Invasive species are one of the greatest threats to biodiversity, reshuffling entire food webs and ecosystems. But some invasive species, whether they migrated because of climate change or accidentally introduced, can end up taking on roles that benefit an ecosystem.
If eradicated on a large scale, it can affect food webs and ecological processes essential for people and native species. Invasive species can even take on the role of extinct species—at least to a point.
For example, in Hawai’i, non-native birds on Oʻahu disperse small-seeded native plants like their extinct predecessors but can’t spread large-seeded native plants as they did.
This is one of the many reasons large-scale eradication programs need to carefully consider any potential ripple effects eradications can have, which should be done through an independent, evidence-based assessment of the risks and benefits.
A risk-benefit assessment yields a range of scenarios, including the risks of doing nothing, that can help decision-makers anticipate risks and make informed decisions on avoiding undesirable outcomes.
The U.S. Fish and Wildlife Service (FWS) is trying to do this through an invasive house mice eradication project on the South Farallon Islands, a wildlife refuge for thousands of seals and sea lions and hundreds of thousands of seabirds, including endemic or exclusive seabirds to Farallon Islands like the ashy storm-petrel.
The California Coastal Commission approved the project and is left for the regional director of FWS to decide. If approved, it could happen as soon as 2023.
The plan is to drop 2,880lb of bait pellets laced with the rodenticide brodifacoum by helicopter. Secondary poisoning of brodifacoum is a high risk since it stays in body tissue for a long time and is highly toxic to birds, mammals and fish—posing a risk to predators.
The threat of secondary poisoning led the state of California to ban brodifacoum for almost all uses. Critics of the project argue that the aerial application of brodifacoum is unfit for a wildlife refuge and threatens marine life if it washes into the ocean.
Like other invasive species, the rodents on the Farallon Islands prey on endemic species and compete for natural resources, but even worse is that they attract burrowing owls. The owls end up staying through the winter, and when the mice population crashes, they turn on ashy storm-petrels, killing hundreds of them every year.
Although ashy storm-petrels aren’t considered endangered, their population has declined by 63 percent over the last 20 years.
Those against the eradication project say there is no way to limit the impact on non-target species and question if the project is worth the risk. The death of non-target species is one of many scenarios addressed in the FWS’s Coastal Consistency Determination report.
Informed by experts in island conservation, island biogeography and biologists who have worked on the Farallon Islands for decades, the report outlines the project’s operational plan to avoid adverse outcomes. The report also took notes from 1,200 rodent eradication attempts on islands worldwide, where more than 700 were successful.
A study published in the Conservation Science and Practice journal on rodenticide eradication programs compared management, operational and environmental factors to determine what contributed to their success or failure.
Out of the 35 eradication attempts they examined, there was a 58 percent success rate when the methods used aligned with best practices, compared to 19 percent when the methods fell below that standard.
The main factors associated with eradication failure included poor planning, poor bait quality, poor treatment of rodent hotspots, inadequate bait coverage, inexperienced pilots with no GPS, deviations from operational plans and human error.
The researchers found that eradication programs need to consider seasonal weather and pay special attention to areas such as cliffs and intertidal areas that provide habitat and food for rodents. They also emphasize carefully evaluating protocols that protect non-target species, such as restricting bait deployment near coastlines.
The FWS plans to drop the bait between October and December when the mice aren’t reproducing, and most wildlife migrate. Seasonal rainfall will follow and help degrade the bait and reduce the threat to non-target species.
Pilots will operate within areas above the Mean High-Water Spring when wind speeds are below 30 knots or 34 miles per hour to minimize bait drifting into the ocean. They’ll also avoid shoreline areas where bait can drift into sensitive intertidal habitats—an area the study says is often an underestimated habitat for rodents.
They’ll hand-bait caves and cliffs and have a Bait Spill Contingency Plan in place, but seabirds and several small bird and raptor species are still at high risk of exposure.
Most seabirds only feed on marine life, such as ashy storm-petrels, while western gulls are opportunistic omnivores that will eat just about anything, making them the most vulnerable.
According to FWS, most studies that discovered fish poisoned after a rodent eradication project had higher bait application rates. Their assessment places a medium risk to marine life, meaning a small number of nearshore fish will be exposed to bait, which should disintegrate within an hour in the water. However, there is a risk that unexpected wind events can discharge poison into the ocean.
For gulls, to avoid a long-term population impact, the assessment found that they must keep the mortality rate at no more than 1,050.
About five weeks before the first bait application, they will haze gulls with techniques such as spotlights, air cannons and kites to deter them from the islands. If the hazing succeeds, the project will proceed, but a concern for some is that there will be a higher death toll if the hazing program unexpectedly fails.
Small bird species that feed on seeds are at high risk of exposure and an increased risk of secondary poisoning if they prey on insects that feed on the bait. Raptors such as hawks and owls are at even higher risk of secondary exposure by preying on poisoned mice, birds or insects.
The assessment explains that since the populations of these species are low, adverse impacts would be minor. However, they plan to capture raptors before and during operation and release them once the islands are safe.
Some deem the eradication plan a simplified solution to a complex problem. It’s 15 years in the making, but studies on eradication programs agree that research organizations should undertake risk-benefit assessments independent of the group proposing invasive species control.
Also, invasive rodents have been on the Farallon Islands for more than a century. Although eradicating them could boost species such as the ashy storm-petrel, the assessment doesn’t include how eradicating them may impact the food web or if they have an ecological role to play.
Image courtesy of Flickr. Originally published by S&S on Mar. 1, 2022.
