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How removal of cats and rats from an island allowed a native predator to threaten a native bird

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Abstract

The question of how to program the removal of two invasive mammals, typically cats and rats, from a marine island without increasing risk to native prey species has received two general answers based on ecological theory: removal of cats must be accompanied by control of their mesopredator prey, and risk is minimized by removing both invaders simultaneously. Nonetheless, a 31-year study showed that in a 82-ha tropical marine bird sanctuary, predation on a native prey, the blue-footed booby, by a native predator, the Atlantic Central American milk snake, apparently diminished after removal of cats then increased 11-fold after the additional removal of black rats. These novel effects are explained in terms of a hypothetical three-link trophic web in which cat removal released rats to increasingly compete with or prey on the snakes that feed on hatchling boobies, and subsequent rat removal released snakes from all remaining predation. The upshot is a disturbing scenario in which approximately 200 milk snakes currently aggregate annually in roughly 1 hectare of booby colony and predate roughly forty percent of the hatchlings. Where the lowest link of an insular trophic web is a native mesopredator that feeds on native prey, the predictions of the classic mesopredator release scenario can be inverted, and removal of invasive mammals may endanger native prey species.

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Availability of data and materials

Dataset is available as supplementary material.

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No custom code was used for analyses, all functions in statistical analyses were acknowledged and respective R packages were cited.

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Acknowledgements

J.L. Osorno and R. Torres helped monitor the booby population; L. Aguilar, M. Peralta, C. Cortés and N. Sori helped monitor the snake population; M. Salmeron advised on handling and marking snakes; and M. Brooke and H. Greene commented on the manuscript. Logistical support was provided by the Mexican Navy, the fishermen of Nayarit and the staff of the Parque Nacional Isla Isabel. Permissions to work on Isla Isabel were granted by the Secretaría del Medio Ambiente y Recursos Naturales.

Funding

Finance was provided by the Universidad Nacional Autónoma de México (PAPIIT Grants IN211491, IN-200702-3, IN206610-3, IN205313 and IN205819), the Consejo Nacional de Ciencia y Tecnología (Grants 81823, 47599, 34500-V, 4722-N9407 and 104313) and the National Geographic Society (Grant 991416).

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Authors and Affiliations

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Contributions

SO: Writing—review & editing, Methodology, Formal analysis, Software, Visualization, Supervision. CR: Conceptualization, Writing—review & editing, Investigation, Data curation, Supervision, Project administration. BM-H: Investigation – data collection, Writing—review & editing. HD: Conceptualization, Methodology, Writing – original draft, Investigation, Funding acquisition, Project administration.

Corresponding author

Correspondence to Santiago Ortega.

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The authors have no competing interests to declare.

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Ethics approval

The study was conducted in Parque Nacional Isla Isabel under the permit SGPA/DGVS/012166/17.

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Ortega, S., Rodríguez, C., Mendoza-Hernández, B. et al. How removal of cats and rats from an island allowed a native predator to threaten a native bird. Biol Invasions 23, 2749–2761 (2021). https://doi.org/10.1007/s10530-021-02533-4

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  • DOI: https://doi.org/10.1007/s10530-021-02533-4

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