Targeted Mop up and Robust Response Tools Can Achieve and Maintain Possum Freedom on the Mainland

Abstract

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.

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

“…At the removal stage, this will require actions targeted at individuals resistant to broadscale control methods (Cruz et al, 2009) (e.g., using detection dogs to find trap‐shy individuals) (Gsell et al, 2010). Achieving complete elimination is a challenge faced by many elimination campaigns (Cruz et al, 2009; Murphy et al, 2019), and our findings reinforce the importance of the active research in this field (e.g., Bell et al, 2019; Cook & Mulgan, 2022; Garvey et al, 2020). Our findings also emphasize the importance of fostering social conditions that allow access to all properties harboring possums (Glen et al, 2017), as even one property supporting a moderate density of possums could cause repopulation.…”

“…Resource constraints require managers to balance the relative cost of actions that ensure complete elimination from a management zone, prevent reinvasion, or rapidly detect incursions (Moore et al, 2010; Rout et al, 2011). Achieving elimination might require substantial up‐front investment and innovation (Cook & Mulgan, 2022; Cruz et al, 2009; Robertson et al, 2017), while maintenance suppression of a residual population can be a significant long‐term investment (Norbury et al, 2014). Similarly, common methods to prevent reinvasion such as arrays of traps or bait along reinvasion routes (buffer zones) (Lustig et al, 2019) or controlling the population of potential immigrants adjacent to the containment area (“source population control”) (Morgan et al, 2006) require commitment for an ongoing, potentially indefinite, period.…”

“…Here, we employ a spatially explicit agent‐based model to predict the reinvasion of an invasive mammal, the Australian common brushtail possum ( Trichosurus vulpecula ), across an urban environment in Aotearoa New Zealand. Possums are the target of a national pest eradication initiative (Predator Free 2050) (Russell et al, 2015) and are beginning to be eliminated from large, unfenced sites on the main islands of the country (Cook & Mulgan, 2022; but see Barron et al, 2023). They are found in cities throughout New Zealand (Adams et al, 2013; Miller et al, 2022; Morgan et al, 2009), and their ecology in New Zealand is well characterized (Cowan & Glen, 2021), presenting a unique opportunity to parameterize an agent‐based model.…”

Rodent management in Aotearoa New Zealand: approaches and challenges to landscape‐scale control

“…In fact, eradication as defined by IUCN (“…complete and permanent removal of all wild populations of a species from a defined area by means of a time‐limited campaign” Genovesi 2001 after Bomford & O'Brien 1995) is currently impossible because reinvasion cannot be stopped with current tools other than fences. Some PF2050 projects now instead aim at “elimination” meaning that “…a small number of animals may exist within a predator‐free site until they are detected and removed” (Cook & Mulgan 2022). At sites seeking suppression, target indices or densities to which species should be suppressed are not stated.…”

Small mice create big problems: Why Predator Free New Zealand should include house mice and other pest species

“…However, PF2050 has been criticized for shortfalls in eradication technology, insufficient finance, low scalability of existing tools, and inadequate consideration of social and ecological complexity, each of which erodes viability of the initiative (King, 2023; Leathwick & Byrom, 2023; Linklater & Steer, 2018; Monks et al., 2023; Parkes et al., 2017; Peltzer et al., 2019). National biosecurity, that is, safeguarding from repeated international pest invasions, for example, has not been assessed despite being a prerequisite for eradication (Kennedy & Broome, 2019), and NZ's ability to manage pest reinvasions locally and regionally remains questionable (Barron et al., 2023). Between 2016/17 and 2020/21 NZ$306.28 million has been spent under the Predator Free umbrella, mostly on pest control which reduces to NZ$212.28 million if investments in habitat restoration and targeted biodiversity protection are subtracted (DOC, 2021).…”