Factors leading to successful island rodent eradications following initial failure

Araceli Samaniego,Gérard Rocamora,David Will,Grant Harper,Keith Springer,Pete Mcclelland,Shane Siers,Richard Griffiths,Keith Broome,Peter Kappes,Russell Palmer,Steve Cranwell

doi:10.1111/csp2.404

Abstract

AbstractIsland rodent eradications are increasingly conducted to eliminate the negative impacts of invasive rodents. The success rate in the tropics has been lower than in temperate regions, triggering research and reviews. Environmental factors unique to the tropics (e.g., land crabs and year‐round rodent breeding) have been associated with eradication failure. Operational factors have also been important, but these have not been comprehensively assessed. The environmental and operational factors using global cases where rodent eradication initially failed and subsequent attempts occurred were compared. It was determined whether operational factors explained the initial failures, whether operational improvements explained subsequent successes, and whether re‐attempting eradication after failure was worthwhile. About 35 eradication attempts on 17 islands, each with 1–2 species from a total of 5 species (Mus musculus and 4 Rattus spp.) were identified. On 14 islands (82%), eradication was achieved on the second (86%) or third attempt (14%). On the remaining 3 islands, eradication was not achieved. Evidence of operational faults for all failed attempts was found (e.g., poor planning, low quality bait, and gaps during bait application). In some cases, operational faults were unequivocally the cause of failure, but in others, it was impossible to discriminate from confounding, environmental factors. Nonetheless, failures appeared to be mainly the result of not exposing all rodents to a lethal dose of toxin, violating a crucial eradication principle. This can cause operational failure on any temperate or tropical island. However, there may be less tolerance for errors such as gaps in bait coverage on tropical islands, mainly due to bait consumption by land crabs. The findings on factors leading to eradication success (e.g., expert reviewed plans, realistic funding and permits, high standard baiting operations) reflect current best practice recommendations. Strict adherence to best practice is expected to increase overall rates of eradication success.

Highlights

Reviews so far have focused on the latter (Griffiths et al, 2019; Holmes, Griffiths, et al, 2015); we focused on the former and set out to investigate the role of operational factors as causes of eradication failure
Of the 17 islands with two or more rodent eradication attempts, success was achieved on 14 islands (82%; range 5–1,020 ha) at the second (86%) or third attempt (14%) (Table 1), despite 9 of these islands (64%) having one or more high risk environmental factors (Appendices S1 and S2)
Our findings indicate that many eradication failures can be attributed to human error

Summary

Introduction

Invasive rodents (Mus musculus, Rattus exulans, R. norvegicus, R. rattus, and R. tanezumi) have been inadvertently spread around the globe by humans; their detrimental impacts on island ecosystems (Angel, Wanless, & Cooper, 2009; Kurle, Croll, & Tershy, 2008; St Clair, 2011; Towns et al, 2009; Towns, Atkinson, & Daugherty, 2006) and the benefits of their removal (e.g., Bellingham et al, 2010; Jones et al, 2016; Rocamora & Henriette, 2015; Towns, 2009; Towns, 2011) are well documented. Pioneered in New Zealand, rodent eradications were largely accidental at first (1960–1976), when rodent reduction efforts unexpectedly resulted in complete extirpation of the target species. Following New Zealand developments, eradications have had a similar history elsewhere (e.g., Rocamora & Henriette, 2015; Samaniego et al, 2011), with increasing success rates over time despite increasing island size (Figure 1). About 600 islands have been cleared of invasive rodents (DIISE, 2019), with many projects comprising complex multi-species eradications (e.g., Macquarie and South Georgia Islands, Springer, 2018; Martin & Richardson, 2019) or operations in challenging habitats such as mangroves (Samaniego et al, 2018). Advances in methodology (e.g., use of helicopters to spread second generation anticoagulants using GPS guidance), confidence from past successes, and positive outcomes driving funding have allowed such increases in size and complexity (Holmes et al, 2015; Howald et al, 2007; Russell & Broome, 2016)

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Results

Conclusion