Oh, rats!

LIHUE — Estimating their numbers is a shot in the dark. But there’s a lot, and it’s safe to say they’re here to stay.

Kyle Pias, field supervisor for the Kauai Forest Bird Recovery Project, compared rats to debt in the United States — an overwhelming and steadily increasing number that hangs overhead.

As they have for centuries, rats continue to pose one of the largest threats to conservation on Kauai — especially for the island’s native bird populations.

“Rodents, like cats, they are predators,” said Thomas J. Ka’iakapu, Kauai wildlife manager for the state Department of Land and Natural Resources. “If we could remove them from the island that would be the right thing to do.”

Accomplishing that, however, would be next to impossible.

“They are everywhere,” said André Raine, coordinator of the Kauai Endangered Seabird Recovery Project. “And they’re never ending.”

Which is exactly why these three men, and many others, continue the work they do, with no intention of giving up or slowing down.

The Hawaiian Islands are home to three rat species. The black rat, or roof rat — the same species that spread the bubonic plague that killed 75 million people worldwide — is between 5 and 7 inches long and an expert climber, commonly found in agriculture areas and forests.

The Polynesian rat is the smallest, 4 to 5 inches, and typically found in fields and coastal areas. And finally, the Norway, also known as the cellar or sewer rat, is Hawaii’s largest, usually 8 to 10 inches long, and found in developed areas, sewers and garbage dumps.

The Polynesian rat first arrived in Hawaii with Polynesians around 400 A.D., while the black and Norway found their way over with Westerners in the 1780s. Out of all the nonnative species introduced in Hawaii, the rat “has, and continues to be, one of the most damaging invasive species,” according to the U.S. Fish and Wildlife Service.

Rats carry harmful diseases. They damage agricultural crops, prey on nesting seabirds, forest birds and sea turtle hatchlings, compete for food and destroy native plants. And to make matters worse, they are prolific breeders, with mothers giving birth to up to 12 offspring every 30 days.

“They are considered a leading cause of the accelerated decline and extirpation of endemic Hawaiian forest birds and a major factor limiting present populations of endangered birds,” according to USFWS. “Of the three rat species found in Hawaii, the black rat, because of its size and arboreal behavior, is considered the greatest threat to native Hawaiian forest birds.”

On Kauai especially, a place known around the world for its endangered species and biodiversity, the problem is very real. And Pias said it is much deeper than rats killing an endangered bird here and there.

“They’re an ecosystemwide impact because they’re killing the native plants, they’re eating fruits and seeds … The lowest level of the food chain, they’re destroying it,” he said. “And that has widespread ramifications for the structure of the forest.”

Suffering seabirds

Raine is among those on Kauai who best understands the detrimental impacts these vermin are having on the island. And he doesn’t downplay it.

Part of his work at KESRP is using infrared cameras to monitor the secret lives of endangered Newell’s Shearwater and Hawaiian Petrels in remote locations of Hono o Na Pali Natural Area Reserve and Upper Limahuli Preserve.

Last year, at one specific Na Pali Coast site, rats alone killed 20 percent of nesting chicks. They visited one specific burrow over 300 times. Somehow, the chick still managed to fledge, according to Raine.

“Almost every single burrow is visited by rats,” Raine said.

The fact that these burrows are in some of the most remote and rugged locations on the island only highlights the extent of the problem. And of course, rats are only part of the battle.

“It’s cats all over the place, rats all over the place,” he said.

Raine described the camera footage he and his team have collected as disheartening and horrible. In one video, a black rat pushes a Shearwater chick from its burrow and attacks it for over five minutes before it is able to deliver a final deadly blow.

“It’s a never-ending battle for these birds to survive up there, and the rats are everywhere,” he said. “From the coast to the highest mountain peak.”

To address the situation, KESRP has begun rat control studies using what are called Goodnature traps, a mechanical trap that resets itself after a rat wanders inside. While technology continues to improve and there are ways to address the issue in specific areas, Raine describes it as an insurmountable problem against a remorseless and — although he doesn’t like to admit it — amazing pest.

“You can remove them from an area, but they’re going to keep coming back,” he said.

Kauai is home to 90 percent of the world’s population of the Newell’s Shearwater, as well as important populations of the Hawaiian Petrel.

Fight for flight in Kauai’s forest

KESRP isn’t the only group battling invasive rats on behalf of Kauai’s endangered species.

The Kauai Forest Bird Recovery Project focuses on research and recovery of three federally endangered species — Puaiohi, Akikiki and Akekee — as well as Kauai’s other native songbirds. For each of the three target species, there are less than 1,000 individual birds, found only in a 25-square-kilometer area of the Alakai Swamp.

The Puaiohi, or Small Kauai Thrush, are believed to have a population as low as 500, according to KFBRP.

Pias said the team has documented incidents of nest predation by rats on both the Puaiohi and Akekee, and the Akikiki has likely fallen victim as well. While he doesn’t have a percentage for the number of birds being killed, Pias said it is safe to say rodents are having significant impacts.

This past breading season, the project received funding to run its own rat control effort, using the same Goodnature traps. They set 40 traps in a grid over a 100-acre area of upper elevation forest and compared numbers to a nearby trap-free control area.

In the area with traps, Pias said they caught about 60 rodents over a four-month period.

While the frequency of rats in the control area grew, the area with the traps stayed the same. In other words, all of KFBRP’s work in that area resulted in keeping the population where it was — not reducing it.

Pias admits that seeing such minimal gains can be discouraging and frustrating.

“When you’re dealing with population numbers as low as we have for some of these birds, every individual is important,” Pias said.

KFBRP is looking into launching a new program in which members of the public could donate a trap to be used in Kauai’s forest. And by concentrating efforts around the endangered birds — for example, setting up several traps around a Puaiohi nest — KFBRP can make strides, Pias said.

Cat connection

Feral cats have gotten a lot of attention on Kauai lately, with the Feral Cat Task Force’s recommendation of having zero feral, abandoned and stray cats on the island by 2025.

One argument that has been made in both online comments and letters to the editor is that the community should consider that a decline in feral cats could lead to an increase in the number of rats around the island.

The Garden Island posed this question to Raine, Pias and Ka’iakapu, and all said there is little research to suggest that would in fact be the case.

Pias said there is a lot of anecdotal information about cats killing lots of rats, but little research to back it up. In fact, studies suggest that rat populations stay relatively the same after cat eradication.

“I very much doubt they have a large impact on the rat problem,” added Raine.

From a conservation standpoint, Ka’iakapu said a best-case scenario would be to get rid of both. The challenge is finding a method that does not result in public backlash.

“It’s a tough one for us because it’s a sensitive issue,” he said.

Ultimately, Raine feels it is a war of attrition, with invasive pests slowly taking out native populations of birds and other wildlife.

“Management is critical,” he said. “If you leave it be, they will cause their extinction.”