Beaked rat: scientists observe predator overload in New Zealand bushes

Scientists have observed some of the highest rat densities in the wild in New Zealand – a remote part of Fiordland recorded 17 of the vermin per hectare.

It occurred during a recent “mega mast” that brought pest numbers to pest proportions, and was seen as part of a large study in which nearly 1,000 rats were captured and microchipped.

Each year, rats, stoats and possums kill 25 million native birds.

Now scientists hope that understanding how altitude and food availability regulate rat numbers will give conservationists a head start in protecting wildlife from rat infestations, which can lead to localized extinctions of native wildlife.

“Rats are generally less common in cold high-altitude forests in New Zealand than in warm lowland forests,” said Dr Jo Carpenter, a postdoctoral researcher at Manaaki Whenua-Landcare Research who has been leading the study.

“But it is not clear if that is because rats cannot withstand cold temperatures, or because there is usually less food, especially in winter, for rats in cold forests at high altitudes.”

To separate the factors, the team of researchers has been intensively monitoring rat population dynamics at high and low elevations on Lake Alabaster in Fiordland.

In reality, high-altitude rats are being fed to see if they can survive the low temperatures when they have enough food.

“This is a really relevant question, because if it’s the temperature that normally prevents rats from living high up, and not food, we might expect to see high-altitude forests harboring more rats as the weather warms up,” said Dr. Adrian Monks, a researcher at Manaaki Whenua. said.

“This could have devastating consequences for some of our birds, who currently use these environments as refuges for pests.”

Meanwhile, the study had direct implications for current pest control programs.

“Profitable pest management relies on knowing where and when pest numbers are high so that management can focus on the right place at the right time,” said Dr. James Griffiths, scientific advisor to the Department of Conservation. .

“This study will help us a lot with that and should result in better pest control and healthier forests.”

Since the study began 14 months ago, 912 individual rats have been captured alive in cage traps at the site.

The researchers gave each rat a unique ID by inserting a microchip under its skin, like the kind used for domestic dogs, and then attached a metal ear tag and released them.

While releasing a rat might be the grain for most conservationists, it allowed the team to calculate how many rats live in an area by looking at the ratio of marked and unmarked rats that they capture.

“This is one of the first studies to estimate the density of rats in the forests of the South Island,” said Manaaki Whenua researcher John Innes.

“Typically, ecologists get relative measures of rat abundance or activity. While this is useful and reasonably easy to do, we often don’t really know how those estimates relate to the number of rats occupying an area.”

The preliminary results have been surprising.

After the beech seed mast in 2019, the rats in Lake Alabaster reached a phenomenal 17 rats per hectare.

These were some of the highest rat densities ever measured on the New Zealand mainland and reflected the incredible ability of rats to multiply rapidly after beech planting.

But did the extra food influence the rats’ ability to survive high up?

“We are still monitoring the population, but it appears that the food helped support the rats through the fall,” Carpenter said.

“However, when we got to winter, the fed rats declined as much as the non-fed rats.

“This suggests that another factor, perhaps temperature or predation by stoats, is limiting rats.”

The study was part of a five-year research program entitled “More Birds in the Wild”, funded by the Endeavor fund of the Ministry of Business, Innovation and Employment.

A key goal of the program was to create a kind of “rat weather forecast,” where conservation managers will be able to predict what rats will do based on the climate and forest on their site.

In turn, this will allow rat control to be done as effectively as possible, resulting in more birds in the bush.

The first findings come after scientists made another breakthrough in the effort against the scourge of New Zealand rats – rebuilding the ship rat genome.

Understanding the genetic makeup of the species now allows researchers to investigate how rats adapt to changing environments and what makes them so successful as invasive predators here and around the world.