Small rodent, big appetite: Researchers identify the dietary impact of invasive mice on the Farallon Islands

On an island 30 miles off the coast of San Francisco, a horde of invasive house mice grabs an ecological wallop much larger than their small stature suggests. These are the conclusions of a study led by Michael Polito, associate professor of oceanography and coastal sciences at LSU, along with researchers at Point Blue Conservation Science, San Jose State University and California State University Channel Islands. The study was published today in PeerJ – Life and Environment.

The island in question is Southeast Farallon Island, part of the Farallon Islands National Wildlife Refuge, home to the largest seabird breeding colony in the contiguous United States and many unique native plant and animal species. House mice are not native to the island but were accidentally introduced in the 19th or early 20th century. Since then, the population has grown to around 50,000 house mice, which inhabit the island about the size of two soccer fields. The study found that mice consume native species and/or compete for food, and therefore supports the US Fish and Wildlife Service’s proposed plan to eradicate mice on all South Farallon Islands.

The research team identified how mice affect this island ecosystem by first gaining a better understanding of mouse abundance and diet.

“Prior to this research, there was a lack of data on what exactly mice ate on the island and how their diet changed over the year,” Polito said.

To study the diets of mice, the scientists used a technique called stable isotope analysis, which tracks the unique chemical signatures of food sources in mouse tissue.

“Actually, mice are what they eat,” Polito said.

In addition, Polito and his colleagues studied the seasonal abundance of introduced mice over a 17-year period and related them to the availability of native seabirds, salamanders, insects and vegetation on the island.

They conclude that mice are very “omnivorous and opportunistic” feeders, whose population numbers and diet vary dramatically throughout the year in response to changes in food availability and seasonal climate. The researchers discovered that in the spring, when the mouse population is low, they mainly eat plants. As summer rolls around, when their numbers begin to increase, the mice start eating more of the native insects and seabirds. In the fall, when the mouse population is booming, their diet shifts more toward insects, putting them in direct competition with the Farallon tree salamander, a species found only on the islands. Mice numbers then decrease during the cooler, wetter conditions of winter.

While it remains unclear to what extent mice actively destroy seabirds or just eat abandoned eggs and carcasses, previous studies have found that the mere presence of mice on the islands attracts migratory predators such as burrowing owls, which then prey on rare native seabirds. The nature of the island environment itself also causes invasive mice to have an outsized impact.

“Native plants and many animals cannot leave the island to escape the mice, and these plants and wildlife have never had to develop defensive behaviors against rodents like mainland species have,” Polito said.

The researchers conclude that mice have wide-ranging influences on the island’s ecosystem due to their high abundance and opportunistic diet.

“Our study provides the latest and most comprehensive understanding of the diet of mice and the impact it has on the native community — particularly the endemic tree salamander,” said Pete Warzybok, Farallon Islands program director at Point Blue Conservation Science and a co- author of the paper. “These results support more than ever the eradication of mice as a critical step in restoring the ecosystem of the Farallon Islands.”