Researchers determine nutritional effects of invasive rats on the Farallon Islands

Researchers determine nutritional effects of invasive rats on the Farallon Islands

View of southeast Farallon Island from the west. Credit: Point Blue Conservation Science

On an island 30 miles off the coast of San Francisco, a hoard of invasive house mice is packing an ecological mass much larger than its miniature figurines might suggest. Those are the conclusions of a study led by LSU Associate Professor of Oceanography and Coastal Sciences Michael Poleto, along with researchers from Point Blue Conservation Science and California State University and San Jose State and Channel Islands University. 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 breeding colony of seabirds in the neighboring United States and many unique native species of flora and fauna. House mice are not native to the island but were unintentionally introduced during the 19th or early 20th century. Since then, the island’s population has grown to about 50,000 house mice inhabiting the island roughly the size of two football fields. The study found that rats consume and/or compete for food with native species, thus providing support for a proposed plan by the US Fish and Wildlife Service to eliminate rats from all of the southern Farallon Islands.

The research team determined how mice affect this island ecosystem by gaining a better understanding of mice abundance and diets.

“Before this research, there was a lack of data on what exactly the rats eat on the island and how their diet has changed over the course of the year,” Poleto said.






House mice were inadvertently introduced to Farallon Islands National Wildlife Refuge, home to the largest breeding colony of seabirds in the contiguous United States, and the population has grown to about 50,000 house mice inhabiting the island roughly the size of two football fields. The study found that rats consume and/or compete for food with native species, thus providing support for a proposed plan by the US Fish and Wildlife Service to eliminate rats from all of the southern Farallon Islands. Credit: Point Blue Conservation Science

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

“Mice are effectively what they eat,” Polito said.

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

Small rodents, big appetite

House mice were inadvertently introduced to Farallon Islands National Wildlife Refuge, home to the largest breeding colony of seabirds in the contiguous United States, and the population has grown to about 50,000 house mice inhabiting the island roughly the size of two football fields. Credit: Point Blue Conservation Science

They conclude that mice are highly “carnivorous” and “opportunistic” eaters, and their population numbers and diets vary greatly throughout the year in response to changes in food availability and seasonal climate. The researchers discovered that in the spring when the number of mice is low, they mainly eat plants. With the onset of summer, when their numbers begin to increase, the mice begin to eat more insects and native seabirds. In the fall, when the rat population is booming, their diet shifts entirely to insects, which puts them in direct competition with the arboreal salamander Farallon, a species found only on islands. Mouse numbers then decline during the cooler, wetter winters.

Small rodents, big appetite

This study found that rats consume and/or compete for food with native species such as the endemic Farallon salamander, thus providing support for a proposed plan by the US Fish and Wildlife Service to eliminate rats from all of the southern Farallon Islands. Credit: Point Blue Conservation Science

While it remains unclear to what extent rats actively prey on seabirds, or simply dispose of abandoned eggs and carcasses, previous studies have found that the mere presence of rats on islands attracts migratory predators, such as burrowing owls, which then prey on rare. Native seabirds. The nature of the island’s environment itself also leads to invasive rats which have a huge impact.

“Native plants and many animals cannot leave the island to get away from mice, and these plants and wildlife have never had to develop defensive behaviors against rodents in the way that mainland species do,” Polito said.

Small rodents, big appetite

An analysis of this study was conducted on the ecosystem of the Farallon Islands, the largest breeding colony of seabirds in the neighboring United States, at Louisiana State University. Credit: Point Blue Conservation Science

The researchers concluded that mice have widespread effects on the island’s ecosystem due to their high abundance and opportunistic diets.

said Pete Worzybock, head of the Farallon Islands Program at Point Blue Conservation Science and co-author of the paper. “These findings present a stronger case than ever for rat eradication as a critical step in restoring the Farallon Islands ecosystem.”


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more information:
Michael J. Polito et al., Population dynamics and resource availability drive seasonal shifts in the consumer and competitive effects of introduced house rats (Mus musculus) on the island ecosystem, berg (2022). DOI: 10.7717 / peerj.13904

Journal information:
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Provided by Louisiana State University


the quote: Small Rodents, Big Appetites: Researchers Determine Dietary Effects of Invasive Rats on Farallon Islands (2022, September 22), Retrieved September 22, 2022 from https://phys.org/news/2022-09-rodent-big-appetite- nutritional effect. html

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