Introduced mice eat an entire island off the US coast

The southeast island of Farallon is small but home to thousands of mice. And rodents are very hungry all year round – to the detriment of the original nature.

Rising from the Pacific Ocean about 30 miles off the California coast, the Farallon Islands are home to unique species of flora and fauna and a breeding ground for countless seabirds.

However, introduced house mice have also been living here since the 19th or early 20th century, and their hunger is becoming a growing problem for the rest of the residents, as observed by Michael Polito of Louisiana State University and his team and described in “PeerJ—Life and Environment”.

Sometimes up to 50,000 mice on the island

In the south-east of the island of Farallon, the working group analyzed how the eating habits of rodents change during the year and what the consequences are for the environment. The island is only a few hectares in size and is sometimes home to up to 50,000 mice. The researchers captured some of them and analyzed their diet using isotopic analysis: Depending on the food consumed, it leaves a typical isotopic signature in the organism.

Unsurprisingly, the animals eat whatever comes their way, but the composition changes throughout the year. In the spring, when the number of mice is still low, they prefer vegetable food, but as soon as they reproduce and their number increases, they turn more and more to animal food: in summer, they enrich growing their diet of insects and seabirds.

Finally, in the fall, the mouse population reaches its maximum; now they almost exclusively eat insects. Finally, in winter, the number of rodents decreases again and they turn to a mixed diet including, among other things, plant seeds.

Appetite for insects puts mice in competition with salamanders

Rodents thus affect the environment in various ways: The loss of plants and seeds hinders the regeneration 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 threatened by introduced mice.

However, it is unclear whether rodents hunt seabirds directly or “only” eat their eggs and chicks or use them as carrion. Mice introduced to the Atlantic island of Gough had learned to hunt even large birds like albatrosses and eat them almost alive.

However, their mere presence on southeast Farallon Island has attracted previously non-native predators there, such as owls, which now also hunt seabirds.

Important breeding sites for seabirds

The island is part of the Farallon Islands National Wildlife Refuge, one of the most important breeding grounds for seabirds off the west coast of the United States.

The “US Fish and Wildlife Service” therefore plans to control the mice. However, such a business is complicated and does not succeed as easily as with rats.

Attempts have already been made to completely eradicate rodents on Gough; However, early follow-up studies had shown that at least a few mice might have survived.

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