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Rodent eradications undertaken on tropical islands are more likely to fail than eradications undertaken at higher latitudes. We report on 12 independent rodent eradication projects undertaken on tropical islands that utilized the results of an in situ bait availability study prior to eradication to inform, a priori, the bait application rate selected for the eradication. These projects also monitored bait availability during the eradication. The results from our analysis verified the utility of bait availability studies to future rodent eradication campaigns and confirmed the influence of two environmental factors that can affect bait availability over time: precipitation prior to the study and the abundance of land crabs at the study site. Our findings should encourage eradication teams to conduct in-depth assessments of the targeted island prior to project implementation. However, we acknowledge the limitations of such studies (two of the projects we reviewed failed and one removed only one of two rodent species present) and provide guidance on how to interpret the results from a bait availability study in planning an eradication. Study design was inconsistent among the twelve cases we reviewed which limited our analysis. We recommend a more standardized approach for measuring bait availability prior to eradication to provide more robust predictions of the rate at which bait availability will decrease during the eradication and to facilitate future comparisons among projects and islands.

Invasive rats are found on most island groups of the world, and usually more than one species has invaded. On tropical islands populations of different invasive rat species can co-exist on very small islands, but the population dynamics of such co-existing rat species, their impact on each other, and the mechanisms of coexistence are not well known. This lack of knowledge is a barrier to improving the success rate of tropical island rat eradications. Through an exhaustive trapping eradication campaign on a small tropical island, we study the population structure of historically established Rattus exulans where R. rattus have colonised within the last fifty years and over-invaded. We contrast this R. exulans population with a nearby island population where R. exulans exist alone. Recently invaded R. rattus numerically and morphologically dominate R. exulans; however stable isotope analyses show that the trophic position of R. exulans remains consistent regardless of the presence of R. rattus, once differences in trophic foundations of islands are accounted for. Although the trophic position of both rat species is indistinguishable, R. rattus is able to dominate R. exulans through interference competition. Our eradication attempt was interrupted by a tropical cyclone and ultimately unsuccessful, but there is some evidence that R. rattus reduced control device availability to R. exulans, which has important implications for multi-species control operations.