Diet and coexistence of Rattus rattus rattus (Linnaeus), Rattus exulans (Peale) and Rattus norvegicus (Berkenhout) on Stewart Island, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Zoology

Abstract

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 Orthoptera, Araneida, Chilopoda and Terricolae were the most common in the diet of R. exulans. Decapoda (freshwater crayfish) were the most frequent diet item of R. norvegicus, and Opiliones, Othoptera, and Araneida were also common. Fruits, especially Prumnopitys ferruginea, and unidentified seeds and rhizomes were the more common plant foods eaten. A greater importance of leaf material than has been previously recorded is suggested in the diet of R. rattus. Birds were rarely eaten by R. rattus and R. norvegicus and not at all by R. exulans. Two instances of R. norvegicus having eaten lizards were recorded. Diet overlap was considerable but significant species differences were found. Diet varied with season for each species and for R.rattus and R. norvegicus there was variation between transects. Coexistence in the various habitats is discussed.