Recent invasions of Micronesia by small mammals

Abstract

The introduced shrew, Suncus murinus, was discovered on Guam in May 1953. Its spread was rapid and probably facilitated by the transport of goods. The shrew was discovered on Saipan in August 1962 and spread from an area of about four square miles around the harbor to cover the rest of the island in about 18 months. Subsequent successful invasions have been reported for Rota (1966) and Moen, Truk Atoll (1967). The shrew has had no observable impact on ground-nesting birds. A marked decline in trap success for Mus muscu/us on Guam from 1958 to 1969 is the only measured change in the vertebrate fauna that can be attributed to the shrew, although changes in densities of skinks may have occurred. The Norway rat, Rattus norvegicus, was known to be present in Saipan and certain other islands for many years but the first record for Guam was not obtained until 1962, apparently a few years after its introduction. In contrast to the shrew, the spread of the Norway rat has been slow-the spotty records covered no more than 20 square miles by 1969. The first record for the Norway rat on Babelthaup, Palau, is noteworthy because of the apparent feral status of the species. Reasons for the different levels of success shown by the shrew and the Norway rat in Micronesia are not known.