See how they run: increased ranging behavior counters potential Allee effects in experimentally introduced house mice on an island

Whether the behaviors of individuals at low population densities differ from those at higher densities is critically informative for predictions of theoretical and experimental ecology, as well as for the conservation management of endangered or introduced species. The globally widespread house mouse Mus musculus is a frequently invasive species; detailed analyses of its low-density behavior will allow for better understanding of population dynamics at early stages of invasions and for generating clearer designs of timely management interventions. Opposite-sex pairs of house mice were released in a small number of replicates at opposite ends of a small, 6 ha island in New Zealand, that had been made experimentally rodent-free, to investigate spatial distributions and ranging behaviors at very low densities by simulating a new invasion of the island with each release trial. Successfully introduced mice (n = 15) of both sexes had twofold greater nightly movements, and tenfold larger range sizes, relative to mice trapped and tracked on the same island prior to eradication. This allowed new arrivals to rapidly and reliably overlap with the range of the opposite sex on the island, especially during the breeding season, suggesting that the driver behind increased ranging was mate searching. Despite the study’s small total sample size, the dramatically increased ranging behaviors exhibited by new arrivals on a mouse-free island imply that this species may avoid potential Allee effects through success in mate searching even at low density. Such behavioral shifts in ranging could contribute to the pervasive success of house mice in founding invasive populations even at extremely low initial densities.