We commend the California Coastal Commission for seeing through the fog of misinformation, distinguishing it from science-based fact, and approving a United States Fish and Wildlife Service plan to remove invasive house mice from the Farallon Islands. The project marks a major milestone in restoring the island’s ecosystem, a process begun 50 years ago. Mouse removal has been achieved on many dozens of islands in recent decades, allowing a remarkable resurgence of nesting seabirds and other native wildlife. Rodent-removal procedures are well tested, and no contraceptive measures are available, as some have inaccurately suggested. Rodent eradication failures have been few and overly emphasized.
When one of us arrived as a volunteer biologist on Southeast Farallon in 1971, later to found the island’s marine research program, there were feral cats, rabbits and house mice, all introduced decades earlier. After the cats and rabbits were removed, black oystercatchers and rhinoceros auklets—not detected as breeding for the previous century—reappeared. The other of us has spent more time on the Farallones during “mouse season,” from September to November, than any other person alive today.
Mice provide a temporary food source for the burrowing owls that appear each fall during their dispersal and migration, along with hundreds of other landbird species documented by daily censuses. More than 99 percent of these migrant birds depart within a day or two, and were it not for the mice, burrowing owls would also continue on. Yet the annual autumnal mouse population explosion and the owls’ arrival coincide, and several owls decide to spend the winter. Once the rains begin, mouse numbers crash and the owls turn to eating ashy storm-petrels, which begin returning to nesting burrows in mid-winter. Storm-petrels have evolved on islands free of land predators and have no defense against alien mammals or owls. Furthermore, many wintering burrowing owls, a threatened species in California, do not gain sufficient nutrition from the petrels and perish in the spring. Thus develops a lose-lose situation created by the presence of mice.
Those opposed to the rodent eradication wonder why we are concerned about the petrels, given that they’re not listed under the Endangered Species Act. Yet the petrel is listed by the International Union for the Conservation of Nature, which has a much broader perspective and is not susceptible to lawsuits against species listings. There are only about 16,000 ashy storm-petrels worldwide (all of them off California), and about half the adults breed on the Farallones. Seabirds with smaller populations are considered critically endangered by the I.U.C.N., and introduced mammals have already caused the extinction of a once co-occurring species, the Guadalupe storm-petrel.
The controversy thus is between those who would like to reverse previous human errors and those who consider it an error to use poison to remove creatures from where they are artificially present, don’t belong and are doing harm. In this debate, a number of misconceptions have provoked opposition to the Farallones project. Here are a few.
Sound science on the Farallones is not available. In fact, there are over 100 peer-reviewed papers and several books about the islands and their wildlife, published over the past 150 years.
The islands’ ashy storm-petrel population is increasing. No direct evidence exists for this. Wintering burrowing owls each kill as many as 100 petrels a year, an unsustainable level of mortality for a long-lived seabird.
The project will have long-lasting negative impacts to the seabird, invertebrate, crab and salmon populations. On the contrary, ridding the island of mice will benefit several invertebrates, salamanders and plants that live nowhere but the Farallones, and will have negligible short-term effects, if any, on the marine ecosystem. There are no land crabs on the Farallones and no Dungeness crabs in the tide pools. Though salmon will never recover from California’s water woes, their food is pelagic krill and fish, absent within the island’s wave wash. Seabird species other than gulls would be absent during the eradication effort.
The drop will kill 3,000 western gulls that will eat the pellets or poisoned, dead mice. This estimate is based on breeding numbers of 30,000 gulls that peak from April to July, but published daily censuses show that during autumn there are typically fewer than 100 gulls, none on territory from mid-September to mid-November, and that they eat marine prey during this time. They don’t commute between the island and coast. Hazing will drive away any gulls while the bait is viable, and dead mice will be picked up. Contrary to opposing claims, no toxic gull carcasses will litter mainland beaches.
Wayward poisoned bait will pollute island waters to poison gulls, fish and the like. Several years ago, a successful mouse eradication project using the same methods as proposed for the Farallones was carried out at Anacapa Island, part of Channel Islands National Park and Marine Sanctuary just 11 miles offshore. Western gulls nest there, too. No evidence indicated these and other dire consequences foretold by opponents.
The scurrying, underfoot mice shown in a video by Fish and Wildlife at the Dec. 16 C.C.C. hearing were baited and staged. Whether the video was staged, we don’t know. But with 500 mice an acre at their autumn peak, the video reflects the way things are. When the population crashes, these mice become frantic, get soaked by winter rains, and turn to killing and cannibalizing. By spring, only a few remain to begin this heart-wrenching cycle again.
Diurnal raptors such as hawks occur commonly at the Farallones and will die upon eating toxic mice. Daily censuses have counted very few rodent-eating diurnal raptors—less than 20 a year—and 97 percent of these pass over the islands without stopping. On average, less than one such raptor winters on the island each year; only four overwintered during the 32-year period 1968 to 1999. While the project is underway, owls and any lingering rodent-eating raptors, such as American kestrels, will be trapped and moved to the mainland.
A few people at the hearing showed an image of a dead bird floating in a tide pool, with fresh rodenticide baits floating nearby. The bird was a booby, which does not eat dead prey, and the rodenticide bait quickly breaks apart when wet. This image was staged, or the bird was dead before the bait drop.
Several prominent individuals opined that the “experimental” use of rodenticide has no place in one of Earth’s most protected stretches of ocean. With 30 years of research on rodent removal from islands and 20 years of research specific to mice on the Farallones, Fish and Wildlife’s plan can hardly be considered experimental. In addition, though the designation of the Farallon National Wildlife Refuge and Central California National Marine Sanctuaries are significant achievements, the waters surrounding the islands are not no-take protected waters, and they are hardly pristine.
We look forward to continued efforts to restore the ecosystem of the Farallon Islands.
David Ainley, a senior ecologist at a Bay Area consulting firm, lives in Marin City and Bolinas. Peter Pyle, a San Francisco resident, works as a biologist for the Institute for Bird Populations.