Rats have thrived since being introduced into New Zealand but a new study shows they not only spread rapidly, new arrivals kept jumping ship.

Using population genetics to trace the genealogical lineage of rats, a team of scientists at the University of Auckland has tracked the spread of the two commonest rat species across the entire country for the first time.

“Thanks to a huge sampling effort from community groups trapping all around New Zealand, we were able to sample over 500 rats to connect the dots for how rats invaded,” says Associate Professor of Statistics at the University of Auckland Rachel Fewster who runs the national rat trap monitoring software CatchIT.

The results showed widespread genetic diversity for the commonest rat species, R. rattus, or ship rat, suggesting at least four different invasions in both the North and South Island and on offshore islands like Great Barrier Island and Stewart Island. Each new invasion was trackable through new genes being introduced into the gene pool each time.

The study found the less common but still widespread R. norvegicus, or Norway rat, had more limited diversity within its gene pool, with two main invasions across New Zealand – one on the North Island and some offshore islands, and the other on the South Island, potentially with both English and Chinese origins.

“The genetics show how quickly rats would have overrun New Zealand when they arrived, and eaten their way through our bird and reptile populations,” says Associate Professor James Russell, who led the study.

“Using these results, today we can trace new arrivals of rats across New Zealand which will be critical if we are serious about reaching our goal of being predator-free by 2050.”

As both common rat species quickly established and rapidly spread by the 19th Century, they displaced R. exulans – kiore or Pacific rat – introduced by Polynesian settlers in the late 13th Century. Eventually the kiore rat became restricted to remote parts of both main islands and a few offshore islands.

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