Wouldn’t it be mice?

Of mice and men in Aotearoa, by Lead Conservation Ranger Ellen Irwin

From Mickey to Reepicheep to Stuart Little, mice are everywhere in our culture, which makes sense, as the house mouse has a commensal relationship with humans, meaning that they benefit from a close association with us. Humans have facilitated their spread around the globe, and consequently, they are the most widely distributed species in the world after us. In areas where they are indigenous, mice play important roles in seed dispersal and as predators and prey. Elsewhere, however, they can have substantial impacts on economies (e.g., millions or even billions of dollars in crop losses) as well as negative impacts on human health through increased disease transmission of various pathogens and parasites.

Mice also have significant effects on ecosystems, particularly on islands where species are not adapted to living with them. They will eat just about anything, from invertebrates to plant material, lizards, birds' eggs, and even the birds themselves: on several islands, mice have been observed consuming seabird chicks many times larger than themselves while the chicks were still alive. On Gough Island, a remote island in the South Atlantic, home to many threatened seabird species, mice kill an estimated 2 million eggs and chicks annually.

Research conducted in Aotearoa has demonstrated the repercussions of mice here, particularly on some of our smaller fauna. At one site, researchers found that mice halved the abundance of ground-dwelling insects, as well as reduced the average body size of species like beetles and wētā by selectively preying on larger individuals. Lizards are sometimes able to escape from larger predators by hiding in small crevices; however, they lose this advantage with mice, as the size of mice enables them to access these spaces as well. One study on Mana Island found that more than 20% of a mouse’s monthly diet could be made up of native skinks, and mice have been observed directly preying on threatened skinks much larger than themselves. In spaces where mice have been removed, various threatened lizard and invertebrate species have increased significantly. This increase in insects and lizards (not to mention seeds and other plant material) also results in more kai for our native manu/birds.

So, what does all this mean for Aotearoa? Unfortunately, mice are a tricky species to eradicate, and options for their control at a landscape scale are fairly limited. Mice breed rapidly and can have tiny home ranges, so whatever devices or methods of control are used must be done very intensively. In conservation here, much of the focus has gone towards the larger mammals like rats and stoats, which undeniably have devastating effects on Aotearoa New Zealand species. However, particularly with Predator Free 2050 targeting many of the very species that can keep mice numbers in check, we desperately need to be thinking about mice; as we know from experience at many fenced mainland sanctuaries, mice populations can explode once competition and predation pressures are removed.

While we are free of other introduced mammals, we do have a small population of mice. The animals are able to squeeze through the mesh as babies and are also possibly dropped in by native predators such as ruru/morepork. We monitor and suppress these mouse populations annually to allow species like the rare Cook Strait giant wētā, pepeketua/Hamilton’s frog, and lizards to survive. Someday in the not-too-distant future, however, we hope to be fully mouse-free and are working towards this goal with our fence refurbishment project. There are still many species that we can’t reintroduce to the valley until we achieve this goal, because those species are so sensitive to mouse predation. The removal of mice, the final introduced mammal species still able to get inside the fence, will enable us to make even more space for rare and threatened taonga species to thrive.

Photo by Lucy Broad