| Summary: | Coexistence and diet of Rattus rattus rattus, Rattus exulans and, Rattus norvegicus on Stewart Island was investigated by snap-trapping and stomach analysis. The trapping period was from June 1982 to December 1986. Four habitats were trapped for three nights every three months. Morphometric and reproductive information was collected throughout the programme. Studies of food availability, involving two forms of invertebrate sampling and plant phenology recordings, were undertaken in the final year of trapping. Overall rat abundance was 4.0 rats/100 trap-nights which is comparable with other findings in mainland forests and on large offshore islands in New Zealand. R. norvegicus abundance was significantly lower than that of R. rattus and R. exulans. Species composition varied considerably between transects and a high degree of disassociation between R. exulans and R. norvegius was recorded. R. norvegics was restricted to areas very close to water. Seasonal variation in rat abundance was marked and was most affected by changes in R. ratrus and R. exulans numbers. Highest rat abundance was in winter of each year. Male R. exulans were heavier (x = 65.61) than females ( = 60.77) and the same was true for R. norvegicus (male = 201.40, female = 165.50). All rat species were smaller than those trapped previously from New Zealand. Few pregnant rats were trapped but seasonal breeding of R. rattus and R. exulans was suggested by annual increases in the proportions of juveniles in winter and spring. Invertebrates and vegetation were the main foods eaten. In terms of frequency of occurrence and mean percentage volume, invertebrates were most important for R. exulans and R. norvegicus. For R. rattus invertebrates were also the most commonly eaten food but vegetation provided greatest volume of diet. Of the three species, R. rattus had a more varied diet and R. exulans was most reliant on invertebrates. Orthoptera, Opiliones, Chilopoda and Coleoptera were the invertebrates most frequently eaten by R. rattus, whereas ...
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