On an island 30 miles off the coast of San Francisco, a mob of invasive house mice is amassing an ecological bounty far greater than their diminutive stature suggests. These are the findings of a study conducted by LSU Associate Professor of Oceanography and Coastal Sciences Michael Polito along with researchers from 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 the southeastern 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 inadvertently introduced in the 1800s or early 1900s. Since then, the population has grown to about 50,000 house mice living on an island roughly the size of two football fields. The study found that mice consume and/or compete for food with native species, and therefore provides support for a plan proposed by the US Fish and Wildlife Service to eradicate mice from all of the South Farallon Islands.
The research team determined how mice affect the island’s ecosystem by first gaining a better understanding of mouse populations and diet.
“Prior to this study, there was a lack of data on exactly what the mice ate on the island and how their diet changed throughout the year,” Polito said.
To study the mice’s diet, the scientists used a technique called stable isotope analysis, which traces the unique chemical signatures of food sources in the mice’s tissues.
“Essentially, mice are what they eat,” Polito said.
In addition, Polito and his colleagues examined the seasonal abundance of introduced mice over a 17-year period and related this to the presence of native seabirds, salamanders, insects and vegetation on the island.
They concluded that mice are highly “omnivorous and pathogenic” eaters, whose population numbers and diets fluctuate dramatically throughout the year in response to changes in food availability and seasonal climate. The researchers found that in the spring, when the mouse population is low, they eat mostly plants. As summer approaches, when their numbers begin to increase, mice begin to feed more on native insects and seabirds. In the fall, when the mouse population increases, their diet shifts more completely to insects, causing them to compete with the Farallon arboreal salamander, a species found only on the islands. Mice numbers decline in the cooler, wetter conditions of winter.
While it remains unclear to what extent the mice are actively killing seabirds or simply scavenging discarded eggs and carcasses, previous research has shown that the mere presence of mice on islands attracts migratory predators such as tawny owls, which then prey on rare native seabirds. The very nature of the island environment also causes invasive mice to have an extraordinary 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 that mainland species do,” Polito said.
The researchers conclude that mice have a wide-ranging impact on the island’s ecosystem due to their large numbers and opportunistic diet.
“Our study provides the newest and most comprehensive understanding of the mice’s diet and the impact they have on the native community – specifically the endemic arboreal salamander,” said Pete Warzibock, head of the Farallon Islands Program at Point Blue Conservation Science and co-author of the paper. “These findings make a stronger case than ever for mouse eradication as an important step in restoring the Farallon Islands ecosystem.”