Detection parameters for managing invasive rats in urban environments

Abstract

Effective mitigation of the impacts of invasive ship rats (Rattus rattus) requires a good understanding of their ecology, but this knowledge is very sparse for urban and peri-urban areas. We radiomarked ship rats in Wellington, New Zealand, to estimate detection parameters (σ, ε0, θ, and g0) that describe the process of an animal encountering a device (bait stations, chew cards and WaxTags) from a distance, and then approaching it and deciding whether to interact with it. We used this information in simulation models to estimate optimal device spacing for eradicating ship rats from Wellington, and for confirming eradication. Mean σ was 25.37 m (SD = 11.63), which equates to a circular home range of 1.21 ha. The mean nightly probability of an individual encountering a device at its home range center (ε0) was 0.38 (SD = 0.11), whereas the probability of interacting with the encountered device (θ) was 0.34 (SD = 0.12). The derived mean nightly probability of an individual interacting with a device at its home range center (g0) was 0.13 (SD = 0.08). Importantly, σ and g0 are intrinsically linked through a negative relationship, thus g0 should be derived from σ using a predictive model including individual variability. Simulations using this approach showed that bait stations deployed for about 500 days using a 25 m × 25 m grid consistently achieved eradication, and that a surveillance network of 3.25 chew cards ha−1 or 3.75 WaxTags ha−1 active for 14 nights would be required to confidently declare eradication. This density could be halved if the surveillance network was deployed for 28 nights or if the prior confidence in eradication was high (0.85). These recommendations take no account of differences in detection parameters between habitats. Therefore, if surveillance suggests that individuals are not encountering devices in certain habitats, device density should be adaptively revised. This approach applies to initiatives globally that aim to optimise eradication with limited funding.

Eradicating an invasive mammal requires local elimination and reduced reinvasion from an urban source population

“…However, we acknowledge that there are gaps in our understanding of urban landscapes and the organisms that occupy them that will affect model outputs. We used mean values of home range size and trapping parameters ( and ), despite knowing that these vary by individual, across habitats, and by device type (Anderson, Rouco, et al, 2022; Latham et al, 2022; Mackenzie et al, 2022). Indeed, new behavioral syndromes can be selected for in urban areas (Lowry et al, 2013), potentially leading to individuals that are more or less likely to be trapped (bold or wary) (Garvey et al, 2020).…”

Spatial ecology of invasive Bennett’s wallaby in South Island, New Zealand

“…In addition to advantages for modelling, there is also the potential for direct operational gains by using the information from our home range, resource selection, and movement analyses to improve the effectiveness and efficacy of control tools, such as optimising the spatial deployment of aerially delivered toxic baits (as has been done for possums (Nugent et al 2012) and rabbits (Latham et al 2016)), and of bait stations (as has been done for ship rats (Rattus rattus; Mackenzie et al 2022)). This is particularly relevant to the aerial application of 1080 baits, when baits are strip (trickle) sown and need to be placed in areas that have the highest use by the target species, a requirement that may be needed for Bennett's wallaby.…”

“…2012) and rabbits (Latham et al . 2016)), and of bait stations (as has been done for ship rats ( Rattus rattus ; Mackenzie et al . 2022)).…”

“…For example, understanding the spatiotemporal population dynamics of the pest can optimise the timing and likelihood of success of an eradication attempt (Harper et al 2015). Similarly, home range information can be used to quantify detection and trappability to guide the optimal spacing and placement of survey devices, traps, and bait stations (Efford 2004;Mackenzie et al 2022). Spatial ecology can highlight habitats and geographical features that may permit reinvasion by the pest.…”

“…2015). Similarly, home range information can be used to quantify detection and trappability to guide the optimal spacing and placement of survey devices, traps, and bait stations (Efford 2004; Mackenzie et al . 2022).…”