Hawaii once considered releasing cats to kill invasive rats on an island; scientists rejected the idea because the cats would attack native birds too

Lehua Island, a 284-acre seabird sanctuary off the northern shore of Niʻihau, faced a conservation problem that had no simple solution. Invasive Pacific rats had established themselves on the island, threatening seabirds and native vegetation and undermining efforts to restore Lehua as a safe haven for wildlife. A 2017 environmental assessment for the Lehua Island Ecosystem Restoration Project examined how the rats could be removed while limiting further damage to the island’s fragile ecosystem. The assessment, prepared for Hawaii’s Department of Land and Natural Resources, also highlighted the danger posed by other predators, including cats. Rather than introducing another predator to hunt the rats, the proposed restoration strategy focused on eradicating them directly. The plan called for the use of rodenticide, with the broader aim of improving conditions for seabirds, native plants and other species that depend on Lehua’s isolated habitat.

Why invasive rats threatened Lehua’s seabird sanctuary

Lehua’s isolation makes it particularly valuable for conservation. The island is a state-designated seabird sanctuary and supports an important colony of seabirds, while its relatively undisturbed environment provides an opportunity to restore native wildlife. Yet the presence of Pacific rats, identified in the assessment as Rattus exulans, had compromised that potential. The rats affected the island in several ways. They are omnivores capable of feeding on plants, seeds and animals, meaning their impact extends beyond individual seabirds. Rat predation can affect different bird species depending on where and how they nest, with burrow-nesting and ground-nesting birds particularly vulnerable. The problem was also not new. An earlier attempt to eradicate rats from Lehua in 2009 had failed to achieve complete removal. By 2017, conservation planners were therefore looking at a more comprehensive approach. The assessment stated that completely removing the rats was necessary if Lehua was to reach its full potential as a seabird sanctuary.

Why cats would not have been a simple solution

At first glance, introducing cats might appear to offer a straightforward answer to an island overrun with rats. Cats are effective predators, and rats can become their prey. But Lehua’s wildlife made that approach particularly risky. The environmental assessment discusses the relationship between rats and other predators, including cats and predatory birds. When rats are present, they can provide an additional food source for predators. This can help sustain predator populations that may also prey on seabirds. That creates a complicated ecological chain. Removing rats could reduce the food available to other predators, while introducing cats could have the opposite effect by adding another predator to an island already containing vulnerable birds. For Lehua, the goal was therefore not simply to find something capable of killing rats. Any solution also had to avoid creating another threat to the seabird population the restoration project was intended to protect.

The plan called for direct rat eradication

Instead of adding cats or another predator, the proposed restoration project centred on eliminating the rats themselves. The preferred alternative called for bait containing the rodenticide diphacinone to be distributed across the island, both aerially and by hand. Brodifacoum could also be used if additional treatment became necessary. The timing was carefully considered. The operation was planned for the dry summer season, when rainfall would be less likely to carry bait towards the surrounding ocean and when rat populations would be at a lower point in their population cycle. The assessment noted that rodenticide had already been used successfully to eradicate rats from more than 400 islands worldwide. It also pointed to successful applications of diphacinone on other Hawaiian islands, including Mokoliʻi, Mokapu and Mokuʻauina. Laboratory and field trials provided further evidence that the approach could work against Pacific rats on Lehua.

Restoring an island without adding another predator

The proposed rat eradication was ultimately about more than removing an invasive rodent. The assessment linked the project to a wider restoration effort that could improve seabird nesting habitat and support the recovery of species such as Band-rumped Storm-Petrels, Hawaiian Petrels and Newell’s Shearwaters. Native coastal plants and insects could also benefit from the removal of rats. Lehua illustrates why conservation on isolated islands can become a delicate balancing act. An invasive species may be destructive, but introducing another predator to control it can create an entirely new ecological problem. For an island whose greatest value lies in protecting native seabirds, simply releasing an efficient hunter was not enough. The restoration plan instead took a more direct route, remove the rats without establishing another non-native predator. By targeting the invasive species itself, conservationists hoped to give Lehua’s native ecosystem a better chance to recover.