dog. The last probable survivor was eliminated on 29 June 2020, 11 months after the initial removal operation. Subsequently, possums entered the site at a rate of 4 per year. These were detected and removed using the same methods. The initial elimination cost NZD 163.75/ha and ongoing detection and response NZD 15.70/ha annually. We compare costs with possum eradications on islands and ongoing suppression on the mainland.

Dispersal is a fundamentally important aspect of animal behaviour, but empirical data describing it are lacking for many species. Here, we report on a field study aimed at measuring post-weaning movement distances of juvenile ship rats (Rattus rattus) and their mother away from a known natal nest site in an area with low conspecific population density. The movement behaviour of invasive species at low density is of particular interest, as it can inform design of surveillance arrays to detect incursion into predator-free areas. Detecting a single invading individual requires intensive effort. An alternative approach is to focus on detecting newlyestablished breeding populations, while they are still spatially-restricted and able to be eliminated with timely and effective incursion response. We released a bio-marked rat mother and litter into an area recently treated with sodium fluroacetate (1080) and monitored their behaviour for 12 weeks. Final capture locations ranged up to 675 m from the release location for the juveniles, with 796 m between known siblings. The total range length for the mother exceeded 1.5 km. Although we found no evidence that the movements of the family as a collective extended further than those of the mother alone, the concept of targeting detection efforts to breeding populations warrants further investigation due to the improved probability of detecting at least one of multiple individuals, rather than a single invader.