Risks posed by rat reproduction and diet to eradications on tropical islands

Abstract

Removing invasive rodents from islands has many ecological and social benefits. However, eradications fail more frequently on tropical than on temperate islands, and causes for these failures are not yet well understood. We addressed two major plausible reasons for eradication failure, testing whether actively reproducing females and pre-weaned pups may not consume bait during an experimental eradication of Rattus exulans on a tropical island (22 ha). We tested four hypotheses, namely that: (1) lactating female rats take longer than non-lactating conspecifics to consume rodent bait, (2) pre-weaned rats orphaned following bait application survive when their mother dies, (3) surviving pre-weaned rats are not exposed to rodent bait and (4) lactating females have different natural diets than non-lactating females. Our experimental eradication was successful, despite conditions that are typically associated with eradication failures: rat density was high (95% CI 65–153 rats/ha), rats were breeding and juveniles of all age-classes were present, land crabs were abundant, bait was only widely available for two nights after bait application (i.e. > 90% of bait disappeared within 36 h; bait was undetectable after 60 h), and alternative foods (e.g. coconut) were abundantly available. We found no evidence to support hypotheses that rat breeding and diet specialisation limit bait uptake because all rats consumed bait. Rat eradication was achieved with a relatively low bait application rate (32 kg/ha; 16 kg/ha per application) and a short (7 day) interval between the two bait applications. We recommend further experimental eradications targeting R. exulans and other invasive Rattus and Mus species on larger islands, but conclude that with comprehensive bait coverage eradications on tropical islands can be successful despite abundant natural food and a high density of reproductively active rats.

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Source: World Weather Online, Moorea station

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Acknowledgements

We thank SOP-Manu for kindly recruiting volunteers, all the volunteers for their hard work despite difficult conditions, Tetiaroa Society for local support, Te Mana o Te Moana for sharing results on their monthly coconut crab surveys and Carl Smith for disseminating our work through ABC/BBC science podcasts and articles. We also thank the Délégation à la Recherche de la Polynésie française and the Haut-Commissariat de la République en Polynésie française for providing the main author a research permit for French Polynesia. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the opinions of the funding institutions. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement.

Funding

This project was supported logistically and financially by Island Conservation, the Royal Society for the Protection of Birds, University of Auckland and Tetiaroa Society (Grant Number Tetiaroa Phase I). These funding sources, through their employees (AS, RG, NH, SO, BS, JR), were involved in study design, collection of data, analysis and interpretation of data, writing of the manuscript and decision to submit the article for publication. The project was also financially supported by National Fish and Wildlife Foundation and The David and Lucile Packard Foundation (Grant Number 59943). These funding sources were not involved in study design, collection of data, analysis and interpretation of data, writing of the manuscript or decision to submit the article for publication.

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AS, RG, NH, JR designed research. AS, MG, JR collected data. BS, AS analysed data. AS wrote the manuscript. All authors provided comments and edits on the manuscript. All authors declare no conflict of interest. This research was conducted under University of Auckland Animal Ethics Committee approval (R1677).

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Correspondence to Araceli Samaniego.

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Samaniego, A., Griffiths, R., Gronwald, M. et al. Risks posed by rat reproduction and diet to eradications on tropical islands. Biol Invasions 22, 1365–1378 (2020). https://doi.org/10.1007/s10530-019-02188-2

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Keywords

  • Best practice
  • Brodifacoum
  • Crabs
  • Eradication failure
  • Rat ecology
  • Rattus exulans