Recovery of introduced Pacific rats following a failed eradication attempt on subtropical Henderson Island, South Pacific Ocean

Rodent eradications in tropical environments are often more challenging and less successful than those intemperate environments. Reduced seasonality and the lack of a defined annual resource pulse influence rodent populationdynamics differently than the well-defined annual cycles on temperate islands, so an understanding of rodent ecologyand population dynamics is important to maximise the chances of eradication success in the tropics. Here, we report onthe recovery of a Pacific rat (Rattus exulans) population on Henderson Island, South Pacific Ocean, following a failederadication operation in 2011. We assessed changes in the rat population using capture rates from snap-trapping andinvestigated seasonality by using capture rates from live-trapping. Following the failed eradication operation in 2011, ratpopulations increased rapidly with annual per capita growth rates, r, of 0.48–5.95, increasing from 60–80 individuals totwo-thirds of the pre-eradication abundance within two years, before decreasing (r = -0.25 – -0.20), presumably as thepopulation fluctuated around its carrying capacity. The long-term changes in rat abundance may, however, be confoundedby short-term fluctuations: four years after the eradication attempt we observed significant variation in rat trapping ratesamong months on the plateau, ranging from 36.6 rats per 100 corrected trap-nights in mid-June to 12.6 in late August.Based on mark-recapture, we also estimated rat density fluctuations in the embayment forest between 20.4 and 42.9 ratsha-1 within one month in 2015, and a much lower rat density on the coral plateau fluctuating between 0.76 and 6.08 ratsha-1 in the span of two months. The causes for the short-term density fluctuations are poorly understood, but as eradicationoperations on tropical and subtropical islands become more frequent, it will be increasingly important to understand thebehaviour and ecology of the invasive species targeted to identify times that maximise eradication success