About 50 kilometers off the coast of California, the Farallon Islands rise from the Pacific Ocean: home to some unique animal and plant species and nesting place of countless seabirds. But since the 19th or early 20th century, introduced house mice have also been living here, whose hunger is increasingly becoming a problem for the rest of the residents, as Michael Polito from Louisiana State University and his team have observed and described in "PeerJ—Life and Environment".
The working group analyzed on Southeast Farallon Island how the nutritional habits of rodents change over the course of the year and what consequences this has for the environment there. The island is only a few hectares in size and at times home is home to up to 50,000 mice. Some of them caught the researchers and analyzed their diet with the help of an isotope analysis: Depending on the absorbent food, this leaves a typical isotope signature in the body.
Not surprisingly, the animals eat everything they come across, but the composition changes throughout the year. In spring, when mouse numbers are still low, they prefer plant-based food. But as soon as they reproduce and their numbers grow, they switch more and more to animal food: In summer, they increasingly enrich their diet with insects and seabirds. In autumn, the mouse population finally reaches its maximum; now they eat almost exclusively insects. Finally, in winter, the number of rodents shrinks again and they turn to a mixed diet, which includes, among other things, plant seeds.
Thus, the rodents adversely affect the environment in various ways: the loss of plants and seeds hinders the rejuvenation of vegetation. Their appetite for insects puts them in direct competition with the Farallon salamander, which lives only on the islands of this archipelago and is additionally threatened by the introduced mice. On the other hand, it is unclear whether the rodents hunt seabirds directly or "only" eat their eggs and chicks or use them as carrion. On the Atlantic island of Gough, introduced mice had learned to hunt even large birds such as albatrosses and eat them almost alive. However, their mere presence on Southeast Farallon Island previously attracted non-native predators such as owls, which are now also hunting seabirds.
The island is part of the Farallon Islands National Wildlife Refuge, one of the most important breeding sites for seabirds in front of the US west coast. The US Fish and Wildlife Service therefore plans to combat the mice. However, such an undertaking is complicated and is not as easy as with rats. On Gough, attempts have already been made to completely remove the rodents; However, the first follow -up examinations had shown that at least individual mice could have survived.
