An Experimental Mouse Invasion

Posted by Helen Nathan

New research by Helen Nathan, Mick Clout, Jamie MacKay, Elaine Murphy, James Russell

IMG_3884How much damage could a couple of mice do on a pest-free island? We used a novel experimental approach to demonstrate the importance of island biosecurity.

Two house mice (one male, one female) were released onto Te Haupa (Saddle) Island, a Department of Conservation scenic reserve which had previously hosted a mouse population, but had recently been declared pest-free. For the following 8 months we returned regularly to the island to undertake live trapping, allowing us to estimate the number of individual mice on the island, and plot the growth of the invasive population over time. We also took genetic samples from captured mice to confirm descent from the two founder individuals released. After 8 months, the experiment concluded and the population was eradicated using a combination of trapping and poison.


Ear-punches were used to mark mice for identification and to provide a genetic sample

We found that population growth was initially rapid, peaking at an estimated 68 individuals 5 months after the release, then stabilising until the end of the 8 month experimental period. This pattern of growth reflects the classic model predicted by invasion biologists, but rarely observed in real-time as, in contrast to our study, the exact point in time when an invasion occurred is usually unknown. A surprise result from our genetic analysis showed that not all of the mice trapped at the end of our experiment were descended from the founding male and female. An unrelated female was first captured 3 months after the start of the invasion, and had produced offspring 1 month later. Genetic analysis suggests that this mouse most likely originated from another island off the north-east coast of New Zealand, almost certainly transported on a boat which had recently visited other nearby islands.

The extremely rapid population growth of the invading mouse population, along with the independent mouse incursion we detected, demonstrate the need for vigilant monitoring of our pest-free sanctuaries. For successful conservation and restoration of sites, invasive species populations must be detected early during colonisation to enable swift elimination, before the population becomes established.

Read the published research online now at Population Ecology

Or in the New Zealand Herald

Helen Nathan is a PhD student in the Centre for Biodiversity and Biosecurity