Ecological Observations on a Feral House Mouse Population Declining to Extinction

It is clearly an unusual opportunity for an ecologist to be able to observe in detail the extinction of a species over a large area. Quite understandably, ecologists tend to study common species which are not likely to disappear during the time of study. This report documents the events occurring in a population of house mice (Mus musculus), living on 55 acre Brooks Island in San Francisco Bay, as it declined to extinction in a period of little over one year. The study was not initiated with the expectation that the population would become extinct, but rather because the mice occurred on the island in very high densities, and it was hoped that something could be learned about density regulation in this species under these congested, yet completely feral, conditions. Almost exactly coincident with the beginning of my studies on Brooks Island house mice in September of 1958, a few individuals of Microtus californicus were accidentally introduced on to the island from a neighboring islet, and rapidly colonized the entire island. The history of the colonization and exploitation of the island by Microtus is documented in detail in an earlier paper (Lidicker and Anderson 1962). Refer also to that report for a more complete description of the island and its recent history. For our purposes here, it is perhaps sufficient to