Dietary niche partitioning and seasonal variation between rat species on an oceanic island.

On oceanic islands, invasive rats represent a significant threat to native species, especially seabirds. However, we do not yet fully understand the ecological impacts of rats or how their dietary niches are affected when more than one rat species is present. Our aim was to examine kiore (Rattus exulans) and ship rat (R. rattus) isotopic niche overlap on Chatham Island, and to determine the threat they might pose to at-risk or threatened native bird species, including endemic species such as the parea (Hemiphaga chathamensis), and an endangered ground-nesting seabird, the tāiko (Pterodroma magenta), in the island's only large remaining tract of forest. Liver stable isotope analysis (δ13C and δ15N) for kiore and ship rats showed some segregation between species in isotopic niche space. Generally, there was low overlap of hypervolumes defined by diet proportions between species, but some seasonal variation in the degree of overlap, and differences between kiore and ship rat niche size that likely facilitate coexistence. Mixing models indicated that birds, but probably not tāiko, were dominant food sources for both species. Proportions of parea and passerines varied across seasons, while invertebrates and plants were also represented. The occurrence of birds in the diet of both rats may be due to either direct predation or scavenging. The results suggest that, as expected, rat control is a critical element of native species management in this remaining forest, most likely acting to protect small forest birds and endemic species like parea, although it remains unclear whether rats are directly impacting endangered populations of ground-nesting birds.