Some of the highest rat densities ever measured on the New Zealand mainland are being recorded in a study run by Manaaki
Whenua – Landcare Research, at a remote Fiordland forest site. The study, run in collaboration with the Department of
Conservation, is showing the remarkable ability of rats to multiply rapidly following beech seeding, such as the “mega
mast” (mass seeding event) that occurred in 2019.
But scientists are hoping that understanding how altitude and food availability regulate rat numbers will give
conservationists the edge in protecting wildlife from rat plagues, which can lead to localised extinctions of native
wildlife.
“Rats are generally less common in cold, high altitude forests across New Zealand than in warm, lowland forests,” says
study lead Dr Jo Carpenter, a postdoctoral researcher at Manaaki Whenua. “But it’s not clear whether that’s because rats
can’t handle cold temperatures, or because there is typically less food, especially in winter, for rats in cold, high
elevation forests.”
To tease these factors apart, the team of researchers have been intensively monitoring rat population dynamics at both
high and low elevations at Lake Alabaster, Fiordland. Rats at high elevation are actually being fed to see whether they
can survive cold temperatures when they have sufficient food.
“This is a really relevant question, because if it’s temperature that normally limits rats from living up high, and not
food, we might expect to see high elevation forests supporting more rats as the climate warms,” warned Manaaki Whenua
researcher Dr Adrian Monks. “This could have devastating consequences for some of our birds, which currently use these
environments as refugia from pests.”
“In the meantime, the study has direct implications for current pest control programmes,” says DOC science advisor Dr
James Griffiths. “Cost-effective pest management relies on knowing where and when pest numbers are high so management
can be focused on the right place at the right time. This study is going to help us a lot with that, and ought to result
in improved pest control and healthier forests.”
Since the study began 14 months ago, 912 individual rats have been live-captured in cage traps at the site. The
researchers give each rat a unique ID by inserting a microchip under the rat’s skin – like the type used for pet dogs –
and placing a metal tag in their ear. The rat is then released. While releasing a rat goes against the grain for most
conservationists, the team can then calculate how many rats live in an area by looking at the proportion of marked and
unmarked rats they capture.
“This is one of the first studies to estimate the density of rats in South Island forests,” comments Manaaki Whenua
researcher John Innes. “Normally, ecologists obtain relative measures of rat abundance or activity. While this is useful
and reasonably easy to do, we often don’t actually know how those estimates relate to the number of rats that occupy an
area.”
The preliminary results have been startling. Following the beech seed mast in 2019, rats at Lake Alabaster reached a
phenomenal 17 rats per hectare. These are some of the highest rat densities ever measured on the New Zealand mainland,
and reflect the incredible ability of rats to multiply rapidly following beech seeding.
But is the extra food making a difference to whether rats can survive up high? “We’re still monitoring the population,”
cautions Carpenter, “but it seems that food helped sustain the rats through the autumn. When we reached winter, though,
the fed rats declined as much as the rats we didn’t feed. This suggests that another factor – perhaps temperature or
predation by stoats – is limiting rats.”
“One of the hardest parts of this study,” says Carpenter, “is letting the rats go.” In order to get a true idea of pest
densities, the team uses live-capture traps, releasing the rats to live another day. But they hope it will all be worth
it in the end. The study is part of a five-year research program titled ‘More Birds in the Bush’, funded by the Ministry
of Business, Innovation, and Employment’s Endeavour fund. A key aim of the program is to create a kind of “weather
forecast about rats”, where conservation managers will be able to predict what rats will do based on the climate and
forest at their site. In turn, this will allow rat control to be done as effectively as possible, resulting in more
birds in the bush.