Population structure and movements of introduced rats in an Hawaiian rainforest

Abstract

We studied populations of introduced rats in three areas in Hawaiian rainforests as part of a larger study to determine the factors limiting endangered bird populations. Species composition among 1 264 rats live trapped was Black Rats Rattus rattus 60.8%, Polynesian Rats R. exulans 38.1 %, and Norway Rats R. norvegicus 1.0%. Capture rates did not differ among the three study areas or four seasonal periods for any of the species. Males comprised 53.0% of 921 Black Rats and 55.3% of 783 Polynesian Rats that were sexed. Juvenile rats comprised 26.2% of Black Rats and 31.7% of Polynesian Rats captured and weighed. Black and Polynesian Rats reproduced throughout the year, but the highest proportion of juveniles in the population was captured in September-November and the lowest proportion in April-June. Mean home range size was 3.6 ha for four Black Rats (three males and one female) and 3.0 ha for three Polynesian Rats (two males and one female). All daytime nests of Black Rats found were above ground, generally in cavities within tree canopies. Mean annual rat activity in tree canopies as measured with tracking tunnels was 43% (range 21?80%). Of 44 rats captured in trees, 43 were Black Rats and one was a Polynesian rat.