ZIP’s predator elimination model involves removing predators from a specific area and then protecting it from reinvasion. After the elimination operation, we intensely monitor the area and quickly respond to any predator detections using a range of techniques, one of which is the targeted application of aerial 1080 around the detection site.
Native birds in the Perth River Valley are recovering just as well in areas that have had one or more of these localised aerial treatments, when compared to surrounding areas that were only treated during the main elimination operation.
Earlier this year we reported positive signs of bird recovery in the Perth River Valley following our operation to eliminate possums, rats and stoats. In this update, we take a closer look at how this recovery looks in areas of the valley that have received different numbers of toxin applications. This work is part of the Predator Free South Westland project.
Setting up a trail camera. A MotoLure that dispenses mayonnaise is placed in the camera’s field of view to attract predators. Photo by Chad Cottle.
In 2019 we removed over 99% of predators in the Perth River Valley by applying aerial 1080 bait to all potential habitat in an elimination operation. We always anticipated that some predators might survive the operation or could reinvade the area over time; after all, an elimination approach requires us to protect the conservation gains we make. Our network of 142 lured trail cameras helps us detect these individuals so we can respond quickly to remove them.
Possums sneaking a snack of mayonnaise are caught in the act by a trail camera.
Usually, our first response to a predator detection is a ground-based operation using bait stations, traps and detection dogs. In some cases, however, the terrain makes this approach impractical and/or a ground-based response is not enough to prevent the emergent predators from spreading. When this happens, we use an aerial treatment of 1080 bait precisely targeted to the localised area where predators have been detected. The confluence of the Perth and Barlow Rivers seems to be particularly prone to reinvasion of rats from across the Barlow River, and has been treated on numerous occasions over the past three years (shaded dark grey in the map below).
Perth River Valley predator elimination project area. Grey shading shows the frequency of localised aerial treatments since 2019. Each dot represents a trail camera set up in front of a lure dispenser.
We sometimes get asked if this use of localised aerial treatments is slowing the recovery of native birds present in the Perth River Valley site. There is plenty of scientific evidence that native bird populations do better when predators are controlled (see references, below). Here, we investigated our camera network to see if there was any difference in native bird recovery between areas that had been treated multiple times and areas that only received the initial elimination treatment. Although trail cameras are primarily used to detect predators, they also detect native birds that happen to pass by. When we compare the proportion of cameras that detect birds, at the same sites, over time, we can get an indication of the relative size of these populations.
We looked at three species— kea, kakaruai/South Island robin, and ngirungiru/South Island tomtit. These species are known to be vulnerable to predation and we therefore expect their populations to recover when predators are removed. They are also the species that spend a lot of time on the forest floor and, because the cameras are low to the ground, are detected most consistently. We know that detections fluctuate over the year, peaking in summer with new fledglings. By comparing two late summer months, these seasonal variations are minimised.
We compared the number of cameras that detected birds in January 2020 (one month before any localised treatments took place) with February 2022 (one month after the most recent treatment for which we have analysed data) in areas that had received either none, between one and two, and three or more localised treatments.
Trail camera detections in January 2020 and February 2022 of kea, kakaruai/South Island robin, and ngirungiru /South Island tomtit in areas of the Perth River Valley that have received different numbers of aerial treatments. Numbers of cameras in each area ranged: 0 treatments, n=25; 1-2 treatments, n=54; and 3+ treatments, n=42
In areas that have received localised aerial treatments, more cameras are detecting kea and ngirungiru than two years ago. Kakaruai detections have remained reasonably stable, which is not surprising given the relatively low number of detections. The very low numbers (or lack) of species detections in areas that have not been treated beyond the initial elimination operation was surprising. However, this result may be explained in part by the habitat type most of these cameras were in (higher altitude in some cases, and the narrow, colder Upper Perth River Valley) where we would expect to find fewer native birds. Across the whole Valley, predator numbers are close to zero, so predation is unlikely to be a factor.
These camera detection results support the increase in birdlife that our field team are seeing and experiencing first-hand. Native bird populations continue to recover in the Perth River Valley, and the localised treatments to maintain predator freedom are not hindering that recovery.
A pair of kea show off in front of a lured trail camera, June 2022
Acknowledgements
Thank you to Matt Hickson of the Department of Conservation for a review of an earlier draft of this update.
References
Kemp, J. R., Mosen, C. C., Elliott, G. P., & Hunter, C. M. (2018). Effects of the aerial application of 1080 to control pest mammals on kea reproductive success. New Zealand Journal of Ecology, 42(2), 158–168.
Miskelly, C. M., Greene, T. C., McMurtrie, P. G., Morrison, K., Taylor, G. A., Tennyson, A. J. D., & Thomas, B. W. (2021). Species turnover in forest bird communities on Fiordland islands following predator eradications. New Zealand Journal of Ecology, 45(2), 1–15.
Schadewinkel, R. B., Senior, A. M., Wilson, D. J., & Jamieson, I. G. (2014). Effects on South Island robins (Petroica australis) from pest control using aerially applied 1080 poison. New Zealand Journal of Ecology, 315–321.
van Heezik, Y., Ray, S. M., Jamieson, I. G., Allen, O., & Schadewinkel, R. (2020). Impacts of aerial 1080 predator control on nest success and adult survival of South Island robins. New Zealand Journal of Ecology, 44(2), 1–11.
