Early colonisation population structure of a Norway rat island invasion

Colonists undergo non-equilibrium processes such as founder effects, inbreeding and changing population size which influence the mating system and demography of a population. Understanding these processes in colonising populations informs management and helps prevent further invasions. We sampled and genotyped most individuals of a Norway rat (Rattus norvegicus) reinvasion on Moturemu island (5 ha) in New Zealand. Population size was most likely between 30 and 33 rats. Genetic methods detected a clear bottleneck signal from the founding population. Parentage assignment revealed promiscuous mating dominated by a few individuals with increasing inbreeding, both putatively a result of small island size. Combining ecological and genetic data from a single sample allowed inferences on population structure and functioning. Invading Norway rats rapidly achieve population structure similar to established island populations despite a small number of colonists and associated inbreeding. Overcoming these initial obstacles to population establishment contributes to the global success of invasive rats.