The genetic structure of the introduced house sparrow populations in Australia and New Zealand is consistent with historical descriptions of multiple introductions to each country

The house sparrow is one of the most widely introduced vertebrate species around the world, making it an important model species for the study of invasion ecology. Population genetic studies of these invasions provide important insights into colonisation processes and adaptive responses occurring during invasion. Here we use microsatellite data to infer the population structure and invasion history of the introduced house sparrow (Passer domesticus) in Australia and New Zealand. Our results identify stronger population structure within Australia in comparison to New Zealand and patterns are consistent with historical records of multiple introduction sites across both countries. Within the five population clusters identified in Australia, we find declines in genetic diversity as we move away from the reported introduction site within each cluster. This pattern is consistent with sequential founder events. Interestingly, an even stronger decline in genetic diversity is seen across Australia as we move away from the Melbourne introduction site; secondary historical reports suggest this site imported a large number of sparrows and was possible the source of a single range expansion across Australia. However, private allele numbers are highest in the north, away from Melbourne, which could be a result of drift increasing the frequency of rare alleles in areas of smaller population size or due to an independent introduction that seeded or augmented the northern population. This study highlights the difficulties of elucidating population dynamics in introduced species with complex introduction histories and suggests that a combination of historical and genetic data can be useful.