Niche differentiation based on diet analysis of three introduced rodents in tropical montane forest

Background/Question/Methods Determining the diets of sympatric rodents can in part define their realized niches.  Food items that appear in the stomachs of introduced rodents not only provide insight into the rodents' trophic positions, but also help identify the species most at risk to consumption by them.  In the Hawaiian Islands, which lacked rodents prior to human arrival ca. 800 years ago, three rodents (Rattus rattus or black rat, R. exulans or Pacific rat, Mus musculus or house mouse) commonly coexist in native habitats where they consume a wide range of plants and animals.  These three species were trapped in montane forest from February 2007 to September 2009; their stomach contents were analyzed to determine their short-term diets (n = 12-95 indiv. per species), and the isotopic fractions of δ¹⁵N and δ¹³C in their bone collagen were analyzed to estimate their long-term (life-time average) diets (n = 11-20 indiv. per species) and trophic positions. 

Results/Conclusions For all three species > 75% of individuals had plants and > 90% had arthropods in their stomachs.  Significant differences in mean relative abundances were found for food items in stomachs of all three rodents, including 81% plant and 14% arthropod for black rats, 60% plant and 38% arthropod for Pacific rats, and 36% plant and 57% arthropod for mice.  Rodents may be dispersing some native and non-native seeds, including the highly invasive Clidemia hirta, because intact seeds were found in some of the stomachs of all three species.  Caterpillars were particularly common in Pacific rats and mice, and aside from a spider, a katydid, and two species of beetle, the majority of identifiable arthropods in all three rodents were non-native species.  No stomachs contained evidence of birds, snails, or lizards, all of which are common at the study site.  The δ¹⁵N and δ¹³C signatures roughly reflected the trophic feeding differences revealed from the stomach contents.  Dietary niche differentiation by coexisting rodent species is evident in this forest, with Pacific rats having an intermediate diet between the mostly carnivorous house mouse and the mostly herbivorous black rat.  Combining these findings with community measurements following large-scale rodent control efforts that began > 1 year ago should improve our assessment of the ultimate impacts of introduced rodents on plant and animal communities.