Invasive rat interactions and over-invasion on a coral atoll

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.